[PATCH v2] Compiler Attributes: Introduce __access_*() function attribute

[Kees Cook <keescook@chromium.org>]

Added in GCC 10.1, the "access" function attribute is used to mark pointer
arguments for how they are expected to be accessed in a given function.
Both their access type (read/write, read-only, or write-only) and bounds
are specified.

These can improve GCC's compile-time diagnostics including -Warray-bounds,
-Wstringop-overflow, etc, and can affect __builtin_dynamic_object_size()
results.

Cc: Miguel Ojeda <ojeda@kernel.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Tom Rix <trix@redhat.com>
Cc: llvm@lists.linux.dev
Signed-off-by: Kees Cook <keescook@chromium.org>
---
 include/linux/compiler_attributes.h | 30 +++++++++++++++++++++++++++++
 1 file changed, 30 insertions(+)

diff --git a/include/linux/compiler_attributes.h b/include/linux/compiler_attributes.h
index 9a9907fad6fd..465be5f072ff 100644
--- a/include/linux/compiler_attributes.h
+++ b/include/linux/compiler_attributes.h
@@ -20,6 +20,36 @@
  * Provide links to the documentation of each supported compiler, if it exists.
  */
 
+/*
+ * Optional: only supported since gcc >= 10
+ * Optional: not supported by Clang
+ *
+ *   gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-access-function-attribute
+ *
+ * While it is legal to provide only the pointer argument position and
+ * access type, the kernel macros are designed to require also the bounds
+ * (element count) argument position; if a function has no bounds argument,
+ * refactor the code to include one.
+ *
+ * These can be used multiple times. For example:
+ *
+ * __access_wo(2, 3) __access_ro(4, 5)
+ * int copy_something(struct context *ctx, u32 *dst, size_t dst_count,
+ *		      const u8 *src, int src_len);
+ *
+ * If "dst" will also be read from, it could use __access_rw(2, 3) instead.
+ *
+ */
+#if __has_attribute(__access__)
+# define __access_rw(ptr, count)	__attribute__((__access__(read_write, ptr, count)))
+# define __access_ro(ptr, count)	__attribute__((__access__(read_only,  ptr, count)))
+# define __access_wo(ptr, count)	__attribute__((__access__(write_only, ptr, count)))
+#else
+# define __access_rw(ptr, count)
+# define __access_ro(ptr, count)
+# define __access_wo(ptr, count)
+#endif
+
 /*
  *   gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-alias-function-attribute
  */
-- 
2.34.1