Re: [PATCH 1/5] Compiler Attributes: Add __alloc_size() for better bounds checking

[Nathan Chancellor]

On 8/17/2021 10:08 PM, Kees Cook wrote:
> GCC and Clang can use the alloc_size attribute to better inform the
> results of __builtin_object_size() (for compile-time constant values).
> Clang can additionally use alloc_size to informt the results of
> __builtin_dynamic_object_size() (for run-time values).
>
> Additionally disables -Wno-alloc-size-larger-than since the allocators
> already reject SIZE_MAX, and the compile-time warnings aren't helpful.

In addition to what Miguel said, it might be helpful to mention that 
this warning is GCC specific, I was a little confused at first as to why 
it was just being added in the GCC only block :)

Otherwise, the attribute addition looks good to me. I will add my tag on v2.

> Cc: Miguel Ojeda <ojeda@xxxxxxxxxx>
> Cc: Nathan Chancellor <nathan@xxxxxxxxxx>
> Cc: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
> Cc: clang-built-linux@xxxxxxxxxxxxxxxx
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> ---
>   Makefile                            | 6 +++++-
>   include/linux/compiler_attributes.h | 6 ++++++
>   2 files changed, 11 insertions(+), 1 deletion(-)
>
> diff --git a/Makefile b/Makefile
> index 1b238ce86ed4..3b6fb740584e 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -1076,9 +1076,13 @@ KBUILD_CFLAGS += $(call cc-disable-warning, stringop-overflow)
>   # Another good warning that we'll want to enable eventually
>   KBUILD_CFLAGS += $(call cc-disable-warning, restrict)
>   
> -# Enabled with W=2, disabled by default as noisy
>   ifdef CONFIG_CC_IS_GCC
> +# Enabled with W=2, disabled by default as noisy
>   KBUILD_CFLAGS += -Wno-maybe-uninitialized
> +
> +# The allocators already balk at large sizes, so silence the compiler
> +# warnings for bounds checks involving those possible values.
> +KBUILD_CFLAGS += -Wno-alloc-size-larger-than
>   endif
>   
>   # disable invalid "can't wrap" optimizations for signed / pointers
> diff --git a/include/linux/compiler_attributes.h b/include/linux/compiler_attributes.h
> index 67c5667f8042..203b0ac62d15 100644
> --- a/include/linux/compiler_attributes.h
> +++ b/include/linux/compiler_attributes.h
> @@ -54,6 +54,12 @@
>   #define __aligned(x)                    __attribute__((__aligned__(x)))
>   #define __aligned_largest               __attribute__((__aligned__))
>   
> +/*
> + *   gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-alloc_005fsize-function-attribute
> + * clang: https://clang.llvm.org/docs/AttributeReference.html#alloc-size
> + */
> +#define __alloc_size(x, ...)		__attribute__((__alloc_size__(x, ## __VA_ARGS__)))
> +
>   /*
>    * Note: users of __always_inline currently do not write "inline" themselves,
>    * which seems to be required by gcc to apply the attribute according