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Sun, 23 Feb 2020 08:55:48 +0000 (UTC) Received: from blackfin.pond.sub.org (ovpn-116-129.ams2.redhat.com [10.36.116.129]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 7BC231001902; Sun, 23 Feb 2020 08:55:38 +0000 (UTC) Received: by blackfin.pond.sub.org (Postfix, from userid 1000) id F0F7411386A6; Sun, 23 Feb 2020 09:55:36 +0100 (CET) From: Markus Armbruster To: Vladimir Sementsov-Ogievskiy Subject: Re: [PATCH v7 03/11] scripts: add coccinelle script to use auto propagated errp References: <20200131130118.1716-1-vsementsov@virtuozzo.com> <20200131130118.1716-4-vsementsov@virtuozzo.com> Date: Sun, 23 Feb 2020 09:55:36 +0100 In-Reply-To: <20200131130118.1716-4-vsementsov@virtuozzo.com> (Vladimir Sementsov-Ogievskiy's message of "Fri, 31 Jan 2020 16:01:10 +0300") Message-ID: <87v9nxwulz.fsf@dusky.pond.sub.org> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/26.3 (gnu/linux) MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] X-Received-From: 205.139.110.61 X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Kevin Wolf , Stefano Stabellini , Michael Roth , qemu-block@nongnu.org, Paul Durrant , Laszlo Ersek , qemu-devel@nongnu.org, Greg Kurz , Gerd Hoffmann , Stefan Hajnoczi , Anthony Perard , xen-devel@lists.xenproject.org, Max Reitz , Philippe =?utf-8?Q?Mathieu-Daud=C3=A9?= , armbru@redhat.com, Stefan Berger Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Vladimir Sementsov-Ogievskiy writes: > Script adds ERRP_AUTO_PROPAGATE macro invocation where appropriate and > does corresponding changes in code (look for details in > include/qapi/error.h) > > Usage example: > spatch --sp-file scripts/coccinelle/auto-propagated-errp.cocci \ > --macro-file scripts/cocci-macro-file.h --in-place --no-show-diff \ > blockdev-nbd.c qemu-nbd.c {block/nbd*,nbd/*,include/block/nbd*}.[hc] > > Signed-off-by: Vladimir Sementsov-Ogievskiy > --- > > CC: Eric Blake > CC: Kevin Wolf > CC: Max Reitz > CC: Greg Kurz > CC: Stefano Stabellini > CC: Anthony Perard > CC: Paul Durrant > CC: Stefan Hajnoczi > CC: "Philippe Mathieu-Daud=C3=A9" > CC: Laszlo Ersek > CC: Gerd Hoffmann > CC: Stefan Berger > CC: Markus Armbruster > CC: Michael Roth > CC: qemu-block@nongnu.org > CC: xen-devel@lists.xenproject.org > > include/qapi/error.h | 3 + > scripts/coccinelle/auto-propagated-errp.cocci | 158 ++++++++++++++++++ > 2 files changed, 161 insertions(+) > create mode 100644 scripts/coccinelle/auto-propagated-errp.cocci > > diff --git a/include/qapi/error.h b/include/qapi/error.h > index b9452d4806..79f8e95214 100644 > --- a/include/qapi/error.h > +++ b/include/qapi/error.h > @@ -141,6 +141,9 @@ > * ... > * } > * > + * For mass conversion use script > + * scripts/coccinelle/auto-propagated-errp.cocci > + * > * > * Receive and accumulate multiple errors (first one wins): > * Error *err =3D NULL, *local_err =3D NULL; Extra blank line. > diff --git a/scripts/coccinelle/auto-propagated-errp.cocci b/scripts/cocc= inelle/auto-propagated-errp.cocci > new file mode 100644 > index 0000000000..fb03c871cb > --- /dev/null > +++ b/scripts/coccinelle/auto-propagated-errp.cocci > @@ -0,0 +1,158 @@ > +// Use ERRP_AUTO_PROPAGATE (see include/qapi/error.h) > +// > +// Copyright (c) 2020 Virtuozzo International GmbH. > +// > +// This program is free software; you can redistribute it and/or modify > +// it under the terms of the GNU General Public License as published by > +// the Free Software Foundation; either version 2 of the License, or > +// (at your option) any later version. > +// > +// This program is distributed in the hope that it will be useful, > +// but WITHOUT ANY WARRANTY; without even the implied warranty of > +// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the > +// GNU General Public License for more details. > +// > +// You should have received a copy of the GNU General Public License > +// along with this program. If not, see . > +// > +// Usage example: > +// spatch --sp-file scripts/coccinelle/auto-propagated-errp.cocci \ > +// --macro-file scripts/cocci-macro-file.h --in-place --no-show-diff \ > +// blockdev-nbd.c qemu-nbd.c {block/nbd*,nbd/*,include/block/nbd*}.