bpf: mark a NULL kfunc argument precisegrafted

check_kfunc_arg() allows bpf_register_is_null() for nullable arguments
w/o marking the underlying scalar register precise. Hence a checkpoint
created on such a path would prune against arbitrary scalar value.

Fixes: 3bda08b63670 ("bpf: Allow NULL buffers in bpf_dynptr_slice(_rw)")
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20260904-register-is-null-precise-fixes-v1-5-0f5a360ff15d@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 90 insertions, 0 deletions

diff --git a/rust/quote/ident_fragment.rs b/rust/quote/ident_fragment.rs
new file mode 100644
index 000000000..d98106f17
--- /dev/null
+++ b/rust/quote/ident_fragment.rs
@@ -0,0 +1,90 @@
+// SPDX-License-Identifier: Apache-2.0 OR MIT
+
+use alloc::borrow::Cow;
+use core::fmt;
+use proc_macro2::{Ident, Span};
+
+/// Specialized formatting trait used by `format_ident!`.
+///
+/// [`Ident`] arguments formatted using this trait will have their `r#` prefix
+/// stripped, if present.
+///
+/// See [`format_ident!`] for more information.
+///
+/// [`format_ident!`]: crate::format_ident
+pub trait IdentFragment {
+    /// Format this value as an identifier fragment.
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result;
+
+    /// Span associated with this `IdentFragment`.
+    ///
+    /// If non-`None`, may be inherited by formatted identifiers.
+    fn span(&self) -> Option<Span> {
+        None
+    }
+}
+
+impl<T: IdentFragment + ?Sized> IdentFragment for &T {
+    fn span(&self) -> Option<Span> {
+        <T as IdentFragment>::span(*self)
+    }
+
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        IdentFragment::fmt(*self, f)
+    }
+}
+
+impl<T: IdentFragment + ?Sized> IdentFragment for &mut T {
+    fn span(&self) -> Option<Span> {
+        <T as IdentFragment>::span(*self)
+    }
+
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        IdentFragment::fmt(*self, f)
+    }
+}
+
+impl IdentFragment for Ident {
+    fn span(&self) -> Option<Span> {
+        Some(self.span())
+    }
+
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        let id = self.to_string();
+        if let Some(id) = id.strip_prefix("r#") {
+            fmt::Display::fmt(id, f)
+        } else {
+            fmt::Display::fmt(&id[..], f)
+        }
+    }
+}
+
+impl<T> IdentFragment for Cow<'_, T>
+where
+    T: IdentFragment + ToOwned + ?Sized,
+{
+    fn span(&self) -> Option<Span> {
+        T::span(self)
+    }
+
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        T::fmt(self, f)
+    }
+}
+
+// Limited set of types which this is implemented for, as we want to avoid types
+// which will often include non-identifier characters in their `Display` impl.
+macro_rules! ident_fragment_display {
+    ($($T:ty),*) => {
+        $(
+            impl IdentFragment for $T {
+                fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+                    fmt::Display::fmt(self, f)
+                }
+            }
+        )*
+    };
+}
+
+ident_fragment_display!(bool, str, String, char);
+ident_fragment_display!(u8, u16, u32, u64, u128, usize);