linux-kernel - Re: [PATCH 3/3] objtool: Support stack layout changes in alternatives

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Message-ID: <alpine.LSU.2.21.2101080914230.6998@pobox.suse.cz>
Date:   Fri, 8 Jan 2021 09:15:43 +0100 (CET)
From:   Miroslav Benes <mbenes@...e.cz>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
cc:     Juergen Gross <jgross@...e.com>, xen-devel@...ts.xenproject.org,
        linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Shinichiro Kawasaki <shinichiro.kawasaki@....com>
Subject: Re: [PATCH 3/3] objtool: Support stack layout changes in
 alternatives

> That comment is indeed now obsolete.  I can squash something like so:
> 
> diff --git a/tools/objtool/check.c b/tools/objtool/check.c
> index 81d56fdef1c3..ce67437aaf3f 100644
> --- a/tools/objtool/check.c
> +++ b/tools/objtool/check.c
> @@ -958,21 +958,8 @@ static int add_call_destinations(struct objtool_file *file)
>  }
>  
>  /*
> - * The .alternatives section requires some extra special care, over and above
> - * what other special sections require:
> - *
> - * 1. Because alternatives are patched in-place, we need to insert a fake jump
> - *    instruction at the end so that validate_branch() skips all the original
> - *    replaced instructions when validating the new instruction path.
> - *
> - * 2. An added wrinkle is that the new instruction length might be zero.  In
> - *    that case the old instructions are replaced with noops.  We simulate that
> - *    by creating a fake jump as the only new instruction.
> - *
> - * 3. In some cases, the alternative section includes an instruction which
> - *    conditionally jumps to the _end_ of the entry.  We have to modify these
> - *    jumps' destinations to point back to .text rather than the end of the
> - *    entry in .altinstr_replacement.
> + * The .alternatives section requires some extra special care over and above
> + * other special sections because alternatives are patched in place.
>   */
>  static int handle_group_alt(struct objtool_file *file,
>  			    struct special_alt *special_alt,

Looks good to me.

Miroslav