linux-kernel - Re: [PATCH 1/2] perf: Fix handling of arch_perf_out_copy

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Date:	Tue, 30 Jul 2013 15:21:14 +0200
From:	Robert Richter <rric@...nel.org>
To:	Jed Davis <jld@...illa.com>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Paul Mackerras <paulus@...ba.org>,
	Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] perf: Fix handling of arch_perf_out_copy_user return
 value.

On 29.07.13 18:12:40, Jed Davis wrote:
> All architectures except x86 use __copy_from_user_inatomic to provide
> arch_perf_out_copy_user; like the other copy_from routines, it returns
> the number of bytes not copied.  perf was expecting the number of bytes
> that had been copied.  This change corrects that, and thereby allows
> PERF_SAMPLE_STACK_USER to be enabled on non-x86 architectures.
> 
> x86 uses copy_from_user_nmi, which deviates from the other copy_from
> routines by returning the number of bytes copied.  (This cancels out
> the effect of perf being backwards; apparently this code has only ever
> been tested on x86.)  This change therefore adds a second wrapper to
> re-reverse it for perf; the next patch in this series will clean it up.
> 
> Signed-off-by: Jed Davis <jld@...illa.com>
> ---
>  arch/x86/include/asm/perf_event.h |  9 ++++++++-
>  kernel/events/internal.h          | 11 ++++++++++-
>  2 files changed, 18 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/include/asm/perf_event.h b/arch/x86/include/asm/perf_event.h
> index 8249df4..ddae5bd 100644
> --- a/arch/x86/include/asm/perf_event.h
> +++ b/arch/x86/include/asm/perf_event.h
> @@ -274,6 +274,13 @@ static inline void perf_check_microcode(void) { }
>   static inline void amd_pmu_disable_virt(void) { }
>  #endif
>  
> -#define arch_perf_out_copy_user copy_from_user_nmi
> +static inline unsigned long copy_from_user_nmi_for_perf(void *to,
> +							const void __user *from,
> +							unsigned long n)
> +{
> +	return n - copy_from_user_nmi(to, from, n);
> +}
> +
> +#define arch_perf_out_copy_user copy_from_user_nmi_for_perf

I like your change of copy_from_user_nmi() to return bytes not copied
since it makes callers simpler and has the same i/f as other copy
functions.

Please do not introduce code that you later remove, instead merge this
patch with your next.

>  
>  #endif /* _ASM_X86_PERF_EVENT_H */
> diff --git a/kernel/events/internal.h b/kernel/events/internal.h
> index ca65997..e61b22c 100644
> --- a/kernel/events/internal.h
> +++ b/kernel/events/internal.h
> @@ -81,6 +81,7 @@ static inline unsigned long perf_data_size(struct ring_buffer *rb)
>  	return rb->nr_pages << (PAGE_SHIFT + page_order(rb));
>  }
>  
> +/* The memcpy_func must return the number of bytes successfully copied. */
>  #define DEFINE_OUTPUT_COPY(func_name, memcpy_func)			\
>  static inline unsigned int						\
>  func_name(struct perf_output_handle *handle,				\
> @@ -122,11 +123,19 @@ DEFINE_OUTPUT_COPY(__output_copy, memcpy_common)
>  
>  DEFINE_OUTPUT_COPY(__output_skip, MEMCPY_SKIP)
>  
> +/* arch_perf_out_copy_user must return the number of bytes not copied. */
>  #ifndef arch_perf_out_copy_user
>  #define arch_perf_out_copy_user __copy_from_user_inatomic
>  #endif
>  
> -DEFINE_OUTPUT_COPY(__output_copy_user, arch_perf_out_copy_user)
> +static inline unsigned long perf_memcpy_from_user(void *to,
> +						  const void __user *from,
> +						  unsigned long n)
> +{
> +	return n - arch_perf_out_copy_user(to, from, n);
> +}
> +
> +DEFINE_OUTPUT_COPY(__output_copy_user, perf_memcpy_from_user)

Better modify DEFINE_OUTPUT_COPY() to deal with bytes-not-copied as
return value for memcpy_func(). Other users of DEFINE_OUTPUT_COPY()
could be fixed easily.

-Robert
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