Merge tag 'trace-v7.3-rc2' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-tracegrafted

Pull tracing fixes from Steven Rostedt:

 - Don't destroy user event fields when removal fails

   User event fields are destroyed before the event is removed from
   visibility. But that can fail leaving the still visible event with no
   fields. Move the destroying of the fields to after the event is
   successfully removed from visibility.

 - Initialize function graph state is fork before calling
   copy_exec_state()

   For non-CLONE_VM forks, copy_exec_state() allocates a new
   task_exec_state. If that allocation fails, ftrace_graph_exit_task()
   will free the tasks ret_stack pointer. Since that pointer is still
   using the parent's ret_stack, it mistakenly frees the parent's
   pointer too.

   Call ftrace_graph_init() on the task first which will NULL out the
   new tasks's ret_stack and if the copy fails, it will not free
   anything.

 - Remove FGRAPH_MAX_INDEX

   The macro FGRAPH_MAX_INDEX was added but never used. Remove it.

 - Save ent_size in function graph printing of nested functions

   The function graph tracer needs to look at the next event to see if
   the next event is the return of the current function entry. If it is,
   it prints a single line:

	ktime_get();

   Otherwise it prints it like a nested function:

	tick_nohz_irq_exit() {
	    ktime_get();
	    kcpustat_irq_exit();
	}

   In order to look at the next event, it must save the current event so
   that it has the information to print from it. It saves the event in
   the iterator descriptor called "ent". What it doesn't save is the
   ent_size of the event which is now used to know if the function graph
   arguments are to be printed. The peek doesn't save the size so the
   size used happens to be that of the size of the last event that was
   seen.

   Save the entry event size in the iterator descriptor so that the
   correct size is used.

 - Fix several errors with freeing data in the histogram code

   The histogram code had a lot of leaked or or incorrect accounting
   when failures happen. Correct them.

 - Fix histogram regression of .percent and .graph modifiers

   Up until 6.3 histogram values could have "percent" or "graph"
   modifiers that changed how they were printed. But a change that added
   restricting histograms values from being strings, stack traces and
   other modifiers inadvertently prevented them from using the percent
   and graph modifiers, which were legal use cases for values.

   Put back the percent and graph modifiers.

 - Fix various typos in the comments

 - Set the trace_clock before initializing a histogram with clock
   argument

   The histogram API allows the user to specific which trace clock to
   use via a "clock=" string. The histogram is set up first before the
   clock is checked. If the passed in clock is not valid, it exits
   without fully fixing up the histogram leaving it on the list and a
   use-after-free can trigger.

   Update the clock argument first and if it fails then exit gracefully
   before the histogram trigger is placed on any lists.

 - Restore :mod: trailer after parsing in ftrace_set_clr_event

   The function ftrace_set_clr_event() modifies the parse string and
   needs to put it back to what was passed in. It searches for ":mod:"
   via a strsep() but fails to put back the first ':' in the string.

   Add back the ':' in the passed in string.

 - Take trace_array reference when opening a tracer options file

   The options files are dynamically created and some tracers add their
   own options. When a tracer adds their own list of options, the
   trace_array holding them has an array to hold the list of options for
   each tracer. This array increases in size via a krealloc(), and the
   new entry gets a newly allocated array to hold the options of the new
   tracer being added.

   The element in each entry of the tracer's option array holds a
   pointer back to the trace_array, a pointer to the tracer it is
   associated to, a pointer to the flags of the option.

   The issue is that these arrays are freed when the trace_array is
   freed when its instance it represents is removed from the instances
   directory. There's a race that an open of one of these options files
   can happen when the instance is being removed.

   Add a new helper function to be called by the open function of the
   options file to iterate all existing trace_arrays under a lock and
   find the one that has the given option element in one of it's tracer
   arrays. If found, then update the associated trace_array's reference
   counter to keep it from being freed. If not found, have the open call
   return -ENODEV.

 - Disable interrupts when acquiring the lock in rb_wake_up_waiters()

   The function rb_wake_up_waiters() assumes it will be called in
   interrupt context and does not disable irqs when taking
   cpu_buffer->reader_lock, which can be called in hard interrupt
   context. The issue is in PREEMPT_RT, this function is called in
   thread context leaving this lock open to a deadlock.

   Take the lock with interrupts disabled.

 - Use rcu_assign_pointer() for tmp_ops filter hash

   The tmp_ops used in update_ftrace_direct_mod() assigns its
   filter_hash field directly, but that field is annotated as __rcu and
   sparse complains. Assign it with rcu_assign_pointer()

 - Fix use-after-free in enable_trigger_private_data_free()

   The trace_event_call is accessed through the event_trigger_data's
   trace_event_file pointer to put the trace_event_call on freeing. The
   issue is that the trace_event_file data may have been freed already
   causing a use-after-free. Add a field to the event_trigger_data that
   points directly to the trace_event_call so that it can decrement its
   reference directly without needing to go through the
   trace_event_file.

