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

[Jiri Olsa <jolsa@redhat.com>]

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?

thanks,
jirka