Messages in this thread | | | Date | Wed, 1 Apr 2020 15:48:20 +0200 (CEST) | From | Miroslav Benes <> | Subject | Re: [PATCH v2 04/10] objtool: check: Ignore empty alternative groups |
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On Wed, 1 Apr 2020, Julien Thierry wrote:
> > > 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); > }
Now that you copy-pasted the code here, I see that I completely missed there is dereference (for obvious reasons) right before the type cast, so all is fine. My mistake, I need more tea.
> 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.
Yes
> 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.
No, I think the check makes sense on its own.
Thanks Miroslav
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