 - Fix accounting of buffer data remote headers

   trace_buffer_desc_size() and trace_remote_alloc_buffer() undercount
   the number of pages is needed for the asked for size as it doesn't
   take into account the meta data on each page. Add a helper function
   to do the calculation properly and use that in these functions.

 - Catch nr_page_va overflow in ring_buffer_desc sizing

   The number of pages per remote ring buffer is capped by
   ring_buffer_desc::nr_page_va (32 bits). A buffer_size large enough to
   overflow that field would silently allocate a descriptor smaller than
   what was asked for.

 - Do not resize the subbuf order if any per_cpu buffer is disabled

   The mmapping of ring buffers disables resizing the subbuffers, but it
   is done per-cpu whereas the subbuf size change is done for all the
   per_cpu buffers under the buffer->mutex. It could change the size of
   some while the mapping is happening on others. Have the resize of the
   subbuf order check all the per_cpu buffers under the lock to see if
   any of them is disabled before starting and causing an inconsistency
   between buffers that are being mapped.

* tag 'trace-v7.3-rc2' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace: (25 commits)
  ring-buffer: Check resize_disabled before publishing the new subbuf order
  tracing/remotes: Catch nr_page_va overflow in ring_buffer_desc sizing
  tracing/remotes: Account for ring buffer page header in size calculation
  tracing: Don't dereference trace_event_file in deferred trigger free
  ftrace: Use rcu_assign_pointer() for tmp_ops filter hash
  ring-buffer: Acquire the lock with irqsave in rb_wake_up_waiters()
  tracing: Take trace_array reference when opening a tracer options file
  tracing: Fix ring_buffer_read_page_size() kernel-doc
  tracing: Restore :mod: trailer after parsing in ftrace_set_clr_event()
  tracing: Fix memory corruption from a "STACKTRACE" histogram key
  tracing: Fix memory corruption from the histogram stacktrace modifier
  tracing: Undo the registration when enabling the histogram trigger fails
  tracing: Take the reference before publishing the named histogram trigger
  tracing: Set the trace clock before registering the histogram trigger
  tracing: Fix typo "preceeded" in comment
  tracing: Fix typo "availabe" in comment
  tracing: Let histogram values keep the percent and graph modifiers
  tracing: Keep the entry count when the histogram stats allocation fails
  tracing: Free histogram the field rejected for a bad modifier
  tracing: Free histogram the var ref when its initialization fails
  ...

