linux-kernel - [BUG] perf: arch_perf_out_copy

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Date:	Wed, 30 Oct 2013 15:37:50 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	will.deacon@....com, fweisbec@...il.com
Cc:	mingo@...nel.org, linux-kernel@...r.kernel.org
Subject: [BUG] perf: arch_perf_out_copy_user default

Hi Frederic,

I just spotted:

#ifndef arch_perf_out_copy_user
#define arch_perf_out_copy_user __copy_from_user_inatomic
#endif

vs:

arch/x86/include/asm/perf_event.h:#define arch_perf_out_copy_user copy_from_user_nmi


Now the problem is that copy_from_user_nmi() and
__copy_from_user_inatomic() have different return semantics.

Furthermore, the macro you use them in DEFINE_OUTPUT_COPY() assumes the
return value is the amount of memory copied; as also illustrated by
memcpy_common().

Trouble is, __copy_from_user_inatomic() returns the number of bytes
_NOT_ copied.

With this, my question to Will is, how did your ARM unwind support
patches ever work? AFAICT they end up using the
__copy_from_user_inatomic() thing.


---
 kernel/events/internal.h | 17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

diff --git a/kernel/events/internal.h b/kernel/events/internal.h
index ca6599723be5..d7a0f753e695 100644
--- a/kernel/events/internal.h
+++ b/kernel/events/internal.h
@@ -110,7 +110,8 @@ func_name(struct perf_output_handle *handle,				\
 	return len;							\
 }
 
-static inline int memcpy_common(void *dst, const void *src, size_t n)
+static inline unsigned long
+memcpy_common(void *dst, const void *src, unsigned long n)
 {
 	memcpy(dst, src, n);
 	return n;
@@ -123,7 +124,19 @@ DEFINE_OUTPUT_COPY(__output_copy, memcpy_common)
 DEFINE_OUTPUT_COPY(__output_skip, MEMCPY_SKIP)
 
 #ifndef arch_perf_out_copy_user
-#define arch_perf_out_copy_user __copy_from_user_inatomic
+#define arch_perf_out_copy_user arch_perf_out_copy_user
+
+static inline unsigned long
+arch_perf_out_copy_user(void *dst, const void *src, unsigned long n)
+{
+	unsigned long ret;
+
+	pagefault_disable();
+	ret = __copy_from_user_inatomic(to, from, n);
+	pagefault_enable();
+
+	return n - ret;
+}
 #endif
 
 DEFINE_OUTPUT_COPY(__output_copy_user, arch_perf_out_copy_user)
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