io_uring/fdinfo: grab ctx->uring_lock around io_uring_show_fdinfo()

Not everything requires locking in there, which is why the 'has_lock'
variable exists. But enough does that it's a bit unwieldy to manage.
Wrap the whole thing in a ->uring_lock trylock, and just return
with no output if we fail to grab it. The existing trylock() will
already have greatly diminished utility/output for the failure case.

This fixes an issue with reading the SQE fields, if the ring is being
actively resized at the same time.

Reported-by: Jann Horn <jannh@google.com>
Fixes: 79cfe9e ("io_uring/register: add IORING_REGISTER_RESIZE_RINGS")
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: axboe

io_uring/fdinfo.c

@@ -86,13 +86,8 @@ static inline void napi_show_fdinfo(struct io_ring_ctx *ctx,
 }
 #endif
 
-/*
- * Caller holds a reference to the file already, we don't need to do
- * anything else to get an extra reference.
- */
-__cold void io_uring_show_fdinfo(struct seq_file *m, struct file *file)
+static void __io_uring_show_fdinfo(struct io_ring_ctx *ctx, struct seq_file *m)
 {
-	struct io_ring_ctx *ctx = file->private_data;
 	struct io_overflow_cqe *ocqe;
 	struct io_rings *r = ctx->rings;
 	struct rusage sq_usage;
@@ -106,7 +101,6 @@ __cold void io_uring_show_fdinfo(struct seq_file *m, struct file *file)
 	unsigned int sq_entries, cq_entries;
 	int sq_pid = -1, sq_cpu = -1;
 	u64 sq_total_time = 0, sq_work_time = 0;
-	bool has_lock;
 	unsigned int i;
 
 	if (ctx->flags & IORING_SETUP_CQE32)
@@ -176,15 +170,7 @@ __cold void io_uring_show_fdinfo(struct seq_file *m, struct file *file)
 		seq_printf(m, "\n");
 	}
 
-	/*
-	 * Avoid ABBA deadlock between the seq lock and the io_uring mutex,
-	 * since fdinfo case grabs it in the opposite direction of normal use
-	 * cases. If we fail to get the lock, we just don't iterate any
-	 * structures that could be going away outside the io_uring mutex.
-	 */
-	has_lock = mutex_trylock(&ctx->uring_lock);
-
-	if (has_lock && (ctx->flags & IORING_SETUP_SQPOLL)) {
+	if (ctx->flags & IORING_SETUP_SQPOLL) {
 		struct io_sq_data *sq = ctx->sq_data;
 
 		/*
@@ -206,7 +192,7 @@ __cold void io_uring_show_fdinfo(struct seq_file *m, struct file *file)
 	seq_printf(m, "SqTotalTime:\t%llu\n", sq_total_time);
 	seq_printf(m, "SqWorkTime:\t%llu\n", sq_work_time);
 	seq_printf(m, "UserFiles:\t%u\n", ctx->file_table.data.nr);
-	for (i = 0; has_lock && i < ctx->file_table.data.nr; i++) {
+	for (i = 0; i < ctx->file_table.data.nr; i++) {
 		struct file *f = NULL;
 
 		if (ctx->file_table.data.nodes[i])
@@ -218,7 +204,7 @@ __cold void io_uring_show_fdinfo(struct seq_file *m, struct file *file)
 		}
 	}
 	seq_printf(m, "UserBufs:\t%u\n", ctx->buf_table.nr);
-	for (i = 0; has_lock && i < ctx->buf_table.nr; i++) {
+	for (i = 0; i < ctx->buf_table.nr; i++) {
 		struct io_mapped_ubuf *buf = NULL;
 
 		if (ctx->buf_table.nodes[i])
@@ -228,7 +214,7 @@ __cold void io_uring_show_fdinfo(struct seq_file *m, struct file *file)
 		else
 			seq_printf(m, "%5u: <none>\n", i);
 	}
-	if (has_lock && !xa_empty(&ctx->personalities)) {
+	if (!xa_empty(&ctx->personalities)) {
 		unsigned long index;
 		const struct cred *cred;
 
@@ -238,7 +224,7 @@ __cold void io_uring_show_fdinfo(struct seq_file *m, struct file *file)
 	}
 
 	seq_puts(m, "PollList:\n");
-	for (i = 0; has_lock && i < (1U << ctx->cancel_table.hash_bits); i++) {
+	for (i = 0; i < (1U << ctx->cancel_table.hash_bits); i++) {
 		struct io_hash_bucket *hb = &ctx->cancel_table.hbs[i];
 		struct io_kiocb *req;
 
@@ -247,9 +233,6 @@ __cold void io_uring_show_fdinfo(struct seq_file *m, struct file *file)
 					task_work_pending(req->tctx->task));
 	}
 
-	if (has_lock)
-		mutex_unlock(&ctx->uring_lock);
-
 	seq_puts(m, "CqOverflowList:\n");
 	spin_lock(&ctx->completion_lock);
 	list_for_each_entry(ocqe, &ctx->cq_overflow_list, list) {
@@ -262,4 +245,23 @@ __cold void io_uring_show_fdinfo(struct seq_file *m, struct file *file)
 	spin_unlock(&ctx->completion_lock);
 	napi_show_fdinfo(ctx, m);
 }
+
+/*
+ * Caller holds a reference to the file already, we don't need to do
+ * anything else to get an extra reference.
+ */
+__cold void io_uring_show_fdinfo(struct seq_file *m, struct file *file)
+{
+	struct io_ring_ctx *ctx = file->private_data;
+
+	/*
+	 * Avoid ABBA deadlock between the seq lock and the io_uring mutex,
+	 * since fdinfo case grabs it in the opposite direction of normal use
+	 * cases.
+	 */
+	if (mutex_trylock(&ctx->uring_lock)) {
+		__io_uring_show_fdinfo(ctx, m);
+		mutex_unlock(&ctx->uring_lock);
+	}
+}
 #endif