Re: [syzbot] Monthly trace report (Oct 2024)

[Jens Axboe <axboe@kernel.dk>]

On 10/11/24 10:00 AM, Steven Rostedt wrote:
> On Fri, 11 Oct 2024 08:25:29 -0700
> syzbot <syzbot+list3bf21e6ac0139f8d008d@syzkaller.appspotmail.com> wrote:
> 
>> Hello trace maintainers/developers,
>>
>> This is a 31-day syzbot report for the trace subsystem.
>> All related reports/information can be found at:
>> https://syzkaller.appspot.com/upstream/s/trace
>>
>> During the period, 1 new issues were detected and 0 were fixed.
>> In total, 10 issues are still open and 38 have been fixed so far.
>>
>> Some of the still happening issues:
>>
>> Ref Crashes Repro Title
>> <1> 34      Yes   INFO: task hung in blk_trace_ioctl (4)
>>                   https://syzkaller.appspot.com/bug?extid=ed812ed461471ab17a0c
> 
> If you check the maintainers file, blktrace.c has:
> 
> BLOCK LAYER
> M:      Jens Axboe <axboe@kernel.dk>
> L:      linux-block@vger.kernel.org

If syzbot can test this one, that would be appreciated. Guess here is
that we're stuck faulting under the debugfs_mutex. I took a look at the
syzbot reproducer, and no not going to attempt to run that... I strongly
suspect that the interesting bits there are:

1) Memory pressure/swap
2) blktrace setup/teardown, obviously

I do wish that once syzbot had a reproducer, it would continue
condensing it down into the most basic reproducer. Once you get into
"let's setup wifi, bluetooth, and tons of other things!" they become
almost impossible to run. And like in this case, I highly doubt they are
either related or useful.


diff --git a/kernel/trace/blktrace.c b/kernel/trace/blktrace.c
index 8fd292d34d89..cefcad120495 100644
--- a/kernel/trace/blktrace.c
+++ b/kernel/trace/blktrace.c
@@ -617,8 +617,8 @@ static int do_blk_trace_setup(struct request_queue *q, char *name, dev_t dev,
 	return ret;
 }
 
-static int __blk_trace_setup(struct request_queue *q, char *name, dev_t dev,
-			     struct block_device *bdev, char __user *arg)
+int blk_trace_setup(struct request_queue *q, char *name, dev_t dev,
+		    struct block_device *bdev, char __user *arg)
 {
 	struct blk_user_trace_setup buts;
 	int ret;
@@ -627,28 +627,18 @@ static int __blk_trace_setup(struct request_queue *q, char *name, dev_t dev,
 	if (ret)
 		return -EFAULT;
 
+	mutex_lock(&q->debugfs_mutex);
 	ret = do_blk_trace_setup(q, name, dev, bdev, &buts);
+	mutex_unlock(&q->debugfs_mutex);
 	if (ret)
 		return ret;
 
 	if (copy_to_user(arg, &buts, sizeof(buts))) {
-		__blk_trace_remove(q);
+		blk_trace_remove(q);
 		return -EFAULT;
 	}
-	return 0;
-}
 
-int blk_trace_setup(struct request_queue *q, char *name, dev_t dev,
-		    struct block_device *bdev,
-		    char __user *arg)
-{
-	int ret;
-
-	mutex_lock(&q->debugfs_mutex);
-	ret = __blk_trace_setup(q, name, dev, bdev, arg);
-	mutex_unlock(&q->debugfs_mutex);
-
-	return ret;
+	return 0;
 }
 EXPORT_SYMBOL_GPL(blk_trace_setup);
 
@@ -673,12 +663,14 @@ static int compat_blk_trace_setup(struct request_queue *q, char *name,
 		.pid = cbuts.pid,
 	};
 
+	mutex_lock(&q->debugfs_mutex);
 	ret = do_blk_trace_setup(q, name, dev, bdev, &buts);
+	mutex_unlock(&q->debugfs_mutex);
 	if (ret)
 		return ret;
 
 	if (copy_to_user(arg, &buts.name, ARRAY_SIZE(buts.name))) {
-		__blk_trace_remove(q);
+		blk_trace_remove(q);
 		return -EFAULT;
 	}
 
@@ -732,12 +724,10 @@ int blk_trace_ioctl(struct block_device *bdev, unsigned cmd, char __user *arg)
 	int ret, start = 0;
 	char b[BDEVNAME_SIZE];
 
-	mutex_lock(&q->debugfs_mutex);
-
 	switch (cmd) {
 	case BLKTRACESETUP:
 		snprintf(b, sizeof(b), "%pg", bdev);
-		ret = __blk_trace_setup(q, b, bdev->bd_dev, bdev, arg);
+		ret = blk_trace_setup(q, b, bdev->bd_dev, bdev, arg);
 		break;
 #if defined(CONFIG_COMPAT) && defined(CONFIG_X86_64)
 	case BLKTRACESETUP32:
@@ -749,17 +739,16 @@ int blk_trace_ioctl(struct block_device *bdev, unsigned cmd, char __user *arg)
 		start = 1;
 		fallthrough;
 	case BLKTRACESTOP:
-		ret = __blk_trace_startstop(q, start);
+		ret = blk_trace_startstop(q, start);
 		break;
 	case BLKTRACETEARDOWN:
-		ret = __blk_trace_remove(q);
+		ret = blk_trace_remove(q);
 		break;
 	default:
 		ret = -ENOTTY;
 		break;
 	}
 
-	mutex_unlock(&q->debugfs_mutex);
 	return ret;
 }
 

-- 
Jens Axboe