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

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Message-ID: <9d0a0051-697c-4e25-9fbf-17786475a0e6@linux.ibm.com>
Date: Thu, 5 Dec 2024 17:35:53 +0530
From: Nilay Shroff <nilay@...ux.ibm.com>
To: Brian Norris <briannorris@...omium.org>
Cc: linux-kernel@...r.kernel.org, kees@...nel.org, nathan@...nel.org,
        yury.norov@...il.com, linux@...ssschuh.net, gjoyce@....com,
        Steffen Klassert <steffen.klassert@...unet.com>,
        Daniel Jordan <daniel.m.jordan@...cle.com>,
        linux-crypto@...r.kernel.org
Subject: Re: [PATCH] cpumask: work around false-postive stringop-overread
 errors



On 12/5/24 01:08, Brian Norris wrote:
> 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@...unet.com> (maintainer:PADATA PARALLEL EXECUTION MECHANISM)
> Daniel Jordan <daniel.m.jordan@...cle.com> (maintainer:PADATA PARALLEL EXECUTION MECHANISM)
> linux-crypto@...r.kernel.org (open list:PADATA PARALLEL EXECUTION MECHANISM)
> linux-kernel@...r.kernel.org (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@linux.ibm.com/
>> Cc: briannorris@...omium.org
>> Cc: kees@...nel.org
>> Cc: nathan@...nel.org
>> Cc: yury.norov@...il.com
>> Cc: linux@...ssschuh.net
>> Cc: gjoyce@....com
>> Signed-off-by: Nilay Shroff <nilay@...ux.ibm.com>
>> ---
>>  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 += $(call cc-disable-warning, stringop-overread)
> 
> Personally, I'd recommend a comment here, noting that these warnings
> seem to produce false positives on GCC 13+. But otherwise, this seems OK
> to me:
> 
> Reviewed-by: Brian Norris <briannorris@...omium.org>
> 
>>  obj-$(CONFIG_PADATA) += padata.o
>>  obj-$(CONFIG_JUMP_LABEL) += jump_label.o
>>  obj-$(CONFIG_CONTEXT_TRACKING) += context_tracking.o
>> -- 
>> 2.45.2
>>
Thank you Brian for your review!

I would incorporate all your comments and spin a new patch and send it out soon...

--Nilay