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Wed, 8 Jan 2025 13:31:07 +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 Z05zFxt+fmcMNwAAD6G6ig (envelope-from ); Wed, 08 Jan 2025 13:31:07 +0000 From: Fabiano Rosas To: Peter Xu Cc: qemu-devel@nongnu.org, Thomas Huth Subject: Re: [PATCH 3/7] migration: Document the effect of vmstate_info_nullptr In-Reply-To: References: <20250107195025.9951-1-farosas@suse.de> <20250107195025.9951-4-farosas@suse.de> Date: Wed, 08 Jan 2025 10:31:05 -0300 Message-ID: <87frlt4eli.fsf@suse.de> MIME-Version: 1.0 Content-Type: text/plain X-Spamd-Result: default: False [-4.30 / 50.00]; BAYES_HAM(-3.00)[100.00%]; NEURAL_HAM_LONG(-1.00)[-1.000]; NEURAL_HAM_SHORT(-0.20)[-1.000]; MIME_GOOD(-0.10)[text/plain]; RCVD_VIA_SMTP_AUTH(0.00)[]; ARC_NA(0.00)[]; MIME_TRACE(0.00)[0:+]; MISSING_XM_UA(0.00)[]; TO_DN_SOME(0.00)[]; MID_RHS_MATCH_FROM(0.00)[]; RCVD_TLS_ALL(0.00)[]; DKIM_SIGNED(0.00)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; FROM_EQ_ENVFROM(0.00)[]; FROM_HAS_DN(0.00)[]; RCPT_COUNT_THREE(0.00)[3]; RCVD_COUNT_TWO(0.00)[2]; FUZZY_BLOCKED(0.00)[rspamd.com]; TO_MATCH_ENVRCPT_ALL(0.00)[]; DBL_BLOCKED_OPENRESOLVER(0.00)[self.data:url, suse.de:email, suse.de:mid, imap1.dmz-prg2.suse.org:helo] Received-SPF: pass client-ip=195.135.223.130; envelope-from=farosas@suse.de; helo=smtp-out1.suse.de X-Spam_score_int: -43 X-Spam_score: -4.4 X-Spam_bar: ---- X-Spam_report: (-4.4 / 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, RCVD_IN_DNSWL_MED=-2.3, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, RCVD_IN_VALIDITY_SAFE_BLOCKED=0.001, 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: 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 Peter Xu writes: > On Tue, Jan 07, 2025 at 04:50:21PM -0300, Fabiano Rosas wrote: >> The migration stream lacks magic numbers at some key points. It's easy >> to mis-parse data. Unfortunately, the VMS_NULLPTR_MARKER continues >> with the trend. A '0' byte is ambiguous and could be interpreted as a >> valid 0x30. >> >> It is maybe not worth trying to change this while keeping backward >> compatibility, so add some words of documentation to clarify. >> >> Signed-off-by: Fabiano Rosas >> --- >> migration/vmstate-types.c | 6 ++++++ >> scripts/analyze-migration.py | 9 +++++++++ >> 2 files changed, 15 insertions(+) >> >> diff --git a/migration/vmstate-types.c b/migration/vmstate-types.c >> index e83bfccb9e..08ed059f87 100644 >> --- a/migration/vmstate-types.c >> +++ b/migration/vmstate-types.c >> @@ -339,6 +339,12 @@ static int put_nullptr(QEMUFile *f, void *pv, size_t size, >> >> const VMStateInfo vmstate_info_nullptr = { >> .name = "uint64", > > Ouch.. So I overlooked this line and this explains why it didn't go via > VMSDFieldGeneric already. Yes, actually I overlooked as well that it should match the size of the data being handled in the get/put functions. My comment below is about NULL -> 0x30 that I think should instead be NULL -> 0x3030303030303030 so we have any chance of looking at this and identifying it's a NULL pointer. When we write 0x30 it might become confusing for people reading the scripts output that their stream has a bunch of '0' in the place where pointers should be. If the MAGIC number were more identifiable, I could change the script to output (null) or 0x0ULL. We also don't really have the concept of a pointer, which I suspect might be the real reason behind all this mess. So we'll see: 0x30 0x30 { .some .struct .here } 0x30 So all this patch was trying to do is document this situation somehow. > > Instead of below comment, do we still have chance to change this to > something like "uint8"? Then I suppose the script will be able to identify > this properly. > >> + >> + /* >> + * Ideally these would actually read/write the size of a pointer, >> + * but we're stuck with just a byte now for backward >> + * compatibility. >> + */ >> .get = get_nullptr, >> .put = put_nullptr, >> }; >> diff --git a/scripts/analyze-migration.py b/scripts/analyze-migration.py >> index f2457b1dde..4292fde424 100755 >> --- a/scripts/analyze-migration.py >> +++ b/scripts/analyze-migration.py >> @@ -388,12 +388,21 @@ def read(self): >> return self.data >> >> class VMSDFieldUInt(VMSDFieldInt): >> + NULL_PTR_MARKER = 0x30 >> + >> def __init__(self, desc, file): >> super(VMSDFieldUInt, self).__init__(desc, file) >> >> def read(self): >> super(VMSDFieldUInt, self).read() >> self.data = self.udata >> + >> + if self.data == self.NULL_PTR_MARKER: >> + # The migration stream encodes NULL pointers as '0' so any >> + # 0x30 in the stream could be a NULL. There's not much we >> + # can do without breaking backward compatibility. >> + pass > > So this change doesn't do anything, right? > > It'll be weird here having it "uint64" but the super().read() will actually > only read 1 byte.. I assume the oneliner change of s/uint64/uint8/ could > be a replacement of this patch, and I hope that'll work too for the script. > So we will still see a bunch of 0x30s but I assume it's ok. > >> + >> return self.data >> >> class VMSDFieldIntLE(VMSDFieldInt): >> -- >> 2.35.3 >>