1 files changed, 863 insertions, 0 deletions

diff --git a/rust/zerocopy-derive/util.rs b/rust/zerocopy-derive/util.rs
new file mode 100644
index 000000000..5c5e9d3bd
--- /dev/null
+++ b/rust/zerocopy-derive/util.rs
@@ -0,0 +1,863 @@
+// SPDX-License-Identifier: (BSD-2-Clause OR Apache-2.0) OR MIT
+//
+// Copyright 2019 The Fuchsia Authors
+//
+// Licensed under a BSD-style license <LICENSE-BSD>, Apache License, Version 2.0
+// <LICENSE-APACHE or https://www.apache.org/licenses/LICENSE-2.0>, or the MIT
+// license <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your option.
+// This file may not be copied, modified, or distributed except according to
+// those terms.
+
+use std::num::NonZeroU32;
+
+use proc_macro2::{Span, TokenStream};
+use quote::{quote, quote_spanned, ToTokens};
+use syn::{
+    parse_quote, spanned::Spanned as _, Data, DataEnum, DataStruct, DataUnion, DeriveInput, Error,
+    Expr, ExprLit, Field, GenericParam, Ident, Index, Lit, LitStr, Meta, Path, Type, Variant,
+    Visibility, WherePredicate,
+};
+
+use crate::repr::{CompoundRepr, EnumRepr, PrimitiveRepr, Repr, Spanned};
+
+pub(crate) struct Ctx {
+    pub(crate) ast: DeriveInput,
+    pub(crate) zerocopy_crate: Path,
+
+    // The value of the last `#[zerocopy(on_error = ...)]` attribute, or `false`
+    // if none is provided.
+    pub(crate) skip_on_error: bool,
+
+    // The span of the last `#[zerocopy(on_error = ...)]` attribute, if any.
+    pub(crate) on_error_span: Option<proc_macro2::Span>,
+}
+
+impl Ctx {
+    /// Attempt to extract a crate path from the provided attributes. Defaults to
+    /// `::zerocopy` if not found.
+    pub(crate) fn try_from_derive_input(ast: DeriveInput) -> Result<Self, Error> {
+        let mut path = parse_quote!(::zerocopy);
+        let mut skip_on_error = false;
+        let mut on_error_span = None;
+
+        for attr in &ast.attrs {
+            if let Meta::List(ref meta_list) = attr.meta {
+                if meta_list.path.is_ident("zerocopy") {
+                    attr.parse_nested_meta(|meta| {
+                        if meta.path.is_ident("crate") {
+                            let expr = meta.value().and_then(|value| value.parse());
+                            if let Ok(Expr::Lit(ExprLit { lit: Lit::Str(lit), .. })) = expr {
+                                if let Ok(path_lit) = lit.parse::<Ident>() {
+                                    path = parse_quote!(::#path_lit);
+                                    return Ok(());
+                                }
+                            }
+
+                            return Err(Error::new(
+                                Span::call_site(),
+                                "`crate` attribute requires a path as the value",
+                            ));
+                        }
+
+                        if meta.path.is_ident("on_error") {
+                            on_error_span = Some(meta.path.span());
+                            let value = meta.value()?;
+                            let s: LitStr = value.parse()?;
+                            match s.value().as_str() {
+                                "skip" => skip_on_error = true,
+                                "fail" => skip_on_error = false,
+                                _ => return Err(Error::new(
+                                    s.span(),
+                                    "unrecognized value for `on_error` attribute from `zerocopy`; expected `skip` or `fail`",
+                                )),
+                            }
+                            return Ok(());
+                        }
+
+                        Err(Error::new(
+                            Span::call_site(),
+                            format!(
+                                "unknown attribute encountered: {}",
+                                meta.path.into_token_stream()
+                            ),
+                        ))
+                    })?;
+                }
+            }
+        }
+
+        Ok(Self { ast, zerocopy_crate: path, skip_on_error, on_error_span })
+    }
+
+    pub(crate) fn with_input(&self, input: &DeriveInput) -> Self {
+        Self {
+            ast: input.clone(),
+            zerocopy_crate: self.zerocopy_crate.clone(),
+            skip_on_error: self.skip_on_error,
+            on_error_span: self.on_error_span,
+        }
+    }
+
+    pub(crate) fn skip_on_error(mut self) -> Self {
+        self.skip_on_error = true;
+        self
+    }
+
+    pub(crate) fn core_path(&self) -> TokenStream {
+        let zerocopy_crate = &self.zerocopy_crate;
+        quote!(#zerocopy_crate::util::macro_util::core_reexport)
+    }
+
+    pub(crate) fn cfg_compile_error(&self) -> TokenStream {
+        // By checking both during the compilation of the proc macro *and* in
+        // the generated code, we ensure that `--cfg
+        // zerocopy_unstable_linux` need only be passed *either* when
+        // compiling this crate *or* when compiling the user's crate. The former
+        // is preferable, but in some situations (such as when cross-compiling
+        // using `cargo build --target`), it doesn't get propagated to this
+        // crate's build by default.
+        if cfg!(zerocopy_unstable_linux) {
+            quote!()
