Re: openat ignores changes to RLIMIT_NOFILE?

[Jens Axboe <axboe@kernel.dk>]

On 3/19/20 6:12 AM, Dmitry Kadashev wrote:
> Hi,
> 
> It seems that openat calls issued via io_uring ignore changes to
> RLIMIT_NOFILE. Maybe a wrong limit is checked. A short reproducer is
> attached, it sets RLIMIT_NOFILE to a very low value and the sync
> openat() call fails with "Too many open files", but io_uring one
> succeeds. The resulting FD is completely usable, I've tried writing to
> it successfully.
> 
> To be clear, originally I've encountered another side of this problem:
> we increase the limit in our code, and io_uring's openat started to
> fail after a while under load, while the sync calls executed on a
> thread pool were working as expected. It's just easier to demo with
> small limit.
> 
> Kernel 5.6-rc2, 5.6-rc6.
> 
> Hope it's the right place to report an issue like this.

Can you try the below patch?


diff --git a/fs/file.c b/fs/file.c
index a364e1a9b7e8..c8a4e4c86e55 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -540,9 +540,14 @@ static int alloc_fd(unsigned start, unsigned flags)
 	return __alloc_fd(current->files, start, rlimit(RLIMIT_NOFILE), flags);
 }
 
+int __get_unused_fd_flags(unsigned flags, unsigned long nofile)
+{
+	return __alloc_fd(current->files, 0, nofile, flags);
+}
+
 int get_unused_fd_flags(unsigned flags)
 {
-	return __alloc_fd(current->files, 0, rlimit(RLIMIT_NOFILE), flags);
+	return __get_unused_fd_flags(flags, rlimit(RLIMIT_NOFILE));
 }
 EXPORT_SYMBOL(get_unused_fd_flags);
 
diff --git a/fs/io_uring.c b/fs/io_uring.c
index c06082bb039a..be5705ff33b4 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -398,6 +398,7 @@ struct io_open {
 	struct filename			*filename;
 	struct statx __user		*buffer;
 	struct open_how			how;
+	unsigned long			nofile;
 };
 
 struct io_files_update {
@@ -2578,6 +2579,7 @@ static int io_openat_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
 		return ret;
 	}
 
+	req->open.nofile = rlimit(RLIMIT_NOFILE);
 	req->flags |= REQ_F_NEED_CLEANUP;
 	return 0;
 }
@@ -2619,6 +2621,7 @@ static int io_openat2_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
 		return ret;
 	}
 
+	req->open.nofile = rlimit(RLIMIT_NOFILE);
 	req->flags |= REQ_F_NEED_CLEANUP;
 	return 0;
 }
@@ -2637,7 +2640,7 @@ static int io_openat2(struct io_kiocb *req, struct io_kiocb **nxt,
 	if (ret)
 		goto err;
 
-	ret = get_unused_fd_flags(req->open.how.flags);
+	ret = __get_unused_fd_flags(req->open.how.flags, req->open.nofile);
 	if (ret < 0)
 		goto err;
 
diff --git a/include/linux/file.h b/include/linux/file.h
index c6c7b24ea9f7..142d102f285e 100644
--- a/include/linux/file.h
+++ b/include/linux/file.h
@@ -85,6 +85,7 @@ extern int f_dupfd(unsigned int from, struct file *file, unsigned flags);
 extern int replace_fd(unsigned fd, struct file *file, unsigned flags);
 extern void set_close_on_exec(unsigned int fd, int flag);
 extern bool get_close_on_exec(unsigned int fd);
+extern int __get_unused_fd_flags(unsigned flags, unsigned long nofile);
 extern int get_unused_fd_flags(unsigned flags);
 extern void put_unused_fd(unsigned int fd);
 
-- 
Jens Axboe