io_uring: grab ->fs as part of async preparation

This passes it in to io-wq, so it assumes the right fs_struct when
executing async work that may need to do lookups.

Cc: stable@vger.kernel.org # 5.3+
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -75,6 +75,7 @@
 #include <linux/fsnotify.h>
 #include <linux/fadvise.h>
 #include <linux/eventpoll.h>
+#include <linux/fs_struct.h>
 
 #define CREATE_TRACE_POINTS
 #include <trace/events/io_uring.h>
@@ -611,6 +612,8 @@ struct io_op_def {
 	unsigned		not_supported : 1;
 	/* needs file table */
 	unsigned		file_table : 1;
+	/* needs ->fs */
+	unsigned		needs_fs : 1;
 };
 
 static const struct io_op_def io_op_defs[] = {
@@ -653,12 +656,14 @@ static const struct io_op_def io_op_defs[] = {
 		.needs_mm		= 1,
 		.needs_file		= 1,
 		.unbound_nonreg_file	= 1,
+		.needs_fs		= 1,
 	},
 	[IORING_OP_RECVMSG] = {
 		.async_ctx		= 1,
 		.needs_mm		= 1,
 		.needs_file		= 1,
 		.unbound_nonreg_file	= 1,
+		.needs_fs		= 1,
 	},
 	[IORING_OP_TIMEOUT] = {
 		.async_ctx		= 1,
@@ -689,6 +694,7 @@ static const struct io_op_def io_op_defs[] = {
 		.needs_file		= 1,
 		.fd_non_neg		= 1,
 		.file_table		= 1,
+		.needs_fs		= 1,
 	},
 	[IORING_OP_CLOSE] = {
 		.needs_file		= 1,
@@ -702,6 +708,7 @@ static const struct io_op_def io_op_defs[] = {
 		.needs_mm		= 1,
 		.needs_file		= 1,
 		.fd_non_neg		= 1,
+		.needs_fs		= 1,
 	},
 	[IORING_OP_READ] = {
 		.needs_mm		= 1,
@@ -733,6 +740,7 @@ static const struct io_op_def io_op_defs[] = {
 		.needs_file		= 1,
 		.fd_non_neg		= 1,
 		.file_table		= 1,
+		.needs_fs		= 1,
 	},
 	[IORING_OP_EPOLL_CTL] = {
 		.unbound_nonreg_file	= 1,
@@ -907,6 +915,16 @@ static inline void io_req_work_grab_env(struct io_kiocb *req,
 	}
 	if (!req->work.creds)
 		req->work.creds = get_current_cred();
+	if (!req->work.fs && def->needs_fs) {
+		spin_lock(&current->fs->lock);
+		if (!current->fs->in_exec) {
+			req->work.fs = current->fs;
+			req->work.fs->users++;
+		} else {
+			req->work.flags |= IO_WQ_WORK_CANCEL;
+		}
+		spin_unlock(&current->fs->lock);
+	}
 }
 
 static inline void io_req_work_drop_env(struct io_kiocb *req)
@@ -919,6 +937,16 @@ static inline void io_req_work_drop_env(struct io_kiocb *req)
 		put_cred(req->work.creds);
 		req->work.creds = NULL;
 	}
+	if (req->work.fs) {
+		struct fs_struct *fs = req->work.fs;
+
+		spin_lock(&req->work.fs->lock);
+		if (--fs->users)
+			fs = NULL;
+		spin_unlock(&req->work.fs->lock);
+		if (fs)
+			free_fs_struct(fs);
+	}
 }
 
 static inline bool io_prep_async_work(struct io_kiocb *req,