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

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Message-ID: <20241112124127.1666300-1-nilay@linux.ibm.com>
Date: Tue, 12 Nov 2024 18:11:24 +0530
From: Nilay Shroff <nilay@...ux.ibm.com>
To: linux-kernel@...r.kernel.org
Cc: Nilay Shroff <nilay@...ux.ibm.com>, briannorris@...omium.org,
        kees@...nel.org, nathan@...nel.org, yury.norov@...il.com,
        linux@...ssschuh.net, gjoyce@....com
Subject: [PATCH] cpumask: work around false-postive stringop-overread errors

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.

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 += $(call cc-disable-warning, stringop-overread)
 obj-$(CONFIG_PADATA) += padata.o
 obj-$(CONFIG_JUMP_LABEL) += jump_label.o
 obj-$(CONFIG_CONTEXT_TRACKING) += context_tracking.o
-- 
2.45.2