Re: [PATCH] perf/core: Fix the mask in perf_output_sample_regs

[Madhavan Srinivasan]

On Thursday 11 August 2016 05:57 PM, Peter Zijlstra wrote:
> Sorry, found it in my inbox while clearing out backlog..
>
> On Sun, Jul 03, 2016 at 11:31:58PM +0530, Madhavan Srinivasan wrote:
> > When decoding the perf_regs mask in perf_output_sample_regs(),
> > we loop through the mask using find_first_bit and find_next_bit functions.
> > While the exisitng code works fine in most of the case,
> > the logic is broken for 32bit kernel (Big Endian).
> > When reading u64 mask using (u32 *)(&val)[0], find_*_bit() assumes it gets
> > lower 32bits of u64 but instead gets upper 32bits which is wrong.
> > Proposed fix is to swap the words of the u64 to handle this case.
> > This is _not_ endianness swap.
> But it looks an awful lot like it..
Hit this issue when testing my perf_arch_regs patchset. Yep exactly
the reason for adding that comment in the commit message.


> > +++ b/kernel/events/core.c
> > @@ -5205,8 +5205,10 @@ perf_output_sample_regs(struct perf_output_handle *handle,
> >   			struct pt_regs *regs, u64 mask)
> >   {
> >   	int bit;
> > +	DECLARE_BITMAP(_mask, 64);
> >   
> > -	for_each_set_bit(bit, (const unsigned long *) &mask,
> > +	bitmap_from_u64(_mask, mask);
> > +	for_each_set_bit(bit, _mask,
> >   			 sizeof(mask) * BITS_PER_BYTE) {
> >   		u64 val;
> > +++ b/lib/bitmap.c
> > +void bitmap_from_u64(unsigned long *dst, u64 mask)
> > +{
> > +	dst[0] = mask & ULONG_MAX;
> > +
> > +	if (sizeof(mask) > sizeof(unsigned long))
> > +		dst[1] = mask >> 32;
> > +}
> > +EXPORT_SYMBOL(bitmap_from_u64);
> Looks small enough for an inline.
>
> Alternatively you can go all the way and add bitmap_from_u64array(), but
> that seems massive overkill.

Ok will make it inline and resend.

Maddy

> Tedious stuff.. I can't come up with anything prettier :/