Re: [PATCH] xfs_repair: avoid segfault if reporting progress early in repair

Eric Sandeen
2013-10-17T23:12:49Z

On 10/17/13 12:50 PM, Eric Sandeen wrote:
> For a very large filesystem, zeroing the log may take some time.
>
> If we ask for progress reports frequently enough that one fires
> before we finish with log zeroing, we try to use a progress format
> which has not yet been set up, and segfault:
>
> # mkfs.xfs -d size=60t,file,name=fsfile
> # xfs_repair -m 9000 -o ag_stride=32 -t 1 fsfile
> Phase 1 - find and verify superblock...
> - reporting progress in intervals of 1 seconds
> Phase 2 - using internal log
> - zero log...
> Segmentation fault
>
> (gdb) bt
> #0 0x0000000000426962 in progress_rpt_thread (p=0x67ad20) at progress.c:234
> #1 0x0000003b98a07851 in start_thread (arg=0x7f19d8e47700) at pthread_create.c:301
> #2 0x0000003b982e767d in ?? ()
> #3 0x0000000000000000 in ?? ()
> (gdb) p msgp
> $1 = (msg_block_t *) 0x67ad20
> (gdb) p msgp->format
> $2 = (progress_rpt_t *) 0x0
> (gdb)
>
> I suppose we could rig up progress reports for log zeroing, but
> that won't usually take terribly long; for now, be defensive
> and init the message->format to NULL, and just return early
> from the progress thread if we've not yet set up any message.
>
> (Sure, global_msgs is global, and ->format is already NULL,
> but to me it's worth being explicit since we will test it).
>
> Signed-off-by: Eric Sandeen <sandeen@redhat.com>
> ---
>
> diff --git a/repair/progress.c b/repair/progress.c
> index ab320dc..45a412e 100644
> --- a/repair/progress.c
> +++ b/repair/progress.c
> @@ -124,6 +124,7 @@ init_progress_rpt (void)
> */
>
> pthread_mutex_init(&global_msgs.mutex, NULL);
> + global_msgs.format = NULL;
> global_msgs.count = glob_agcount;
> global_msgs.interval = report_interval;
> global_msgs.done = prog_rpt_done;
> @@ -169,6 +170,10 @@ progress_rpt_thread (void *p)
> msg_block_t *msgp = (msg_block_t *)p;
> __uint64_t percent;
>
> + /* It's possible to get here very early w/ no progress msg set */
> + if (!msgp->format)
> + return NULL;
> +
> if ((msgbuf = (char *)malloc(DURATION_BUF_SIZE)) == NULL)
> do_error (_("progress_rpt: cannot malloc progress msg buffer\n"));

Dammit:

CID 1107596: Data race condition (MISSING_LOCK)

/repair/progress.c: 127 ( missing_lock)
124 */
125
126 pthread_mutex_init(&global_msgs.mutex, NULL);
>>> CID 1107596: Data race condition (MISSING_LOCK)
>>> Accessing "global_msgs.format" without holding lock "msg_block_s.mutex". Elsewhere, "global_msgs.format" is accessed with "msg_block_s.mutex" held 2 out of 2 times.
127 global_msgs.format = NULL;
128 global_msgs.count = glob_agcount;
129 global_msgs.interval = report_interval;
130 global_msgs.done = prog_rpt_done;
131 global_msgs.total = &prog_rpt_total;

Probably best to just drop the new NULL assignment, since it's a global init'd to 0 anyway, to shut up coverity?

-Eric