[added,to,the,4.1,stable,tree] x86/oprofile: Fix bogus GCC-8 warning in nmi_setup()

Message ID 20180301152116.1486-510-alexander.levin@microsoft.com
State New
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  • [added,to,the,4.1,stable,tree] x86/oprofile: Fix bogus GCC-8 warning in nmi_setup()
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Commit Message

Sasha Levin March 1, 2018, 3:27 p.m.
From: Arnd Bergmann <arnd@arndb.de>

This patch has been added to the 4.1 stable tree. If you have any
objections, please let us know.




[ Upstream commit 85c615eb52222bc5fab6c7190d146bc59fac289e ]

GCC-8 shows a warning for the x86 oprofile code that copies per-CPU
data from CPU 0 to all other CPUs, which when building a non-SMP
kernel turns into a memcpy() with identical source and destination

 arch/x86/oprofile/nmi_int.c: In function 'mux_clone':
 arch/x86/oprofile/nmi_int.c:285:2: error: 'memcpy' source argument is the same as destination [-Werror=restrict]
   memcpy(per_cpu(cpu_msrs, cpu).multiplex,
          per_cpu(cpu_msrs, 0).multiplex,
          sizeof(struct op_msr) * model->num_virt_counters);
 arch/x86/oprofile/nmi_int.c: In function 'nmi_setup':
 arch/x86/oprofile/nmi_int.c:466:3: error: 'memcpy' source argument is the same as destination [-Werror=restrict]
 arch/x86/oprofile/nmi_int.c:470:3: error: 'memcpy' source argument is the same as destination [-Werror=restrict]

I have analyzed a number of such warnings now: some are valid and the
GCC warning is welcome. Others turned out to be false-positives, and
GCC was changed to not warn about those any more. This is a corner case
that is a false-positive but the GCC developers feel it's better to keep
warning about it.

In this case, it seems best to work around it by telling GCC
a little more clearly that this code path is never hit with
an IS_ENABLED() configuration check.

Cc:stable as we also want old kernels to build cleanly with GCC-8.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Jessica Yu <jeyu@kernel.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Martin Sebor <msebor@gcc.gnu.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Robert Richter <rric@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: oprofile-list@lists.sf.net
Cc: stable@vger.kernel.org
Link: http://lkml.kernel.org/r/20180220205826.2008875-1-arnd@arndb.de
Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84095
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
 arch/x86/oprofile/nmi_int.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/oprofile/nmi_int.c b/arch/x86/oprofile/nmi_int.c
index 1d2e6392f5fa..f24bd7249536 100644
--- a/arch/x86/oprofile/nmi_int.c
+++ b/arch/x86/oprofile/nmi_int.c
@@ -471,7 +471,7 @@  static int nmi_setup(void)
 		goto fail;
 	for_each_possible_cpu(cpu) {
-		if (!cpu)
+		if (!IS_ENABLED(CONFIG_SMP) || !cpu)
 		memcpy(per_cpu(cpu_msrs, cpu).counters,