blktrace: ensure our debugfs dir exists

[ Upstream commit b431ef837e3374da0db8ff6683170359aaa0859c ]

We make an assumption that a debugfs directory exists, but since
this can fail ensure it exists before allowing blktrace setup to
complete. Otherwise we end up stuffing blktrace files on the debugfs
root directory. In the worst case scenario this *in theory* can create
an eventual panic *iff* in the future a similarly named file is created
prior on the debugfs root directory. This theoretical crash can happen
due to a recursive removal followed by a specific dentry removal.

This doesn't fix any known crash, however I have seen the files
go into the main debugfs root directory in cases where the debugfs
directory was not created due to other internal bugs with blktrace
now fixed.

blktrace is also completely useless without this directory, so
this ensures to userspace we only setup blktrace if the kernel
can stuff files where they are supposed to go into.

debugfs directory creations typically aren't checked for, and we have
maintainers doing sweep removals of these checks, but since we need this
check to ensure proper userspace blktrace functionality we make sure
to annotate the justification for the check.

Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Sasha Levin <sashal@kernel.org>

diff --git a/kernel/trace/blktrace.c b/kernel/trace/blktrace.c
index 588e8e3960197e7c4ca34055271895ddd77e7e07..1bd6563939e596f1a07ec771b2f824b0e9b68374 100644 (file)
--- a/kernel/trace/blktrace.c
+++ b/kernel/trace/blktrace.c
@@ -536,6 +536,18 @@ static int do_blk_trace_setup(struct request_queue *q, char *name, dev_t dev,
 #endif
                bt->dir = dir = debugfs_create_dir(buts->name, blk_debugfs_root);
 
+       /*
+        * As blktrace relies on debugfs for its interface the debugfs directory
+        * is required, contrary to the usual mantra of not checking for debugfs
+        * files or directories.
+        */
+       if (IS_ERR_OR_NULL(dir)) {
+               pr_warn("debugfs_dir not present for %s so skipping\n",
+                       buts->name);
+               ret = -ENOENT;
+               goto err;
+       }
+
        bt->dev = dev;
        atomic_set(&bt->dropped, 0);
        INIT_LIST_HEAD(&bt->running_list);