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

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Date:   Wed, 1 Apr 2020 14:53:42 +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 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.

Is this a preparatory patch for arm64, where this could happen? If yes, it 
would be better to mention it in the changelog.

Miroslav