[hc] > + > +@rule0@ > +// Add invocation to errp-functions where necessary > +// We should skip functions with "Error *const *errp" > +// parameter, but how to do it with coccinelle? > +// I don't know, so, I skip them by function name regex. > +// It's safe: if we did not skip some functions with > +// "Error *const *errp", ERRP_AUTO_PROPAGATE invocation > +// will fail to compile, because of const violation. Not skipping a function we should skip fails to compile. What about skipping a function we should not skip? > +identifier fn !~ "error_append_.*_hint"; > +identifier local_err, ERRP; A few of our coccinelle scripts use ALL_CAPS for meta-variables. Most don't. Either is fine with me. Mixing the two styles feels a bit confusing, though. > +@@ > + > + fn(..., Error **ERRP, ...) > + { > ++ ERRP_AUTO_PROPAGATE(); > + <+... > + when !=3D ERRP_AUTO_PROPAGATE(); > +( > + error_append_hint(ERRP, ...); > +| > + error_prepend(ERRP, ...); > +| > + Error *local_err =3D NULL; > +) > + ...+> > + } Misses error_vprepend(). Currently harmless, but as long as we commit the script, we better make it as robust as we reasonably can. The previous patch explains this Coccinelle script's intent: To achieve these goals, later patches will add invocations of this macro at the start of functions with either use error_prepend/error_append_hint (solving 1) or which use local_err+error_propagate to check errors, switching those functions to use *errp instead (solving 2 and 3). This rule matches "use error_prepend/error_append_hint" directly. It appears to use presence of a local Error * variable as proxy for "use local_err+error_propagate to check errors". Hmm. We obviously have such a variable when we use "local_err+error_propagate to check errors". But we could also have such variables without use of error_propagate(). In fact, error.h documents such use: * Call a function and receive an error from it: * Error *err =3D NULL; * foo(arg, &err); * if (err) { * handle the error... * } where "handle the error" frees it. I figure such uses typically occur in functions without an Error **errp parameter. This rule doesn't apply then. But they could occur even in functions with such a parameter. Consider: void foo(Error **errp) { Error *err =3D NULL; bar(&err); if (err) { error_free(err); error_setg(errp, "completely different error"); } } Reasonable enough when bar() gives us an error that's misleading in this context, isn't it? The script transforms it like this: void foo(Error **errp) { - Error *err =3D NULL; + ERRP_AUTO_PROPAGATE(); - bar(&err); - if (err) { - error_free(err); + bar(errp); + if (*errp) { + error_free_errp(errp); error_setg(errp, "completely different error"); } } Unwanted. Now, if this script applied in just a few dozen places, we could rely on eyeballing its output to catch unwanted transformations. Since it applies in so many more, I don't feel comfortable relying on reviewer eyeballs. Can we make rule0 directly match error_propagate(errp, local_err) somehow? Another observation: the rule does not match error_reportf_err() and warn_reportf_err(). These combine error_prepend(), error_report()/warn_report() and error_free(), for convenience. Don't their users need ERRP_AUTO_PROPAGATE() just like error_prepend()'s users? > + > +@@ > +// Switch unusual (Error **) parameter names to errp > +// (this is necessary to use ERRP_AUTO_PROPAGATE). Please put your rule comments right before the rule, i.e. before the @-line introducing metavariable declarations, not after. Same elsewhere. > +identifier rule0.fn; > +identifier rule0.ERRP !=3D errp; > +@@ > + > + fn(..., > +- Error **ERRP > ++ Error **errp > + ,...) > + { > + <... > +- ERRP > ++ errp > + ...> > + } This normalizes errp parameter naming. It matches exactly when rule0 matches (and inserts ERRP_AUTO_PROPAGATE()) and the Error ** parameter is unusual. Good. > + > +@rule1@ > +// We want to patch error propagation in functions regardless of > +// whether the function already uses ERRP_AUTO_PROPAGATE prior to > +// applying rule0, hence this one does not inherit from it. I'm not sure I get this comment. Let's see what the rule does. > +identifier fn !