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  2. Fixing stack trace function names by argument introspection

Re: Fixing stack trace function names by argument introspection

marko@kevac.org
 



On Mon, Jul 23, 2018 at 7:48 AM, Y Song <ys114321@...> wrote:

We did not have such an example in BCC. In Facebook, we have a bpf
program to catch
stack traces for python programs. It is very similar to what you want
to achieve in the above.

Can you share it with me? Maybe I can use it as an example.
 
Basically, you need to walk the stack by yourself. Since verifier do
not support unbounded loops,
you need to have a fully-unrollable loop with progma unroll.

I have never used pragma unroll before, but I understand what it is supposed to do.
Quick search gives me usages for CUDA and several little known examples for gcc/clang.

 
During each loop iteration, you can access the frame pointer, you need
some mechanism to
get the real function name based on that level frame pointer and then
you move on
to the next. In bpf program, you can access current task structure,
which contains some
data related to TLS which could be used by the bpf program.


As far as I understand it would work if my program is built with frame pointers. In that case going throigh stack trace shold be straightforward. Never done it before though :-)
But usually programs are build omitting frame pointers. In that case you need additional info from DWARF and code is much more complex. Right?
Are you suggesting implementing all this?

Sorry for newbie questions :-)
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