diff mbox series

[v2,1/4] pidfd: support PIDFD_NONBLOCK in pidfd_open()

Message ID 20200902102130.147672-2-christian.brauner@ubuntu.com
State Accepted
Commit 6da73d15258a1e5e86d03d4ffba8776d17a8a287
Headers show
Series [v2,1/4] pidfd: support PIDFD_NONBLOCK in pidfd_open() | expand

Commit Message

Christian Brauner Sept. 2, 2020, 10:21 a.m. UTC 
Introduce PIDFD_NONBLOCK to support non-blocking pidfd file descriptors.

Ever since the introduction of pidfds and more advanced async io various
programming languages such as Rust have grown support for async event
libraries. These libraries are created to help build epoll-based event loops
around file descriptors. A common pattern is to automatically make all file
descriptors they manage to O_NONBLOCK.

For such libraries the EAGAIN error code is treated specially. When a function
is called that returns EAGAIN the function isn't called again until the event
loop indicates the the file descriptor is ready. Supporting EAGAIN when
waiting on pidfds makes such libraries just work with little effort. In the
following patch we will extend waitid() internally to support non-blocking
pidfds.

This introduces a new flag PIDFD_NONBLOCK that is equivalent to O_NONBLOCK.
This follows the same patterns we have for other (anon inode) file descriptors
such as EFD_NONBLOCK, IN_NONBLOCK, SFD_NONBLOCK, TFD_NONBLOCK and the same for
close-on-exec flags.

Link: https://lore.kernel.org/lkml/20200811181236.GA18763@localhost/
Link: https://github.com/joshtriplett/async-pidfd
Cc: Kees Cook <keescook@chromium.org>
Cc: Sargun Dhillon <sargun@sargun.me>
Cc: Oleg Nesterov <oleg@redhat.com>
Suggested-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
---
/* v2 */
- Christian Brauner <christian.brauner@ubuntu.com>:
  - Improve commit message.
---
 include/uapi/linux/pidfd.h | 12 ++++++++++++
 kernel/pid.c               | 12 +++++++-----
 2 files changed, 19 insertions(+), 5 deletions(-)
 create mode 100644 include/uapi/linux/pidfd.h

Comments

Oleg Nesterov Sept. 3, 2020, 2:31 p.m. UTC | #1 
On 09/02, Christian Brauner wrote:
>
> -static int pidfd_create(struct pid *pid)
> +static int pidfd_create(struct pid *pid, unsigned int flags)
>  {
>  	int fd;
>  
>  	fd = anon_inode_getfd("[pidfd]", &pidfd_fops, get_pid(pid),
> -			      O_RDWR | O_CLOEXEC);
> +			      flags | O_RDWR | O_CLOEXEC);
>  	if (fd < 0)
>  		put_pid(pid);
>  
> @@ -565,7 +567,7 @@ SYSCALL_DEFINE2(pidfd_open, pid_t, pid, unsigned int, flags)
>  	int fd;
>  	struct pid *p;
>  
> -	if (flags)
> +	if (flags & ~PIDFD_NONBLOCK)
>  		return -EINVAL;
>  
>  	if (pid <= 0)
> @@ -576,7 +578,7 @@ SYSCALL_DEFINE2(pidfd_open, pid_t, pid, unsigned int, flags)
>  		return -ESRCH;
>  
>  	if (pid_has_task(p, PIDTYPE_TGID))
> -		fd = pidfd_create(p);
> +		fd = pidfd_create(p, flags);
>  	else
>  		fd = -EINVAL;
>  

Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Josh Triplett Sept. 3, 2020, 11:50 p.m. UTC | #2 
On Wed, Sep 02, 2020 at 12:21:27PM +0200, Christian Brauner wrote:
> Introduce PIDFD_NONBLOCK to support non-blocking pidfd file descriptors.
> 
> Ever since the introduction of pidfds and more advanced async io various
> programming languages such as Rust have grown support for async event
> libraries. These libraries are created to help build epoll-based event loops
> around file descriptors. A common pattern is to automatically make all file
> descriptors they manage to O_NONBLOCK.
> 
> For such libraries the EAGAIN error code is treated specially. When a function
> is called that returns EAGAIN the function isn't called again until the event
> loop indicates the the file descriptor is ready. Supporting EAGAIN when
> waiting on pidfds makes such libraries just work with little effort. In the
> following patch we will extend waitid() internally to support non-blocking
> pidfds.
> 
> This introduces a new flag PIDFD_NONBLOCK that is equivalent to O_NONBLOCK.
> This follows the same patterns we have for other (anon inode) file descriptors
> such as EFD_NONBLOCK, IN_NONBLOCK, SFD_NONBLOCK, TFD_NONBLOCK and the same for
> close-on-exec flags.
> 
> Link: https://lore.kernel.org/lkml/20200811181236.GA18763@localhost/
> Link: https://github.com/joshtriplett/async-pidfd
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Sargun Dhillon <sargun@sargun.me>
> Cc: Oleg Nesterov <oleg@redhat.com>
> Suggested-by: Josh Triplett <josh@joshtriplett.org>
> Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

Reviewed-by: Josh Triplett <josh@joshtriplett.org>
diff mbox series

Patch

diff --git a/include/uapi/linux/pidfd.h b/include/uapi/linux/pidfd.h
new file mode 100644
index 000000000000..5406fbc13074
--- /dev/null
+++ b/include/uapi/linux/pidfd.h
@@ -0,0 +1,12 @@ 
+/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
+
+#ifndef _UAPI_LINUX_PIDFD_H
+#define _UAPI_LINUX_PIDFD_H
+
+#include <linux/types.h>
+#include <linux/fcntl.h>
+
+/* Flags for pidfd_open().  */
+#define PIDFD_NONBLOCK O_NONBLOCK
+
+#endif /* _UAPI_LINUX_PIDFD_H */
diff --git a/kernel/pid.c b/kernel/pid.c
index b2562a7ce525..74ddbff1a6ba 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -43,6 +43,7 @@ 
 #include <linux/sched/task.h>
 #include <linux/idr.h>
 #include <net/sock.h>
+#include <uapi/linux/pidfd.h>
 
 struct pid init_struct_pid = {
 	.count		= REFCOUNT_INIT(1),
@@ -522,7 +523,8 @@  struct pid *find_ge_pid(int nr, struct pid_namespace *ns)
 /**
  * pidfd_create() - Create a new pid file descriptor.
  *
- * @pid:  struct pid that the pidfd will reference
+ * @pid:   struct pid that the pidfd will reference
+ * @flags: flags to pass
  *
  * This creates a new pid file descriptor with the O_CLOEXEC flag set.
  *
@@ -532,12 +534,12 @@  struct pid *find_ge_pid(int nr, struct pid_namespace *ns)
  * Return: On success, a cloexec pidfd is returned.
  *         On error, a negative errno number will be returned.
  */
-static int pidfd_create(struct pid *pid)
+static int pidfd_create(struct pid *pid, unsigned int flags)
 {
 	int fd;
 
 	fd = anon_inode_getfd("[pidfd]", &pidfd_fops, get_pid(pid),
-			      O_RDWR | O_CLOEXEC);
+			      flags | O_RDWR | O_CLOEXEC);
 	if (fd < 0)
 		put_pid(pid);
 
@@ -565,7 +567,7 @@  SYSCALL_DEFINE2(pidfd_open, pid_t, pid, unsigned int, flags)
 	int fd;
 	struct pid *p;
 
-	if (flags)
+	if (flags & ~PIDFD_NONBLOCK)
 		return -EINVAL;
 
 	if (pid <= 0)
@@ -576,7 +578,7 @@  SYSCALL_DEFINE2(pidfd_open, pid_t, pid, unsigned int, flags)
 		return -ESRCH;
 
 	if (pid_has_task(p, PIDTYPE_TGID))
-		fd = pidfd_create(p);
+		fd = pidfd_create(p, flags);
 	else
 		fd = -EINVAL;