+        } else if let Some(span) = self.on_error_span {
+            let core = self.core_path();
+            let error_message =
+                "`on_error` is experimental; pass '--cfg zerocopy_unstable_linux' to enable";
+            quote::quote_spanned! {span=>
+                #[allow(unused_attributes, unexpected_cfgs)]
+                const _: () = {
+                    #[cfg(not(zerocopy_unstable_linux))]
+                    #core::compile_error!(#error_message);
+                };
+            }
+        } else {
+            quote!()
+        }
+    }
+
+    pub(crate) fn error_or_skip<E>(&self, error: E) -> Result<TokenStream, E> {
+        if self.skip_on_error {
+            Ok(self.cfg_compile_error())
+        } else {
+            Err(error)
+        }
+    }
+}
+
+pub(crate) trait DataExt {
+    /// Extracts the names and types of all fields. For enums, extracts the
+    /// names and types of fields from each variant. For tuple structs, the
+    /// names are the indices used to index into the struct (ie, `0`, `1`, etc).
+    ///
+    /// FIXME: Extracting field names for enums doesn't really make sense. Types
+    /// makes sense because we don't care about where they live - we just care
+    /// about transitive ownership. But for field names, we'd only use them when
+    /// generating is_bit_valid, which cares about where they live.
+    fn fields(&self) -> Vec<(&Visibility, TokenStream, &Type)>;
+
+    fn variants(&self) -> Vec<(Option<&Variant>, Vec<(&Visibility, TokenStream, &Type)>)>;
+
+    fn tag(&self) -> Option<Ident>;
+}
+
+impl DataExt for Data {
+    fn fields(&self) -> Vec<(&Visibility, TokenStream, &Type)> {
+        match self {
+            Data::Struct(strc) => strc.fields(),
+            Data::Enum(enm) => enm.fields(),
+            Data::Union(un) => un.fields(),
+        }
+    }
+
+    fn variants(&self) -> Vec<(Option<&Variant>, Vec<(&Visibility, TokenStream, &Type)>)> {
+        match self {
+            Data::Struct(strc) => strc.variants(),
+            Data::Enum(enm) => enm.variants(),
+            Data::Union(un) => un.variants(),
+        }
+    }
+
+    fn tag(&self) -> Option<Ident> {
+        match self {
+            Data::Struct(strc) => strc.tag(),
+            Data::Enum(enm) => enm.tag(),
+            Data::Union(un) => un.tag(),
+        }
+    }
+}
+
+impl DataExt for DataStruct {
+    fn fields(&self) -> Vec<(&Visibility, TokenStream, &Type)> {
+        map_fields(&self.fields)
+    }
+
+    fn variants(&self) -> Vec<(Option<&Variant>, Vec<(&Visibility, TokenStream, &Type)>)> {
+        vec![(None, self.fields())]
+    }
+
+    fn tag(&self) -> Option<Ident> {
+        None
+    }
+}
+
+impl DataExt for DataEnum {
+    fn fields(&self) -> Vec<(&Visibility, TokenStream, &Type)> {
+        map_fields(self.variants.iter().flat_map(|var| &var.fields))
+    }
+
+    fn variants(&self) -> Vec<(Option<&Variant>, Vec<(&Visibility, TokenStream, &Type)>)> {
+        self.variants.iter().map(|var| (Some(var), map_fields(&var.fields))).collect()
+    }
+
+    fn tag(&self) -> Option<Ident> {
+        Some(Ident::new("___ZerocopyTag", Span::call_site()))
+    }
+}
+
+impl DataExt for DataUnion {
+    fn fields(&self) -> Vec<(&Visibility, TokenStream, &Type)> {
+        map_fields(&self.fields.named)
+    }
+
+    fn variants(&self) -> Vec<(Option<&Variant>, Vec<(&Visibility, TokenStream, &Type)>)> {
+        vec![(None, self.fields())]
+    }
+
+    fn tag(&self) -> Option<Ident> {
+        None
+    }
+}
+
+fn map_fields<'a>(
+    fields: impl 'a + IntoIterator<Item = &'a Field>,
+) -> Vec<(&'a Visibility, TokenStream, &'a Type)> {
+    fields
+        .into_iter()
+        .enumerate()
+        .map(|(idx, f)| {
+            (
+                &f.vis,
+                f.ident
+                    .as_ref()
+                    .map(ToTokens::to_token_stream)
+                    .unwrap_or_else(|| Index::from(idx).to_token_stream()),
+                &f.ty,
+            )
+        })
+        .collect()
+}
+
+pub(crate) fn to_ident_str(t: &impl ToString) -> String {
+    let s = t.to_string();
+    if let Some(stripped) = s.strip_prefix("r#") {
+        stripped.to_string()
+    } else {
+        s
+    }
+}
+
+/// This enum describes what kind of padding check needs to be generated for the
+/// associated impl.
+pub(crate) enum PaddingCheck {
+    /// Check that the sum of the fields' sizes exactly equals the struct's
+    /// size.
+    Struct,
+    /// Check that a `repr(C)` struct has no padding.
+    ReprCStruct,
+    /// Check that the size of each field exactly equals the union's size.
+    Union,
+    /// Check that every variant of the enum contains no padding.
+    ///
+    /// Because doing so requires a tag enum, this padding check requires an
+    /// additional `TokenStream` which defines the tag enum as `___ZerocopyTag`.
+    Enum { tag_type_definition: TokenStream },
+}
+
+impl PaddingCheck {
+    /// Returns the idents of the trait to use and the macro to call in order to
+    /// validate that a type passes the relevant padding check.
+    pub(crate) fn validator_trait_and_macro_idents(&self) -> (Ident, Ident) {
+        let (trt, mcro) = match self {
+            PaddingCheck::Struct => ("PaddingFree", "struct_padding"),
