On 27/06/15 12:10, Will Deacon wrote:
> Commit 922d0e4d9f04 ("perf tools: Adjust symbols in VDSO") changed the
> ELF symbol parsing so that the vDSO is treated the same as ET_EXEC and
> ET_REL binaries despite being an ET_DYN. This was a partial workaround
> to deal with older x86 vDSOs being prelinked at a high address that
> didn't correspond to the map, so using object-relative offsets and
> adding the base of the map allowed symbol resolution to succeed.
>
> Unfortunately, this causes objdump not to produce any output in
> conjunction with perf annotate, which cheerfully passes the absolute
> address of the map symbol.
>
> This patch fixes the problem by avoiding adjustment of vDSO symbols and
> instead setting the map->pgoff field to correspond to the virtual load
> address specified in the vDSO ELF header.
>
> Cc: Vladimir Nikulichev <n...@tbricks.com>
> Cc: Adrian Hunter <adrian.hun...@intel.com>
> Cc: Namhyung Kim <namhy...@kernel.org>
> Cc: Andy Lutomirski <l...@amacapital.net>
> Reported-by: Kristina Martsenko <kristina.martse...@arm.com>
> Signed-off-by: Will Deacon <will.dea...@arm.com>
> ---
>
> v1->v2: Adjust map->pgoff in ELF loader to avoid breaking symbol lookup
> on older kernels.
>
> tools/perf/util/map.c | 5 ++---
> tools/perf/util/symbol-elf.c | 9 ++++++++-
> 2 files changed, 10 insertions(+), 4 deletions(-)
>
> diff --git a/tools/perf/util/map.c b/tools/perf/util/map.c
> index a14f08f41686..6ba38293be88 100644
> --- a/tools/perf/util/map.c
> +++ b/tools/perf/util/map.c
> @@ -173,10 +173,9 @@ struct map *map__new(struct machine *machine, u64 start,
> u64 len,
> filename = newfilename;
> }
>
> - if (vdso) {
> - pgoff = 0;
> + if (vdso)
> dso = vdso__dso_findnew(machine, thread);
> - } else
> + else
> dso = __dsos__findnew(&machine->user_dsos, filename);
>
> if (dso == NULL)
> diff --git a/tools/perf/util/symbol-elf.c b/tools/perf/util/symbol-elf.c
> index a7ab6063e038..83f8ba232575 100644
> --- a/tools/perf/util/symbol-elf.c
> +++ b/tools/perf/util/symbol-elf.c
> @@ -706,7 +706,6 @@ int symsrc__init(struct symsrc *ss, struct dso *dso,
> const char *name,
> GElf_Shdr shdr;
> ss->adjust_symbols = (ehdr.e_type == ET_EXEC ||
> ehdr.e_type == ET_REL ||
> - dso__is_vdso(dso) ||
> elf_section_by_name(elf, &ehdr, &shdr,
> ".gnu.prelink_undo",
> NULL) != NULL);
> @@ -824,6 +823,14 @@ int dso__load_sym(struct dso *dso, struct map *map,
> sec = syms_ss->symtab;
> shdr = syms_ss->symshdr;
>
> + /*
> + * Older x86 kernels prelink the vDSO at a high address, so
> + * we need to reflect that in map->pgoff in order to talk to
> + * objdump.
> + */
> + if (dso__is_vdso(dso))
> + map->pgoff = shdr.sh_addr - shdr.sh_offset;
In the case of perf tools, maps map memory addresses to file offsets.
That is used to read from the object file, so you can't change the map.
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