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Fri, 24 Apr 2020 07:44:47 +0000 (UTC) Received: from blackfin.pond.sub.org (ovpn-113-6.ams2.redhat.com [10.36.113.6]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 61227600EF; Fri, 24 Apr 2020 07:44:47 +0000 (UTC) Received: by blackfin.pond.sub.org (Postfix, from userid 1000) id D602911358BC; Fri, 24 Apr 2020 09:44:45 +0200 (CEST) From: Markus Armbruster To: Eric Blake Subject: Re: [PATCH 10/13] qapi: Clean up visitor's recovery from input with invalid type References: <20200423160036.7048-1-armbru@redhat.com> <20200423160036.7048-11-armbru@redhat.com> <435f2d5d-e732-a236-bf13-991d5c3510e6@redhat.com> <89636246-6a5c-476d-6016-977f39cb653c@redhat.com> Date: Fri, 24 Apr 2020 09:44:45 +0200 In-Reply-To: <89636246-6a5c-476d-6016-977f39cb653c@redhat.com> (Eric Blake's message of "Thu, 23 Apr 2020 13:18:04 -0500") Message-ID: <877dy5pbte.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 2.79 on 10.5.11.11 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Received-SPF: pass client-ip=207.211.31.120; envelope-from=armbru@redhat.com; helo=us-smtp-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/04/24 03:07:34 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 207.211.31.120 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: qemu-devel@nongnu.org, mdroth@linux.vnet.ibm.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Eric Blake writes: > On 4/23/20 1:06 PM, Eric Blake wrote: >> On 4/23/20 11:00 AM, Markus Armbruster wrote: >>> An alternate type's visit_type_FOO() fails when it runs into an >>> invalid ->type.=C2=A0 If it's an input visit, we then need to free the = the >>> object we got from visit_start_alternate().=C2=A0 We do that with >>> qapi_free_FOO(), which uses the dealloc visitor. >>> >>> Trouble is that object is in a bad state: its ->type is invalid.=C2=A0 = So >>> the dealloc visitor will run into the same error again, and the error >>> recovery skips deallocating the alternate's (invalid) alternative. >>> This is a roundabout way to g_free() the alternate. >>> >>> Simplify: replace the qapi_free_FOO() by g_free(). >>> >>> Signed-off-by: Markus Armbruster >>> --- >>> =C2=A0 scripts/qapi/visit.py | 2 +- >>> =C2=A0 1 file changed, 1 insertion(+), 1 deletion(-) >>> >> >> Required looking at what gets generated into qapi_free_FOO() as well >> as when visit_start_alternate() can fail, but makes sense. >> >> Reviewed-by: Eric Blake > > Actually, I'm having second thoughts. As an example, look at the generat= ed: > >> void visit_type_BlockDirtyBitmapMergeSource(Visitor *v, const char *name= , BlockDirtyBitmapMergeSource **obj, Error **errp) >> { >> Error *err =3D NULL; >> >> visit_start_alternate(v, name, (GenericAlternate **)obj, sizeof(**ob= j), >> &err); >> if (err) { >> goto out; >> } >> if (!*obj) { >> goto out_obj; > [1] >> } >> switch ((*obj)->type) { >> case QTYPE_QSTRING: >> visit_type_str(v, name, &(*obj)->u.local, &err); > [2] >> break; >> case QTYPE_QDICT: >> visit_start_struct(v, name, NULL, 0, &err); >> if (err) { >> break; > [3] >> } >> visit_type_BlockDirtyBitmap_members(v, &(*obj)->u.external, &err= ); >> if (!err) { >> visit_check_struct(v, &err); > [4] >> } >> visit_end_struct(v, NULL); >> break; >> case QTYPE_NONE: >> abort(); >> default: >> error_setg(&err, QERR_INVALID_PARAMETER_TYPE, name ? name : "nul= l", >> "BlockDirtyBitmapMergeSource"); > [5] >> } >> out_obj: >> visit_end_alternate(v, (void **)obj); >> if (err && visit_is_input(v)) { >> qapi_free_BlockDirtyBitmapMergeSource(*obj); > > If we got here, we must have failed at any of the points mentioned above. > > If [1], visit_start_alternate() failed, but *obj is NULL and both > qapi_free_FOO(NULL) and g_free(NULL) are safe. > > If [2], visit_type_str() failed, so *obj is allocated but the embedded > string (here, u.local) was left NULL. qapi_free_FOO() then does > nothing further than g_free(obj). > > If [3], visit_start_struct() failed, the embedded dict (here, > u.external) was left NULL. qapi_free_FOO() then does nothing further > than g_free(obj). > > If [5], we have the wrong ->type. As pointed out by this commit, > qapi_free_FOO() does nothing further than g_free(obj). > > But what happens in [4]? Here, the embedded dict was allocated, but > we then failed while parsing its members. That leaves us in a > partially-allocated state, and g_free(NULL) does NOT recursively visit > that partial allocation. I think this patch is prone to a memory leak > unless you _also_ patch things to free any dict branch on failure > (perhaps during the QTYPE_QDICT case label, rather than here at the > end). You're right. Let's change cleanup only for the default case, like this: default: error_setg(&err, QERR_INVALID_PARAMETER_TYPE, name ? name : "nul= l", "BlockDirtyBitmapMergeSource"); + g_free(*obj); + *obj =3D NULL; } out_obj: visit_end_alternate(v, (void **)obj); if (err && visit_is_input(v)) { qapi_free_BlockDirtyBitmapMergeSource(*obj); *obj =3D NULL; } out: error_propagate(errp, err); } Thanks!