+            PaddingCheck::ReprCStruct => ("DynamicPaddingFree", "repr_c_struct_has_padding"),
+            PaddingCheck::Union => ("PaddingFree", "union_padding"),
+            PaddingCheck::Enum { .. } => ("PaddingFree", "enum_padding"),
+        };
+
+        let trt = Ident::new(trt, Span::call_site());
+        let mcro = Ident::new(mcro, Span::call_site());
+        (trt, mcro)
+    }
+
+    /// Sometimes performing the padding check requires some additional
+    /// "context" code. For enums, this is the definition of the tag enum.
+    pub(crate) fn validator_macro_context(&self) -> Option<&TokenStream> {
+        match self {
+            PaddingCheck::Struct | PaddingCheck::ReprCStruct | PaddingCheck::Union => None,
+            PaddingCheck::Enum { tag_type_definition } => Some(tag_type_definition),
+        }
+    }
+}
+
+#[derive(Clone)]
+pub(crate) enum Trait {
+    KnownLayout,
+    HasTag,
+    HasField {
+        variant_id: Box<Expr>,
+        field: Box<Type>,
+        field_id: Box<Expr>,
+    },
+    ProjectField {
+        variant_id: Box<Expr>,
+        field: Box<Type>,
+        field_id: Box<Expr>,
+        invariants: Box<Type>,
+    },
+    Immutable,
+    TryFromBytes,
+    FromZeros,
+    FromBytes,
+    IntoBytes,
+    Unaligned,
+    Sized,
+    ByteHash,
+    ByteEq,
+    SplitAt,
+}
+
+impl ToTokens for Trait {
+    fn to_tokens(&self, tokens: &mut TokenStream) {
+        // According to [1], the format of the derived `Debug`` output is not
+        // stable and therefore not guaranteed to represent the variant names.
+        // Indeed with the (unstable) `fmt-debug` compiler flag [2], it can
+        // return only a minimalized output or empty string. To make sure this
+        // code will work in the future and independent of the compiler flag, we
+        // translate the variants to their names manually here.
+        //
+        // [1] https://doc.rust-lang.org/1.81.0/std/fmt/trait.Debug.html#stability
+        // [2] https://doc.rust-lang.org/beta/unstable-book/compiler-flags/fmt-debug.html
+        let s = match self {
+            Trait::HasField { .. } => "HasField",
+            Trait::ProjectField { .. } => "ProjectField",
+            Trait::KnownLayout => "KnownLayout",
+            Trait::HasTag => "HasTag",
+            Trait::Immutable => "Immutable",
+            Trait::TryFromBytes => "TryFromBytes",
+            Trait::FromZeros => "FromZeros",
+            Trait::FromBytes => "FromBytes",
+            Trait::IntoBytes => "IntoBytes",
+            Trait::Unaligned => "Unaligned",
+            Trait::Sized => "Sized",
+            Trait::ByteHash => "ByteHash",
+            Trait::ByteEq => "ByteEq",
+            Trait::SplitAt => "SplitAt",
+        };
+        let ident = Ident::new(s, Span::call_site());
+        let arguments: Option<syn::AngleBracketedGenericArguments> = match self {
+            Trait::HasField { variant_id, field, field_id } => {
+                Some(parse_quote!(<#field, #variant_id, #field_id>))
+            }
+            Trait::ProjectField { variant_id, field, field_id, invariants } => {
+                Some(parse_quote!(<#field, #invariants, #variant_id, #field_id>))
+            }
+            Trait::KnownLayout
+            | Trait::HasTag
+            | Trait::Immutable
+            | Trait::TryFromBytes
+            | Trait::FromZeros
+            | Trait::FromBytes
+            | Trait::IntoBytes
+            | Trait::Unaligned
+            | Trait::Sized
+            | Trait::ByteHash
+            | Trait::ByteEq
+            | Trait::SplitAt => None,
+        };
+        tokens.extend(quote!(#ident #arguments));
+    }
+}
+
+impl Trait {
+    pub(crate) fn crate_path(&self, ctx: &Ctx) -> Path {
+        let zerocopy_crate = &ctx.zerocopy_crate;
+        let core = ctx.core_path();
+        match self {
+            Self::Sized => parse_quote!(#core::marker::#self),
+            _ => parse_quote!(#zerocopy_crate::#self),
+        }
+    }
+}
+
+pub(crate) enum TraitBound {
+    Slf,
+    Other(Trait),
+}
+
+pub(crate) enum FieldBounds<'a> {
+    None,
+    All(&'a [TraitBound]),
+    Trailing(&'a [TraitBound]),
+    Explicit(Vec<WherePredicate>),
+}
+
+impl<'a> FieldBounds<'a> {
+    pub(crate) const ALL_SELF: FieldBounds<'a> = FieldBounds::All(&[TraitBound::Slf]);
+    pub(crate) const TRAILING_SELF: FieldBounds<'a> = FieldBounds::Trailing(&[TraitBound::Slf]);
+}
+
+pub(crate) enum SelfBounds<'a> {
+    None,
+    All(&'a [Trait]),
+}
+
+// FIXME(https://github.com/rust-lang/rust-clippy/issues/12908): This is a false
+// positive. Explicit lifetimes are actually necessary here.
+#[allow(clippy::needless_lifetimes)]
+impl<'a> SelfBounds<'a> {
+    pub(crate) const SIZED: Self = Self::All(&[Trait::Sized]);
+}
+
+/// Normalizes a slice of bounds by replacing [`TraitBound::Slf`] with `slf`.
+pub(crate) fn normalize_bounds<'a>(
+    slf: &'a Trait,
+    bounds: &'a [TraitBound],
+) -> impl 'a + Iterator<Item = Trait> {
+    bounds.iter().map(move |bound| match bound {
+        TraitBound::Slf => slf.clone(),
+        TraitBound::Other(trt) => trt.clone(),
+    })
+}
+
+pub(crate) struct ImplBlockBuilder<'a> {
+    ctx: &'a Ctx,
+    data: &'a dyn DataExt,
+    trt: Trait,
+    field_type_trait_bounds: FieldBounds<'a>,
+    self_type_trait_bounds: SelfBounds<'a>,
