Re: [PATCH] ci: update 'static-analysis' to Ubuntu 22.04

From: Jeff King <peff@peff.net>
Date: Aug 26, 2022, 07:48 UTC
Message-ID: <Ywh6v8oo3nRl/O5h@coredump.intra.peff.net>
In-Reply-To: <220825.86ilmg4mil.gmgdl@evledraar.gmail.com>

On Thu, Aug 25, 2022 at 12:47:53PM +0200, Ævar Arnfjörð Bjarmason wrote:

> What's happening here is that coccinelle can no longer properly parse
> the file after the UNUSED() macros were applied to refs.c.

Ugh. Thanks for finding this.

I'm a little disappointed in coccinelle here. I mean, I get that it probably can't just use the output of "gcc -E"; it wants to see and transform the code as written (and maybe even translate macros). But if it doesn't understand the macros enough to see inside them, there are a lot of things it is going to get confused by.

A simple example like this shows that it doesn't really see inside them:

  $ cat foo.c
  #define foo(x) bar(x)
  static void fun(int x) { foo(x); }

  $ cat foo.cocci
  expression E;
  @@
  - bar(E);
  + changed(E);

I wonder what it makes of code-generation macros like for_each_string_list_item(). Or all of commit-slab.

Likewise, the manner in which it fails is unexpected. I could see it getting confused about the line in question, but why would that cause it to fail to see the of a strbuf in another function entirely?

I realize that pseudo-parsing C is an incredibly hard problem. But I also wonder how robust the tool actually is in practice. We've had a lot of mystery "now why didn't it catch this case" incidents over the years. Just running the tip of master with --verbose-parsing yields quite a few "bad" and "parse error" entries in the log. I wonder how much that contributes.

> We should probably coerce coccinelle into stopping in general if it's
> encountering "BAD:!!!!!" parse errors behind the scenes, as it clearly
> results in broken logic, but offhand (and from briefly browsing the
> manpage) I don't know a way to do that.

You could detect the BAD entries from the log and complain before returning the exit code to make. But since there are several instances already, I suspect it's a losing battle. We need some way to actually _fix_ them. And in the meantime, it does seem to _mostly_ work, as it clearly has some kind of resync logic.

> But the fix here isn't to delete unused.cocci, but to hold off on the
> UNUSEwork D() patches until we figure out how to make coccinelle jive with
> them.

Yeah, my general skepticism and disappointment above notwithstanding, this seems like the best path forward from here. I tried a few other tricks (like --macro-file and --iso-file), but if its parser chokes, I don't think there's much we can do about it. Even if we wrote a patch to coccinelle itself (and I have no interest in doing that myself), it would take a while to become available.

-Peff