Messages in this thread | | | Date | Wed, 25 Oct 2017 14:50:33 +0530 | From | "Naveen N. Rao" <> | Subject | Re: [RFC] perf tool: Fix memory corruption because of zero length symbols |
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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.
- Naveen
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