+    padding_check: Option<PaddingCheck>,
+    param_extras: Vec<GenericParam>,
+    inner_extras: Option<TokenStream>,
+    outer_extras: Option<TokenStream>,
+}
+
+impl<'a> ImplBlockBuilder<'a> {
+    pub(crate) fn new(
+        ctx: &'a Ctx,
+        data: &'a dyn DataExt,
+        trt: Trait,
+        field_type_trait_bounds: FieldBounds<'a>,
+    ) -> Self {
+        Self {
+            ctx,
+            data,
+            trt,
+            field_type_trait_bounds,
+            self_type_trait_bounds: SelfBounds::None,
+            padding_check: None,
+            param_extras: Vec::new(),
+            inner_extras: None,
+            outer_extras: None,
+        }
+    }
+
+    pub(crate) fn self_type_trait_bounds(mut self, self_type_trait_bounds: SelfBounds<'a>) -> Self {
+        self.self_type_trait_bounds = self_type_trait_bounds;
+        self
+    }
+
+    pub(crate) fn padding_check<P: Into<Option<PaddingCheck>>>(mut self, padding_check: P) -> Self {
+        self.padding_check = padding_check.into();
+        self
+    }
+
+    pub(crate) fn param_extras(mut self, param_extras: Vec<GenericParam>) -> Self {
+        self.param_extras.extend(param_extras);
+        self
+    }
+
+    pub(crate) fn inner_extras(mut self, inner_extras: TokenStream) -> Self {
+        self.inner_extras = Some(inner_extras);
+        self
+    }
+
+    pub(crate) fn outer_extras<T: Into<Option<TokenStream>>>(mut self, outer_extras: T) -> Self {
+        self.outer_extras = outer_extras.into();
+        self
+    }
+
+    pub(crate) fn build(self) -> TokenStream {
+        // In this documentation, we will refer to this hypothetical struct:
+        //
+        //   #[derive(FromBytes)]
+        //   struct Foo<T, I: Iterator>
+        //   where
+        //       T: Copy,
+        //       I: Clone,
+        //       I::Item: Clone,
+        //   {
+        //       a: u8,
+        //       b: T,
+        //       c: I::Item,
+        //   }
+        //
+        // We extract the field types, which in this case are `u8`, `T`, and
+        // `I::Item`. We re-use the existing parameters and where clauses. If
+        // `require_trait_bound == true` (as it is for `FromBytes), we add where
+        // bounds for each field's type:
+        //
+        //   impl<T, I: Iterator> FromBytes for Foo<T, I>
+        //   where
+        //       T: Copy,
+        //       I: Clone,
+        //       I::Item: Clone,
+        //       T: FromBytes,
+        //       I::Item: FromBytes,
+        //   {
+        //   }
+        //
+        // NOTE: It is standard practice to only emit bounds for the type
+        // parameters themselves, not for field types based on those parameters
+        // (e.g., `T` vs `T::Foo`). For a discussion of why this is standard
+        // practice, see https://github.com/rust-lang/rust/issues/26925.
+        //
+        // The reason we diverge from this standard is that doing it that way
+        // for us would be unsound. E.g., consider a type, `T` where `T:
+        // FromBytes` but `T::Foo: !FromBytes`. It would not be sound for us to
+        // accept a type with a `T::Foo` field as `FromBytes` simply because `T:
+        // FromBytes`.
+        //
+        // While there's no getting around this requirement for us, it does have
+        // the pretty serious downside that, when lifetimes are involved, the
+        // trait solver ties itself in knots:
+        //
+        //     #[derive(Unaligned)]
+        //     #[repr(C)]
+        //     struct Dup<'a, 'b> {
+        //         a: PhantomData<&'a u8>,
+        //         b: PhantomData<&'b u8>,
+        //     }
+        //
+        //     error[E0283]: type annotations required: cannot resolve `core::marker::PhantomData<&'a u8>: zerocopy::Unaligned`
+        //      --> src/main.rs:6:10
+        //       |
+        //     6 | #[derive(Unaligned)]
+        //       |          ^^^^^^^^^
+        //       |
+        //       = note: required by `zerocopy::Unaligned`
+
+        let type_ident = &self.ctx.ast.ident;
+        let trait_path = self.trt.crate_path(self.ctx);
+        let fields = self.data.fields();
+        let variants = self.data.variants();
+        let tag = self.data.tag();
+        let zerocopy_crate = &self.ctx.zerocopy_crate;
+
+        fn bound_tt(ty: &Type, traits: impl Iterator<Item = Trait>, ctx: &Ctx) -> WherePredicate {
+            let traits = traits.map(|t| t.crate_path(ctx));
+            parse_quote!(#ty: #(#traits)+*)
+        }
+        let field_type_bounds: Vec<_> = match (self.field_type_trait_bounds, &fields[..]) {
+            (FieldBounds::All(traits), _) => fields
+                .iter()
+                .map(|(_vis, _name, ty)| {
+                    bound_tt(ty, normalize_bounds(&self.trt, traits), self.ctx)
+                })
+                .collect(),
+            (FieldBounds::None, _) | (FieldBounds::Trailing(..), []) => vec![],
+            (FieldBounds::Trailing(traits), [.., last]) => {
+                vec![bound_tt(last.2, normalize_bounds(&self.trt, traits), self.ctx)]
+            }
+            (FieldBounds::Explicit(bounds), _) => bounds,
+        };
+
+        let padding_check_bound = self
+            .padding_check
+            .map(|check| {
