On Wed, Aug 18, 2021 at 04:19:12PM -0700, Andrew Morton wrote: > 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 Oh, er, where did "Makefile: remove stale cc-option checks" go? Ah, I see now: https://lore.kernel.org/mm-commits/20210814215814.W_qqW%25akpm@xxxxxxxxxxxxxxxxxxxx/T/#u Looks like I just happened to pick the wrong linux-next. ;) Thanks for the fix-up! -- Kees Cook
