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

[Brian Norris]

Hi Nilay,

I see you didn't CC the maintainers for this file. You might consider
looking through:
https://www.kernel.org/doc/html/latest/process/submitting-patches.html
And:

$ scripts/get_maintainer.pl -f kernel/padata.c
Steffen Klassert <steffen.klassert@xxxxxxxxxxx> (maintainer:PADATA PARALLEL EXECUTION MECHANISM)
Daniel Jordan <daniel.m.jordan@xxxxxxxxxx> (maintainer:PADATA PARALLEL EXECUTION MECHANISM)
linux-crypto@xxxxxxxxxxxxxxx (open list:PADATA PARALLEL EXECUTION MECHANISM)
linux-kernel@xxxxxxxxxxxxxxx (open list:PADATA PARALLEL EXECUTION MECHANISM)

I'll leave the full contents intact below for their sake, with a few
inline comments as well:

On Tue, Nov 12, 2024 at 06:11:24PM +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,
>       |                                  ~~~~~~~~~~~~~~^~~~~~~~
> 
> Apparentrly, above errors only menifests 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 erros 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.

You might consider running this paragraph through a spelling checker if you submit a
v2.

> Link: https://lore.kernel.org/all/7cbbd751-8332-4ab2-afa7-8c353834772a@xxxxxxxxxxxxx/
> Cc: briannorris@xxxxxxxxxxxx
> Cc: kees@xxxxxxxxxx
> Cc: nathan@xxxxxxxxxx
> Cc: yury.norov@xxxxxxxxx
> Cc: linux@xxxxxxxxxxxxxx
> Cc: gjoyce@xxxxxxx
> Signed-off-by: Nilay Shroff <nilay@xxxxxxxxxxxxx>
> ---
>  kernel/Makefile | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/kernel/Makefile b/kernel/Makefile
> index 87866b037fbe..e5adba7a30f1 100644
> --- a/kernel/Makefile
> +++ b/kernel/Makefile
> @@ -120,6 +120,7 @@ obj-$(CONFIG_CFI_CLANG) += cfi.o
>  obj-$(CONFIG_PERF_EVENTS) += events/
>  
>  obj-$(CONFIG_USER_RETURN_NOTIFIER) += user-return-notifier.o
> +CFLAGS_padata.o += $(call cc-disable-warning, stringop-overread)

Personally, I'd recommend a comment here, noting that these warnings
seem to produce false positives on GCC 13+. But otherwise, this seems OK
to me:

Reviewed-by: Brian Norris <briannorris@xxxxxxxxxxxx>

>  obj-$(CONFIG_PADATA) += padata.o
>  obj-$(CONFIG_JUMP_LABEL) += jump_label.o
>  obj-$(CONFIG_CONTEXT_TRACKING) += context_tracking.o
> -- 
> 2.45.2
>