+                // Parse the repr for `align` and `packed` modifiers. Note that
+                // `Repr::<PrimitiveRepr, NonZeroU32>` is more permissive than
+                // what Rust supports for structs, enums, or unions, and thus
+                // reliably extracts these modifiers for any kind of type.
+                let repr =
+                    Repr::<PrimitiveRepr, NonZeroU32>::from_attrs(&self.ctx.ast.attrs).unwrap();
+                let core = self.ctx.core_path();
+                let option = quote! { #core::option::Option };
+                let nonzero = quote! { #core::num::NonZeroUsize };
+                let none = quote! { #option::None::<#nonzero> };
+                let repr_align =
+                    repr.get_align().map(|spanned| {
+                        let n = spanned.t.get();
+                        quote_spanned! { spanned.span => (#nonzero::new(#n as usize)) }
+                    }).unwrap_or(quote! { (#none) });
+                let repr_packed =
+                    repr.get_packed().map(|packed| {
+                        let n = packed.get();
+                        quote! { (#nonzero::new(#n as usize)) }
+                    }).unwrap_or(quote! { (#none) });
+                let variant_types = variants.iter().map(|(_, fields)| {
+                    let types = fields.iter().map(|(_vis, _name, ty)| ty);
+                    quote!([#((#types)),*])
+                });
+                let validator_context = check.validator_macro_context();
+                let (trt, validator_macro) = check.validator_trait_and_macro_idents();
+                let t = tag.iter();
+                parse_quote! {
+                    (): #zerocopy_crate::util::macro_util::#trt<
+                        Self,
+                        {
+                            #validator_context
+                            #zerocopy_crate::#validator_macro!(Self, #repr_align, #repr_packed, #(#t,)* #(#variant_types),*)
+                        }
+                    >
+                }
+            });
+
+        let self_bounds: Option<WherePredicate> = match self.self_type_trait_bounds {
+            SelfBounds::None => None,
+            SelfBounds::All(traits) => {
+                Some(bound_tt(&parse_quote!(Self), traits.iter().cloned(), self.ctx))
+            }
+        };
+
+        let zerocopy_bounds =
+            field_type_bounds
+                .into_iter()
+                .chain(padding_check_bound)
+                .chain(self_bounds)
+                .map(|bound| {
+                    if self.ctx.skip_on_error {
+                        parse_quote!(for<'zc> #bound)
+                    } else {
+                        bound.clone()
+                    }
+                })
+                .collect::<Vec<_>>();
+
+        let bounds = self
+            .ctx
+            .ast
+            .generics
+            .where_clause
+            .as_ref()
+            .map(|where_clause| where_clause.predicates.iter())
+            .into_iter()
+            .flatten()
+            .chain(zerocopy_bounds.iter());
+
+        // The parameters with trait bounds, but without type defaults.
+        let mut params: Vec<_> = self
+            .ctx
+            .ast
+            .generics
+            .params
+            .clone()
+            .into_iter()
+            .map(|mut param| {
+                match &mut param {
+                    GenericParam::Type(ty) => ty.default = None,
+                    GenericParam::Const(cnst) => cnst.default = None,
+                    GenericParam::Lifetime(_) => {}
+                }
+                parse_quote!(#param)
+            })
+            .chain(self.param_extras)
+            .collect();
+
+        // For MSRV purposes, ensure that lifetimes precede types precede const
+        // generics.
+        params.sort_by_cached_key(|param| match param {
+            GenericParam::Lifetime(_) => 0,
+            GenericParam::Type(_) => 1,
+            GenericParam::Const(_) => 2,
+        });
+
+        // The identifiers of the parameters without trait bounds or type
+        // defaults.
+        let param_idents = self.ctx.ast.generics.params.iter().map(|param| match param {
+            GenericParam::Type(ty) => {
+                let ident = &ty.ident;
+                quote!(#ident)
+            }
+            GenericParam::Lifetime(l) => {
+                let ident = &l.lifetime;
+                quote!(#ident)
+            }
+            GenericParam::Const(cnst) => {
+                let ident = &cnst.ident;
+                quote!({#ident})
+            }
+        });
+
+        let inner_extras = self.inner_extras;
+        let allow_trivial_bounds =
+            if self.ctx.skip_on_error { quote!(#[allow(trivial_bounds)]) } else { quote!() };
+        let impl_tokens = quote! {
+            #allow_trivial_bounds
+            unsafe impl < #(#params),* > #trait_path for #type_ident < #(#param_idents),* >
+            where
+                #(#bounds,)*
+            {
+                fn only_derive_is_allowed_to_implement_this_trait() {}
+
+                #inner_extras
+            }
+        };
+
+        let outer_extras = self.outer_extras.filter(|e| !e.is_empty());
+        let cfg_compile_error = self.ctx.cfg_compile_error();
