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SubjectRe: [BUG] perf: arch_perf_out_copy_user default
On Wed, Oct 30, 2013 at 03:37:50PM +0100, Peter Zijlstra wrote:
> 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.

Aie, sorry about that, I did a wrong assumption indeed.

>
> 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;

Would it make sense to rather make copy_from_user_nmi() to use a return value
pattern that is closer to those of the existing copy_from_user_*() ?

This way we avoid future mistakes of that kind.

Thanks.

> +}
> #endif
>
> DEFINE_OUTPUT_COPY(__output_copy_user, arch_perf_out_copy_user)


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