linux-kernel - [PATCH 4/7] x86/cpu: Elide KCSAN for cpu

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Message-ID: <20220526105957.944756116@infradead.org>
Date:   Thu, 26 May 2022 12:52:56 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     x86@...nel.org, jpoimboe@...hat.com
Cc:     linux-kernel@...r.kernel.org, peterz@...radead.org,
        elver@...gle.com, jbaron@...mai.com, rostedt@...dmis.org,
        ardb@...nel.org, mark.rutland@....com,
        kernel test robot <lkp@...el.com>
Subject: [PATCH 4/7] x86/cpu: Elide KCSAN for cpu_has() and friends

As x86 uses the <asm-generic/bitops/instrumented-*.h> headers, the
regular forms of all bitops are instrumented with explicit calls to
KASAN and KCSAN checks. As these are explicit calls, these are not
suppressed by the noinstr function attribute.

This can result in calls to those check functions in noinstr code, which
objtool warns about:

vmlinux.o: warning: objtool: enter_from_user_mode+0x24: call to __kcsan_check_access() leaves .noinstr.text section
vmlinux.o: warning: objtool: syscall_enter_from_user_mode+0x28: call to __kcsan_check_access() leaves .noinstr.text section
vmlinux.o: warning: objtool: syscall_enter_from_user_mode_prepare+0x24: call to __kcsan_check_access() leaves .noinstr.text section
vmlinux.o: warning: objtool: irqentry_enter_from_user_mode+0x24: call to __kcsan_check_access() leaves .noinstr.text section

Prevent this by using the arch_*() bitops, which are the underlying
bitops without explciit instrumentation.

Reported-by: kernel test robot <lkp@...el.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Link: https://lkml.kernel.org/r/20220502111216.290518605@infradead.org
---
 arch/x86/include/asm/cpufeature.h |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/arch/x86/include/asm/cpufeature.h
+++ b/arch/x86/include/asm/cpufeature.h
@@ -51,7 +51,7 @@ extern const char * const x86_power_flag
 extern const char * const x86_bug_flags[NBUGINTS*32];
 
 #define test_cpu_cap(c, bit)						\
-	 test_bit(bit, (unsigned long *)((c)->x86_capability))
+	 arch_test_bit(bit, (unsigned long *)((c)->x86_capability))
 
 /*
  * There are 32 bits/features in each mask word.  The high bits