Re: [RFC] perf tool: Fix memory corruption because of zero length symbols

[Arnaldo Carvalho de Melo]

Em Wed, Oct 25, 2017 at 02:50:33PM +0530, Naveen N. Rao escreveu:
> On 2017/10/25 08:28AM, Jiri Olsa wrote:
> > On Tue, Oct 24, 2017 at 07:50:06PM +0530, Ravi Bangoria wrote:
> > > Perf top is often crashing at very random locations on powerpc.
> > > After investigating, I found the crash only happens when sample
> > > is of zero length symbol. Powerpc kernel has many such symbols
> > > which does not contain length details in vmlinux binary and thus
> > > start and end addresses of such symbols are same.
> > > 
> > > Structure
> > > 
> > >   struct sym_hist {
> > >         u64                   nr_samples;
> > >         u64                   period;
> > >         struct sym_hist_entry addr[0];
> > >   };
> > > 
> > > has last member 'addr[]' of size zero. 'addr[]' is an array of
> > > addresses that belongs to one symbol (function). If function
> > > consist of 100 instructions, 'addr' points to an array of 100
> > > 'struct sym_hist_entry' elements. For zero length symbol, it
> > > points to the *empty* array, i.e. no members in the array and
> > > thus offset 0 is also invalid for such array.
> > > 
> > >   static int __symbol__inc_addr_samples(...)
> > >   {
> > >         ...
> > >         offset = addr - sym->start;
> > >         h = annotation__histogram(notes, evidx);
> > >         h->nr_samples++;
> > >         h->addr[offset].nr_samples++;
> > >         h->period += sample->period;
> > >         h->addr[offset].period += sample->period;
> > >         ...
> > >   }
> > > 
> > > Here, when 'addr' is same as 'sym->start', 'offset' becomes 0,
> > > which is valid for normal symbols but *invalid* for zero length
> > > symbols and thus updating h->addr[offset] causes memory corruption.
> > 
> > I think this will work, however what's the idea behind
> > zero sized symbols? I mean when we get samples for it,
> > what are they for.. is it like alias, whats the purpose?
> 
> This was discussed here:
> https://lkml.org/lkml/2016/10/10/148
> 
> We aren't setting the size for our assembly routines.

Applying, after adding this as a Link: tag and Namhyung's and Jiri's
acked-by, will try to have it in perf/urgent soon, i.e. for 4.14, if
still possible.

- Arnaldo