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

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Date:   Thu, 7 Jan 2021 12:18:35 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Miroslav Benes <mbenes@...e.cz>
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

On Thu, Jan 07, 2021 at 02:22:27PM +0100, Miroslav Benes wrote:
> On Tue, 22 Dec 2020, Josh Poimboeuf wrote:
> 
> > BTW, another benefit of these changes is that, thanks to some related
> > cleanups (new fake nops and alt_group struct) objtool can finally be rid
> > of fake jumps, which were a constant source of headaches.
> 
> \o/
> 
> You may also want to remove/edit the comment right before 
> handle_group_alt() now that fake jumps are gone.
> 
> Anyway, I walked through the patch (set) and I think it should work fine 
> (but I am not confident enough to give it Reviewed-by. My head spins :)). 
> I even like the change.
> 
> Also, 1/3 is a benefit on its own, so if nothing else, it could go in.

Thanks for the review!

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,