~ "error_append_.*_hint"; > +identifier local_err; > +symbol errp; > +@@ > + > + fn(..., Error **errp, ...) > + { > + <... > +- Error *local_err =3D NULL; > + ...> > + } rule1 matches like rule0, except the Error ** parameter match is tightened from any C identifier to the C identifier errp, and the function body match tightened from "either use error_prepend/error_append_hint or which use local_err+error_propagate to check errors" to just the latter. I figure tightening the Error ** parameter match has no effect, because we already normalized the parameter name. So rule1 deletes variable local_err where rule0 applied. Correct? > + > +@@ > +// Handle pattern with goto, otherwise we'll finish up > +// with labels at function end which will not compile. > +identifier rule1.fn, rule1.local_err; > +identifier OUT; > +@@ > + > + fn(...) > + { > + <... > +- goto OUT; > ++ return; > + ...> > +- OUT: > +- error_propagate(errp, local_err); > + } This is one special case of error_propagate() deletion. It additionally gets rid of a goto we no longer want. For the general case, see below. The rule applies only where rule1 just deleted the variable. Thus, the two rules work in tandem. Makes sense. > + > +@@ > +identifier rule1.fn, rule1.local_err; This rule also works in tandem with rule1. > +expression list args; // to reindent error_propagate_prepend What is the comment trying to tell me? > +@@ > + > + fn(...) > + { > + <... > +( > +- error_free(local_err); > +- local_err =3D NULL; > ++ error_free_errp(errp); Reminder: static inline void error_free_errp(Error **errp) { assert(errp && *errp); error_free(*errp); *errp =3D NULL; } Now let's examine the actual change. The assertion's first half trivially holds, ERRP_AUTO_PROPAGATE() ensures it. The second half is new. We now crash when we haven't set an error. Why is this safe? Note that error_free(local_err) does nothing when !local_err. The zapping of the variable pointing to the Error just freed is unchanged. > +| > +- error_free(local_err); > ++ error_free_errp(errp); Here, the zapping is new. Zapping dangling pointers is obviously safe. Needed, or else the automatic error_propagate() due to ERRP_AUTO_PROPAGATE() would propagate the dangling pointer. > +| > +- error_report_err(local_err); > ++ error_report_errp(errp); The only difference to the previous case is that we also report the error. The previous case has a buddy that additionally matches *errp =3D NULL. Why not this one? > +| > +- warn_report_err(local_err); > ++ warn_report_errp(errp); Likewise. What about error_reportf_err(), warn_reportf_err()? Up to here, this rule transforms the various forms of error_free(). Next: error_propagate(). > +| > +- error_propagate_prepend(errp, local_err, args); > ++ error_prepend(errp, args); > +| > +- error_propagate(errp, local_err); rule0's adding of ERRP_AUTO_PROPAGATE() made error_propagate() redundant. This is the general case of error_propagate() deletion. I'd put the plain error_propagate() first, variations second, like you do with error_free(). If neither of these two patterns match on a path from ERRP_AUTO_PROPAGATE() to return, we effectively insert error_propagate() where it wasn't before. Does nothing when the local error is null there. Bug fix when it isn't: it's at least a memory leak, and quite possibly worse. Identifying these bug fixes would be nice, but I don't have practical ideas on how to do that. Can we explain this in the commit message? > +) > + ...> > + } > + > +@@ > +identifier rule1.fn, rule1.local_err; > +@@ > + > + fn(...) > + { > + <... > +( > +- &local_err > ++ errp > +| > +- local_err > ++ *errp > +) > + ...> > + } Also in tandem with rule1, fixes up uses of local_err. Good. > + > +@@ > +identifier rule1.fn; > +@@ > + > + fn(...) > + { > + <... > +- *errp !=3D NULL > ++ *errp > + ...> > + } Still in tandem with rule1, normalizes style. Good.