Re: [syzbot] WARNING in do_mkdirat

[Aleksandr Nogikh]

Hi Eric, Ted,

On Sat, Dec 31, 2022 at 5:58 PM Theodore Ts'o <tytso@xxxxxxx> wrote:
>
> On Thu, Dec 29, 2022 at 01:17:31PM -0800, Eric Biggers wrote:
> > Thanks Aleksandr.  From what I can see, the fix is working for new filesystem
> > bugs: the filesystem(s) involved get added to the title and the recipients.
> >
> > One question: what happens to all the open bugs, like this one ("WARNING in
> > do_mkdirat") that were reported before the syzbot fix?  Are they going to be
> > re-reported correctly?  Perhaps any bug whose reproducer includes
> > "syz_mount_image" and was reported before the date of this fix should be
> > invalidated more aggressively than usual, so that it can be re-reported?

I fear that the community will not be super excited to see those tons
of fs bug reports again :)

Soon it'll become possible to see the subsystems on the dashboard and
to filter bugs based on them, hopefully this will help those bugs not
get completely lost.

>
> As a related request/wish, it would be nice if those dashboard pages
> that were created before the new-style reporting which includes the
> file system image, strace otuput, etc., could get regenerated.  For example:
>
> https://syzkaller.appspot.com/bug?id=be6e90ce70987950e6deb3bac8418344ca8b96cd

I've deployed the change -- now it should display all the download
links on the bug info page. If we have reported a download link /
strace log in an email, it should be there. For yet older bugs, it's
trickier. We regenerate reproducers once in 100 days, so if the old
bugs keep on happening, the information will appear there over time as
well.

>
> Even if someone has already submitted a proposed fix, I often like to
> double-check that the fix is really fixing the true root cause of the
> problem, as opposed to just making a superficial change that blocks
> the current syzbot reproducer, but which will eventually be tripped
> again because code is still vulnerable.  (For example, we might block
> a straightforward reproducer by adding a check at mount time, but if
> the superblocks get corrupted during the journal replay, we'd still be
> vulnerable.)  And having access to the corrupted file system image,
> and other associated reporting data, is often super-helpful in that
> regard.

Thank you very much for the feedback below!

>
> Also, can we at some point have the C reproducer actually using proper
> C strings instead of hex digits?  It will make the reproducer much
> more human understandable, as well making it easier to edit the string
> when the developer is trying to do a better job minimizing the test
> case than syzbot.  For example:
>
>   memcpy(
>       (void*)0x20000000,
>       "\x6e\x6f\x75\x73\x65\x72\x5f\x78\x61\x74\x74\x72\x2c\x61\x63\x6c\x2c\x64"
>       "\x65\x62\x75\x67\x5f\x77\x61\x6e\x74\x5f\x65\x78\x74\x72\x61\x5f\x69\x73"
>       "\x69\x7a\x65\x3d\x30\x78\x30\x30\x30\x30\x30\x30\x30\x30\x30\x30\x30\x30"
>       "\x30\x30\x38\x30\x2c\x6c\x61\x7a\x79\x74\x69\x6d\x65\x2c\x6e\x6f\x62\x68"
>       "\x2c\x71\x75\x6f\x74\x61\x2c\x00\x3d\x93\x09\x61\x36\x5d\x73\x58\x9c",
>       89);
>
> Would be *much* more understable if it were:
>
>   memcpy(
>       (void*)0x20000000,
>       "nouser_xattr,acl,debug_want_extra_isize=0x0000000000000080,lazytime,nobh,quota,",
>       80);

I've filed an issue to keep track on the progress:
https://github.com/google/syzkaller/issues/3605

>
> Of course, something like:
>
>    char mount_options[] = "nouser_xattr,acl,debug_want_extra_isize=0x0000000000000080,lazytime,nobh,quota,";
>
> Would be even better (and more portable) than using random hex
> addresses, but just simply using ASCII strings would be a good first
> step.
>
> Of course, filling in C structures instead of just a random memcpy of
> hex garbage would be even *more* awesome, bunt I'll take what I can
> get.  :-)
>
> Another opportunity for improvement is to try minimizing mount
> options, so it becomes more obvious which ones are required.  For
> example, in the above example, a minimized mount option string would
> have been:
>
>   memcpy((void*)0x20000000, "debug_want_extra_isize=0x80,lazytime," 38);
>
> Having a more minimized reproducer would improve the reliability of
> the bisect, as well as making it easier for the developer to figure
> out the true root cause of the problem.

I've filed an issue: https://github.com/google/syzkaller/issues/3606

--
Best Regards,
Aleksandr

>
> Cheers,
>
>                                         - Ted
>