+        const_block([Some(cfg_compile_error), Some(impl_tokens), outer_extras])
+    }
+}
+
+// A polyfill for `Option::then_some`, which was added after our MSRV.
+//
+// The `#[allow(unused)]` is necessary because, on sufficiently recent toolchain
+// versions, `b.then_some(...)` resolves to the inherent method rather than to
+// this trait, and so this trait is considered unused.
+//
+// FIXME(#67): Remove this once our MSRV is >= 1.62.
+#[allow(unused)]
+trait BoolExt {
+    fn then_some<T>(self, t: T) -> Option<T>;
+}
+
+impl BoolExt for bool {
+    fn then_some<T>(self, t: T) -> Option<T> {
+        if self {
+            Some(t)
+        } else {
+            None
+        }
+    }
+}
+
+pub(crate) fn const_block(items: impl IntoIterator<Item = Option<TokenStream>>) -> TokenStream {
+    let items = items.into_iter().flatten();
+    quote! {
+        #[allow(
+            // FIXME(#553): Add a test that generates a warning when
+            // `#[allow(deprecated)]` isn't present.
+            deprecated,
+            // Required on some rustc versions due to a lint that is only
+            // triggered when `derive(KnownLayout)` is applied to `repr(C)`
+            // structs that are generated by macros. See #2177 for details.
+            private_bounds,
+            non_local_definitions,
+            non_camel_case_types,
+            non_upper_case_globals,
+            non_snake_case,
+            non_ascii_idents,
+            clippy::missing_inline_in_public_items,
+        )]
+        #[deny(ambiguous_associated_items)]
+        // While there are not currently any warnings that this suppresses
+        // (that we're aware of), it's good future-proofing hygiene.
+        #[automatically_derived]
+        const _: () = {
+            #(#items)*
+        };
+    }
+}
+pub(crate) fn generate_tag_enum(ctx: &Ctx, repr: &EnumRepr, data: &DataEnum) -> TokenStream {
+    let zerocopy_crate = &ctx.zerocopy_crate;
+    let variants = data.variants.iter().map(|v| {
+        let ident = &v.ident;
+        if let Some((eq, discriminant)) = &v.discriminant {
+            quote! { #ident #eq #discriminant }
+        } else {
+            quote! { #ident }
+        }
+    });
+
+    // Don't include any `repr(align)` when generating the tag enum, as that
+    // could add padding after the tag but before any variants, which is not the
+    // correct behavior.
+    let repr = match repr {
+        EnumRepr::Transparent(span) => quote::quote_spanned! { *span => #[repr(transparent)] },
+        EnumRepr::Compound(c, _) => quote! { #c },
+    };
+
+    quote! {
+        #repr
+        #[allow(dead_code)]
+        pub enum ___ZerocopyTag {
+            #(#variants,)*
+        }
+
+        // SAFETY: `___ZerocopyTag` has no fields, and so it does not permit
+        // interior mutation.
+        unsafe impl #zerocopy_crate::Immutable for ___ZerocopyTag {
+            fn only_derive_is_allowed_to_implement_this_trait() {}
+        }
+    }
+}
+pub(crate) fn enum_size_from_repr(repr: &EnumRepr) -> Result<usize, Error> {
+    use CompoundRepr::*;
+    use PrimitiveRepr::*;
+    use Repr::*;
+    match repr {
+        Transparent(span)
+        | Compound(
+            Spanned {
+                t: C | Rust | Primitive(U32 | I32 | U64 | I64 | U128 | I128 | Usize | Isize),
+                span,
+            },
+            _,
+        ) => Err(Error::new(
+            *span,
+            "`FromBytes` only supported on enums with `#[repr(...)]` attributes `u8`, `i8`, `u16`, or `i16`",
+        )),
+        Compound(Spanned { t: Primitive(U8 | I8), span: _ }, _align) => Ok(8),
+        Compound(Spanned { t: Primitive(U16 | I16), span: _ }, _align) => Ok(16),
+    }
+}
+
+#[cfg(test)]
+pub(crate) mod testutil {
+    use proc_macro2::TokenStream;
+    use syn::visit::{self, Visit};
+
+    /// Checks for hygiene violations in the generated code.
+    ///
+    /// # Panics
+    ///
+    /// Panics if a hygiene violation is found.
+    pub(crate) fn check_hygiene(ts: TokenStream) {
+        struct AmbiguousItemVisitor;
+
+        impl<'ast> Visit<'ast> for AmbiguousItemVisitor {
+            fn visit_path(&mut self, i: &'ast syn::Path) {
+                if i.segments.len() > 1 && i.segments.first().unwrap().ident == "Self" {
+                    panic!(
+                    "Found ambiguous path `{}` in generated output. \
+                     All associated item access must be fully qualified (e.g., `<Self as Trait>::Item`) \
+                     to prevent hygiene issues.",
+                    quote::quote!(#i)
+                );
+                }
+                visit::visit_path(self, i);
+            }
+        }
+
+        let file = syn::parse2::<syn::File>(ts).expect("failed to parse generated output as File");
+        AmbiguousItemVisitor.visit_file(&file);
+    }
+
+    #[test]
+    fn test_check_hygiene_success() {
+        check_hygiene(quote::quote! {
+            fn foo() {
+                let _ = <Self as Trait>::Item;
+            }
+        });
+    }
+
+    #[test]
+    #[should_panic(expected = "Found ambiguous path `Self :: Ambiguous`")]
+    fn test_check_hygiene_failure() {
+        check_hygiene(quote::quote! {
+            fn foo() {
+                let _ = Self::Ambiguous;
+            }
+        });
+    }
+}