Re: [PATCH 1/3] x86: record relocation offset

Arnaldo Carvalho de Melo
2009-12-30T21:59:11Z

Em Wed, Dec 30, 2009 at 06:39:36PM -0200, Arnaldo Carvalho de Melo escreveu:
> Em Wed, Dec 30, 2009 at 11:45:30AM -0800, H. Peter Anvin escreveu:
> > The kernel already knows where it is loaded -- obviously, by sheer
> > necessity -- and knows how it was itself configured, and as such we
> > can do this calculation in C code without modifying boot_params or
> > the early bootstrap.
>
> Problem is that at 'perf record' time we may not have access to the
> vmlinux file, and thus not be able to figure out the relocation applied
> in that boot.
>
> Then, at a later time, and possibly on another machine, on another arch,
> we try to map back IPs to symbols, the /proc/kallsyms is completely
> unrelated and we now have a vmlinux unrelocated...
>
> So we need a way to get the relocation applied at 'perf record' time and
> encode it in the perf.data header. Ideas about how to do that?

Well, I guess we could do the _stext trick again, storing its value,
taken from /proc/kallsyms, into the perf.data header, then figuring out
the relocation by looking at its value in the vmlinux symtab.

There were concerns in the past about relying on _stext, IIRC, James?

- Arnaldo