Re: [PATCH v2] perf: Fix segbase for ld.lld linked objects

Arnaldo Carvalho de Melo
2022-06-03T19:24:08Z

Em Fri, May 27, 2022 at 11:51:54AM -0700, Ian Rogers escreveu:
> On Fri, May 27, 2022 at 11:20 AM Fangrui Song <maskray@google.com> wrote:
> >
> > segbase is the address of .eh_frame_hdr and table_data is segbase plus
> > the header size. find_proc_info computes segbase as `map->start +
> > segbase - map->pgoff` which is wrong when
> >
> > * .eh_frame_hdr and .text are in different PT_LOAD program headers
> > * and their p_vaddr difference does not equal their p_offset difference
> >
> > Since 10.0, ld.lld's default --rosegment -z noseparate-code layout has
> > such R and RX PT_LOAD program headers.
> >
> > ld.lld (default) => perf report fails to unwind `perf record
> > --call-graph dwarf` recorded data
> > ld.lld --no-rosegment => ok (trivial, no R PT_LOAD)
> > ld.lld -z separate-code => ok but by luck: there are two PT_LOAD but
> > their p_vaddr difference equals p_offset difference
> >
> > ld.bfd -z noseparate-code => ok (trivial, no R PT_LOAD)
> > ld.bfd -z separate-code (default for Linux/x86) => ok but by luck:
> > there are two PT_LOAD but their p_vaddr difference equals p_offset
> > difference
> >
> > To fix the issue, compute segbase as dso's base address plus
> > PT_GNU_EH_FRAME's p_vaddr. The base address is computed by iterating
> > over all dso-associated maps and then subtract the first PT_LOAD p_vaddr
> > (the minimum guaranteed by generic ABI) from the minimum address.
> >
> > In libunwind, find_proc_info transitively called by unw_step is cached,
> > so the iteration overhead is acceptable.
> >
> > Reported-by: Sebastian Ullrich <sebasti@nullri.ch>
> > Link: https://github.com/ClangBuiltLinux/linux/issues/1646
> > Signed-off-by: Fangrui Song <maskray@google.com>
> > Cc: Ian Rogers <irogers@google.com>
>
> Reviewed-by: Ian Rogers <irogers@google.com>

Thanks, applied.

- Arnaldo