Re: [PATCHv2] cpumask: work around false-postive stringop-overread errors

Greg KH
2024-12-07T11:43:23Z

On Thu, Dec 05, 2024 at 06:04:09PM +0530, Nilay Shroff wrote:
> While building the powerpc code using gcc 13, I came across following
> errors generated for kernel/padata.c file:
>
> CC kernel/padata.o
> In file included from ./include/linux/string.h:390,
> from ./arch/powerpc/include/asm/paca.h:16,
> from ./arch/powerpc/include/asm/current.h:13,
> from ./include/linux/thread_info.h:23,
> from ./include/asm-generic/preempt.h:5,
> from ./arch/powerpc/include/generated/asm/preempt.h:1,
> from ./include/linux/preempt.h:79,
> from ./include/linux/spinlock.h:56,
> from ./include/linux/swait.h:7,
> from ./include/linux/completion.h:12,
> from kernel/padata.c:14:
> In function ‘bitmap_copy’,
> inlined from ‘cpumask_copy’ at ./include/linux/cpumask.h:839:2,
> inlined from ‘__padata_set_cpumasks’ at kernel/padata.c:730:2:
> ./include/linux/fortify-string.h:114:33: error: ‘__builtin_memcpy’ reading between 257 and 536870904 bytes from a region of size 256 [-Werror=stringop-overread]
> 114 | #define __underlying_memcpy __builtin_memcpy
> | ^
> ./include/linux/fortify-string.h:633:9: note: in expansion of macro ‘__underlying_memcpy’
> 633 | __underlying_##op(p, q, __fortify_size); \
> | ^~~~~~~~~~~~~
> ./include/linux/fortify-string.h:678:26: note: in expansion of macro ‘__fortify_memcpy_chk’
> 678 | #define memcpy(p, q, s) __fortify_memcpy_chk(p, q, s, \
> | ^~~~~~~~~~~~~~~~~~~~
> ./include/linux/bitmap.h:259:17: note: in expansion of macro ‘memcpy’
> 259 | memcpy(dst, src, len);
> | ^~~~~~
> kernel/padata.c: In function ‘__padata_set_cpumasks’:
> kernel/padata.c:713:48: note: source object ‘pcpumask’ of size [0, 256]
> 713 | cpumask_var_t pcpumask,
> | ~~~~~~~~~~~~~~^~~~~~~~
> In function ‘bitmap_copy’,
> inlined from ‘cpumask_copy’ at ./include/linux/cpumask.h:839:2,
> inlined from ‘__padata_set_cpumasks’ at kernel/padata.c:730:2:
> ./include/linux/fortify-string.h:114:33: error: ‘__builtin_memcpy’ reading between 257 and 536870904 bytes from a region of size 256 [-Werror=stringop-overread]
> 114 | #define __underlying_memcpy __builtin_memcpy
> | ^
> ./include/linux/fortify-string.h:633:9: note: in expansion of macro ‘__underlying_memcpy’
> 633 | __underlying_##op(p, q, __fortify_size); \
> | ^~~~~~~~~~~~~
> ./include/linux/fortify-string.h:678:26: note: in expansion of macro ‘__fortify_memcpy_chk’
> 678 | #define memcpy(p, q, s) __fortify_memcpy_chk(p, q, s, \
> | ^~~~~~~~~~~~~~~~~~~~
> ./include/linux/bitmap.h:259:17: note: in expansion of macro ‘memcpy’
> 259 | memcpy(dst, src, len);
> | ^~~~~~
> kernel/padata.c: In function ‘__padata_set_cpumasks’:
> kernel/padata.c:713:48: note: source object ‘pcpumask’ of size [0, 256]
> 713 | cpumask_var_t pcpumask,
> | ~~~~~~~~~~~~~~^~~~~~~~
>
> Apparently, above errors only manifests with GCC 13.x and config option
> CONFIG_FORTIFY_SOURCE. Furthermore, if I use gcc 11.x or gcc 12.x then I
> don't encounter above errors. Prima facie, these errors appear to be false-
> positive. Brian informed me that currently some efforts are underway by
> GCC developers to emit more verbose information when GCC detects string
> overflow errors and that might help to further narrow down the root cause
> of this error. So for now, silence these errors using -Wno-stringop-
> overread gcc option while building kernel/padata.c file until we find the
> root cause.

I'm hitting this now on Linus's tree using gcc14 on x86-64 so this isn't
just a problem with your arch.

Let me try this patch locally and see if it helps...

thanks,

greg k-h