On Wed, 18 Aug 2021 14:40:15 -0700 Kees Cook <keescook@xxxxxxxxxxxx> 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 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.) > > ... > > --- 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 Makefile has changed. I did this: --- a/Makefile~compiler-attributes-add-__alloc_size-for-better-bounds-checking +++ a/Makefile @@ -1003,6 +1003,12 @@ KBUILD_CFLAGS += $(call cc-disable-warni # Enabled with W=2, disabled by default as noisy KBUILD_CFLAGS += $(call cc-disable-warning, maybe-uninitialized) +ifdef CONFIG_CC_IS_GCC +# 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 KBUILD_CFLAGS += -fno-strict-overflow _
