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 inform the results of __builtin_dynamic_object_size() (for run-time values). Because GCC sees the frequent use of struct_size() as an allocator size argument, and notices it can return SIZE_MAX (the overflow indication), it complains about these call sites may overflow (since SIZE_MAX is greater than the default -Walloc-size-larger-than=PTRDIFF_MAX). This isn't helpful since we already know a SIZE_MAX will be caught at run-time (this was an intentional design). Instead, just disable this check as it is both a false positive and redundant. (Clang does not have this warning option.) 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 72f9e2b0202c..34cffcdfd5dc 100644 --- a/Makefile +++ b/Makefile @@ -1078,9 +1078,13 @@ 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 -- 2.30.2
