Re: [BUG] ext4/block null pointer crashes in linux-next

[Dennis Zhou <dennis@kernel.org>]

Hi Vladis,

On Mon, Oct 15, 2018 at 07:28:48PM -0400, valdis.kletnieks@vt.edu wrote:
> So I finally had a chance to find a replicator and finish bisecting this and:
> 
> [/usr/src/linux-next] git bisect good
> e2b0989954ae7c80609f77e7ce203bea6d2c54e1 is the first bad commit
> commit e2b0989954ae7c80609f77e7ce203bea6d2c54e1
> Author: Dennis Zhou (Facebook) <dennisszhou@gmail.com>
> Date:   Tue Sep 11 14:41:35 2018 -0400
> 
>     blkcg: cleanup and make blk_get_rl use blkg_lookup_create
> 
> I was able to do a bit of sleuthing with strace, and I tracked it down to one of
> several execve() calls that 'rpm' makes with my replicating test case.
> 
>  grep execve /root/rpm-exec-strace 
> execve("/usr/bin/rpm", ["rpm", "-Uvh", "--force", "dracut-049-4.git20181010.fc30.x8"...], 0x7ffc9d967d80 /* 33 vars */) = 0
> [pid 119212] execve("/bin/sh", ["/bin/sh", "/usr/src/redhat/tmp/rpm-tmp.w7fu"..., "0", "0"], 0x7ffdfe17d480 /* 33 vars */) = 0
> [pid 119213] execve("/sbin/ldconfig", ["/sbin/ldconfig"], 0x558ccf928ac0 /* 33 vars */) = 0
> [pid 119216] execve("/bin/sh", ["/bin/sh", "/usr/src/redhat/tmp/rpm-tmp.bIKt"..., "0", "0"], 0x7ffdfe17d480 /* 33 vars */) = 0
> [pid 119217] execve("/usr/bin/systemd-run", ["/usr/bin/systemd-run", "/usr/bin/systemctl", "start", "man-db-cache-update"], 0x56360645d290 /* 33 vars */) = 0
> [pid 119221] execve("/bin/sh", ["/bin/sh", "/usr/src/redhat/tmp/rpm-tmp.OGWg"..., "0", "0"], 0x7ffdfe17d480 /* 33 vars */) = 0
> [pid 119920] execve("/usr/bin/systemctl", ["/usr/bin/systemctl", "daemon-reload"], 0x55c0f5d43c30 /* 33 vars */) = 0
> 
> The ldconfig and systemctl commands run just fine stand-alone, so I'm suspecting the
> calls to run the temp files - it's quite possible that execve() gets invoked on them before
> writeback has actually gotten the data to the disk - though that shouldn't matter.
> 
> But I managed to trigger a different traceback. I cd /usr/src/redhat/tmp, and I
> did an 'rm *' - and never got a prompt back. Traceback out of pstore below.
> 
> Now here's the weird part - I'd already unmounted, fsck'ed, and remounted the
> file system before the 'rm *'.  And thinking that there was one file with a
> busted inode that passed fsck.ext4's sniff test, I did:
> 
> cd /usr/src/redhat/tmp
> for i in `find . -type f`; do sleep 5; echo $i; rm $i; done
> 
> and that worked just fine. Nothing left in that directory but . and ..  
> I then re-ran my rpm-based replicator and it blew up again.
> 
> Traceback of the rm crash (I have *no* idea why it has systemd-tmpfile as Comm:
> as none of the tmpfile config reference /usr/src at all, and the config says it
> shouldn't have been running at the time of the crash, and I can't replicate as
> the directory is now empty...)
> 

Thanks for testing and reporting this! Do you mind sending me your
reproducer?

Thanks,
Dennis