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, 50 insertions, 0 deletions

diff --git a/rust/ffi.rs b/rust/ffi.rs
new file mode 100644
index 000000000..f961e9728
--- /dev/null
+++ b/rust/ffi.rs
@@ -0,0 +1,50 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Foreign function interface (FFI) types.
+//!
+//! This crate provides mapping from C primitive types to Rust ones.
+//!
+//! The Rust [`core`] crate provides [`core::ffi`], which maps integer types to the platform default
+//! C ABI. The kernel does not use [`core::ffi`], so it can customise the mapping that deviates from
+//! the platform default.
+
+#![no_std]
+
+macro_rules! alias {
+    ($($name:ident = $ty:ty;)*) => {$(
+        #[allow(non_camel_case_types, missing_docs)]
+        pub type $name = $ty;
+
+        // Check size compatibility with `core`.
+        const _: () = assert!(
+            ::core::mem::size_of::<$name>() == ::core::mem::size_of::<::core::ffi::$name>()
+        );
+    )*}
+}
+
+alias! {
+    // `core::ffi::c_char` is either `i8` or `u8` depending on architecture. In the kernel, we use
+    // `-funsigned-char` so it's always mapped to `u8`.
+    c_char = u8;
+
+    c_schar = i8;
+    c_uchar = u8;
+
+    c_short = i16;
+    c_ushort = u16;
+
+    c_int = i32;
+    c_uint = u32;
+
+    // In the kernel, `intptr_t` is defined to be `long` in all platforms, so we can map the type to
+    // `isize`.
+    c_long = isize;
+    c_ulong = usize;
+
+    c_longlong = i64;
+    c_ulonglong = u64;
+}
+
+pub use core::ffi::c_void;
+
+pub use core::ffi::CStr;