Messages in this thread | | | Subject | Re: [PATCH v2 04/10] objtool: check: Ignore empty alternative groups | From | Julien Thierry <> | Date | Wed, 1 Apr 2020 14:43:22 +0100 |
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On 4/1/20 1:53 PM, Miroslav Benes wrote: > On Fri, 27 Mar 2020, Julien Thierry wrote: > >> Atlernative section can contain entries for alternatives with no >> instructions. Objtool will currently crash when handling such an entry. >> >> Just skip that entry, but still give a warning to discourage useless >> entries. >> >> Signed-off-by: Julien Thierry <jthierry@redhat.com> >> --- >> tools/objtool/check.c | 6 ++++++ >> 1 file changed, 6 insertions(+) >> >> diff --git a/tools/objtool/check.c b/tools/objtool/check.c >> index 74353b2c39ce..5c03460f1f07 100644 >> --- a/tools/objtool/check.c >> +++ b/tools/objtool/check.c >> @@ -904,6 +904,12 @@ static int add_special_section_alts(struct objtool_file *file) >> } >> >> if (special_alt->group) { >> + if (!special_alt->orig_len) { >> + WARN_FUNC("empty alternative entry", >> + orig_insn->sec, orig_insn->offset); >> + continue; >> + } >> + >> ret = handle_group_alt(file, special_alt, orig_insn, >> &new_insn); >> if (ret) > > Probably the first time I am looking at alternatives handling in objtool, > so I must be missing something, but is this even possible now? I mean > get_alt_entry() in special.c sets alt->orig_len when alt->group is true > (which means .alternatives section) to something which cannot be zero. >
What I see is:
if (alt->group) { alt->orig_len = *(unsigned char *)(sec->data->d_buf + offset + entry->orig_len); alt->new_len = *(unsigned char *)(sec->data->d_buf + offset + entry->new_len); }
And as far as I can tell, "alt->orig_len" can be 0 if the entry in the .altinstructions section of the .o file has the length set to 0.
I don't know how the alternative section generation works on x86, but on arm64 it's just a computed assembly offset which can be 0.
> Is this a preparatory patch for arm64, where this could happen? If yes, it > would be better to mention it in the changelog. >
It used to happen on arm64, but the fix [1] was picked.
I can add that link to the commit if necessary.
[1] https://lkml.org/lkml/2020/1/9/708
Cheers,
-- Julien Thierry
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