+ Fangrui
On Tue, Oct 17, 2023 at 4:10 AM Andrii Nakryiko
<andrii.nakryiko@xxxxxxxxx> wrote:
>
> On Mon, Oct 16, 2023 at 11:28 AM Andrii Nakryiko <andrii@xxxxxxxxxx> wrote:
> >
> > Fix too eager assumption that SHT_GNU_verdef ELF section is going to be
> > present whenever binary has SHT_GNU_versym section. It seems like either
> > SHT_GNU_verdef or SHT_GNU_verneed can be used, so failing on missing
> > SHT_GNU_verdef actually breaks use cases in production.
> >
> > One specific reported issue, which was used to manually test this fix,
> > was trying to attach to `readline` function in BASH binary.
> >
> > Cc: Hengqi Chen <hengqi.chen@xxxxxxxxx>
> > Reported-by: Liam Wisehart <liamwisehart@xxxxxxxx>
> > Fixes: bb7fa09399b9 ("libbpf: Support symbol versioning for uprobe")
> > Signed-off-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
> > ---
> > tools/lib/bpf/elf.c | 16 ++++++++++------
> > 1 file changed, 10 insertions(+), 6 deletions(-)
> >
>
> Hengqi,
>
> Please take a look when you get a chance. I'm not very familiar with
> symbol versioning details, but it seems like we made a too strong
> assumption about verdef always being present. In bash's case we have
> VERNEED, but not VERDEF, and that seems to be ok:
>
Yes, both VERNEED and VERDEF are optional.
> [ 8] .gnu.version VERSYM 000000000001c9ca 01c9ca
> 00130c 02 A 6 0 2
> [ 9] .gnu.version_r VERNEED 000000000001dcd8 01dcd8
> 0000b0 00 A 7 2 8
>
> So perhaps we need to complete the implementation to take VERNEED into
> account. And also let's add a test that can catch an issue like this
> going forward. Thanks!
>
AFAIK, VERNEED contains version requirements for shared libraries.
> > diff --git a/tools/lib/bpf/elf.c b/tools/lib/bpf/elf.c
> > index 2a158e8a8b7c..2a62bf411bb3 100644
> > --- a/tools/lib/bpf/elf.c
> > +++ b/tools/lib/bpf/elf.c
> > @@ -141,14 +141,15 @@ static int elf_sym_iter_new(struct elf_sym_iter *iter,
> > iter->versyms = elf_getdata(scn, 0);
> >
> > scn = elf_find_next_scn_by_type(elf, SHT_GNU_verdef, NULL);
> > - if (!scn) {
> > - pr_debug("elf: failed to find verdef ELF sections in '%s'\n", binary_path);
> > - return -ENOENT;
> > - }
> > - if (!gelf_getshdr(scn, &sh))
> > + if (!scn)
> > + return 0;
> > +
> > + iter->verdefs = elf_getdata(scn, 0);
> > + if (!iter->verdefs || !gelf_getshdr(scn, &sh)) {
> > + pr_warn("elf: failed to get verdef ELF section in '%s'\n", binary_path);
> > return -EINVAL;
> > + }
> > iter->verdef_strtabidx = sh.sh_link;
> > - iter->verdefs = elf_getdata(scn, 0);
> >
> > return 0;
> > }
> > @@ -199,6 +200,9 @@ static const char *elf_get_vername(struct elf_sym_iter *iter, int ver)
> > GElf_Verdef verdef;
> > int offset;
> >
> > + if (!iter->verdefs)
> > + return NULL;
> > +
> > offset = 0;
> > while (gelf_getverdef(iter->verdefs, offset, &verdef)) {
> > if (verdef.vd_ndx != ver) {
> > --
> > 2.34.1
> >
Anyway, this change look good to me, so
Acked-by: Hengqi Chen <hengqi.chen@xxxxxxxxx>
--
Hengqi
