linux-kernel - [PATCH 3.18 08/13] x86/oprofile: Fix bogus GCC-8 warning in nmi

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Message-Id: <20180226201527.599704954@linuxfoundation.org>
Date:   Mon, 26 Feb 2018 21:15:36 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Arnd Bergmann <arnd@...db.de>,
        Jessica Yu <jeyu@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Martin Sebor <msebor@....gnu.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Robert Richter <rric@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        oprofile-list@...ts.sf.net, Ingo Molnar <mingo@...nel.org>
Subject: [PATCH 3.18 08/13] x86/oprofile: Fix bogus GCC-8 warning in nmi_setup()

3.18-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Arnd Bergmann <arnd@...db.de>

commit 85c615eb52222bc5fab6c7190d146bc59fac289e upstream.

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
pointers:

 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@...db.de>
Cc: Jessica Yu <jeyu@...nel.org>
Cc: Kees Cook <keescook@...omium.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Martin Sebor <msebor@....gnu.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Robert Richter <rric@...nel.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: oprofile-list@...ts.sf.net
Cc: stable@...r.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@...nel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 arch/x86/oprofile/nmi_int.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- 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)
 			continue;
 
 		memcpy(per_cpu(cpu_msrs, cpu).counters,