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bh=W4WwdGjH3+By8fNFreo6uZWnqbXxcU/ieOCK2CwDNo8=; b=0iG1ftUiwHgCt3JtzhNZJeh3mqNdwkZMHCQTinNkogmFGY/BRrpWihbykzb9qkTnMDXymj prqtxt617za/LWAg== Received: from imap1.dmz-prg2.suse.org (localhost [127.0.0.1]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits) server-digest SHA256) (No client certificate requested) by imap1.dmz-prg2.suse.org (Postfix) with ESMTPS id 950663FFE7; Thu, 12 Mar 2026 14:15:16 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id iFf6FnTKsmmAZwAAD6G6ig (envelope-from ); Thu, 12 Mar 2026 14:15:16 +0000 From: Fabiano Rosas To: Alexandr Moshkov , qemu-devel@nongnu.org Cc: Peter Maydell , Vladimir Sementsov-Ogievskiy , "yc-core@yandex-team.ru" , Peter Xu , Alexandr Moshkov Subject: Re: [PATCH v3] vmstate: fix subsection load name check In-Reply-To: <20260312102626.891359-1-dtalexundeer@yandex-team.ru> References: <20260312102626.891359-1-dtalexundeer@yandex-team.ru> Date: Thu, 12 Mar 2026 11:15:14 -0300 Message-ID: <878qbxyw0d.fsf@suse.de> MIME-Version: 1.0 Content-Type: text/plain X-Rspamd-Action: no action X-Rspamd-Server: rspamd2.dmz-prg2.suse.org X-Spamd-Result: default: False [-4.51 / 50.00]; BAYES_HAM(-3.00)[99.99%]; NEURAL_HAM_LONG(-1.00)[-1.000]; R_DKIM_ALLOW(-0.20)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; NEURAL_HAM_SHORT(-0.20)[-1.000]; MIME_GOOD(-0.10)[text/plain]; MX_GOOD(-0.01)[]; ARC_NA(0.00)[]; RCVD_VIA_SMTP_AUTH(0.00)[]; TO_DN_EQ_ADDR_SOME(0.00)[]; MISSING_XM_UA(0.00)[]; FUZZY_RATELIMITED(0.00)[rspamd.com]; TO_DN_SOME(0.00)[]; MIME_TRACE(0.00)[0:+]; RCVD_TLS_ALL(0.00)[]; MID_RHS_MATCH_FROM(0.00)[]; SPAMHAUS_XBL(0.00)[2a07:de40:b281:104:10:150:64:97:from]; FROM_EQ_ENVFROM(0.00)[]; FROM_HAS_DN(0.00)[]; RCPT_COUNT_SEVEN(0.00)[7]; RCVD_COUNT_TWO(0.00)[2]; TO_MATCH_ENVRCPT_ALL(0.00)[]; DBL_BLOCKED_OPENRESOLVER(0.00)[yandex-team.ru:email,suse.de:dkim,suse.de:mid,imap1.dmz-prg2.suse.org:helo,imap1.dmz-prg2.suse.org:rdns]; DKIM_SIGNED(0.00)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; DKIM_TRACE(0.00)[suse.de:+] X-Rspamd-Queue-Id: 15ED14D1EF Received-SPF: pass client-ip=2a07:de40:b251:101:10:150:64:1; envelope-from=farosas@suse.de; helo=smtp-out1.suse.de X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: qemu development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Alexandr Moshkov writes: Sorry, but I'll have to nitpick on the commit message, I can't parse this without looking at the code. > When loading a subset, its name is checked for the parent prefix. The What do you mean by subset? > following bug may occur here: > > Let's say there is a vmstate named "virtio-blk", it has a subsection > named "virtio-blk/subsection", and it also has another vmstate named > "virtio" in the fields. > Then, during the migration, when trying to load this subsection for What is "this subsection" in this context? 'virtio-blk/subsection' ? Or do you mean the 'virtio' field. > "virtio", the prefix condition will pass for "virtio-blk/subsection" and What's "the prefix condition" exactly? Could you be more explicit? > then the migration will break, because this vmstate does not have such a > subsection. > > In other words, if a field inside vmstate1 is set via vmstate2 with a > name that is a prefix of the parent vmstate, then the field can "steal" > a subsection belonging to the parent state. > This is what I read from the above paragraph. Is this it? vmstate1 "abc" vmstate2 "abcdef" field -> vmstate2 subsection > Looks like it happens because migration stream for "virtio-blk" looks > like this: > > [virtio-blk header] [virtio-blk fields] [virtio-blk subsections] > > "virtio-blk" contains "virtio" field, so migration stream is: > > [virtio-blk header] [virtio header] [virtio fields] [virtio > subsections] [virtio-blk subsections] > So the code sees the [virtio-blk subsections] as a continuation of the preceding [virtio subsections] because the string "virtio", which is the parent of [virtio subsection] is matched in the string "virtio-blk"? > And when we load the subsections of the "virtio" device, > vmstate_subsection_load() uses qemu_peek_byte() to try to figure out if > this is his subsection. This is where we encounter an error. > > Thus, the error occurs due to the fact that vmsd does not know how many > subsections it has when loading (this does not appear anywhere in the > migration stream), so it tries to load all the appropriate ones by > names. > > To fix this issue, we call vmstate_get_subsection() and, in case of > fail, ignore it, pop the VMSD stack and will try to define this > subsection for outer VMSD > > Signed-off-by: Alexandr Moshkov > --- > migration/vmstate.c | 4 +--- > 1 file changed, 1 insertion(+), 3 deletions(-) > > diff --git a/migration/vmstate.c b/migration/vmstate.c > index 4d28364f7b..8f3a69c26e 100644 > --- a/migration/vmstate.c > +++ b/migration/vmstate.c > @@ -639,9 +639,7 @@ static int vmstate_subsection_load(QEMUFile *f, const VMStateDescription *vmsd, > sub_vmsd = vmstate_get_subsection(vmsd->subsections, idstr); > if (sub_vmsd == NULL) { > trace_vmstate_subsection_load_bad(vmsd->name, idstr, "(lookup)"); > - error_setg(errp, "VM subsection '%s' in '%s' does not exist", > - idstr, vmsd->name); > - return -ENOENT; > + return 0; > } > qemu_file_skip(f, 1); /* subsection */ > qemu_file_skip(f, 1); /* len */