linux-kernel - Re: [PATCH v2 04/10] objtool: check: Ignore empty alternative groups

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Date:   Wed, 1 Apr 2020 15:48:20 +0200 (CEST)
From:   Miroslav Benes <mbenes@...e.cz>
To:     Julien Thierry <jthierry@...hat.com>
cc:     linux-kernel@...r.kernel.org, jpoimboe@...hat.com,
        peterz@...radead.org, raphael.gault@....com
Subject: Re: [PATCH v2 04/10] objtool: check: Ignore empty alternative
 groups

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@...hat.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