On 2026-07-16 13:11 +0200, Jan Kara wrote:
> On Thu 16-07-26 13:27:22, Li Chen wrote:
> > PIDFD_THREAD shares O_EXCL. do_dentry_open() clears O_EXCL after
> > pidfs_export_open() validates the flags, so open_by_handle_at()
> > silently turns a thread pidfd into a process pidfd.
> >
> > Restore PIDFD_THREAD on the opened file, matching pidfs_alloc_file(),
> > and cover the restored flag with F_GETFL.
> >
> > Signed-off-by: Li Chen <[email protected]>
>
> Makes sense. Just looking at this code, what about PIDFD_AUTOKILL (aka
> O_TRUNC?)? Reading the commit introducing it 07c3ef58223e it appears it was
> never meant to be directly requested by userspace so probably
> pidfs_export_open() needs to reject it. But that's unrelated issue to your
> patch so feel free to add:
Good point. But pidfs_export_open() is guarded by
pidfs_export_permission() which does:
#define VALID_FILE_HANDLE_OPEN_FLAGS \
(O_RDONLY | O_WRONLY | O_RDWR | O_NONBLOCK | O_CLOEXEC | O_EXCL)
if (oflags & ~(VALID_FILE_HANDLE_OPEN_FLAGS | O_LARGEFILE))
return -EINVAL;
and so is safe. But I've added the following patch on top which also
splats if that is passed!
>From 99a859485394eca8cf6271de09ae72ac3b846bda Mon Sep 17 00:00:00 2001
From: Christian Brauner <[email protected]>
Date: Wed, 22 Jul 2026 13:56:28 +0200
Subject: [PATCH] pidfs: add pidfs_dentry_open() helper
Both pidfs_alloc_file() and pidfs_export_open() need to force O_RDWR
and reapply the pidfd flags that do_dentry_open() strips. Move the
common logic into a helper.
PIDFD_AUTOKILL is now part of the restore mask in the file handle path
as well, but pidfs_export_permission() rejects O_TRUNC, so this is a
no-op there. But warn nonetheless.
Signed-off-by: Christian Brauner (Amutable) <[email protected]>
---
fs/pidfs.c | 42 +++++++++++++++++++++---------------------
1 file changed, 21 insertions(+), 21 deletions(-)
diff --git a/fs/pidfs.c b/fs/pidfs.c
index c20ffd747ff5..695215aa2a58 100644
--- a/fs/pidfs.c
+++ b/fs/pidfs.c
@@ -915,6 +915,20 @@ static struct dentry *pidfs_fh_to_dentry(struct
super_block *sb,
return path.dentry;
}
+static struct file *pidfs_dentry_open(const struct path *path,
+ unsigned int flags,
+ const struct cred *cred)
+{
+ struct file *file;
+
+ /* pidfds are always O_RDWR. */
+ file = dentry_open(path, flags | O_RDWR, cred);
+ /* do_dentry_open() strips O_EXCL and O_TRUNC. */
+ if (!IS_ERR(file))
+ file->f_flags |= flags & (PIDFD_THREAD | PIDFD_AUTOKILL);
+ return file;
+}
+
/*
* Make sure that we reject any nonsensical flags that users pass via
* open_by_handle_at(). Note that PIDFD_THREAD is defined as O_EXCL, and
@@ -939,18 +953,14 @@ static int pidfs_export_permission(struct
handle_to_path_ctx *ctx,
static struct file *pidfs_export_open(const struct path *path, unsigned int
oflags)
{
- struct file *file;
-
/*
- * Clear O_LARGEFILE as open_by_handle_at() forces it and raise
- * O_RDWR as pidfds always are.
+ * Opening via file handle may never raise PIDFD_AUTOKILL. That can
+ * only be done at task creation!
*/
- oflags &= ~O_LARGEFILE;
- file = dentry_open(path, oflags | O_RDWR, current_cred());
- /* do_dentry_open() strips O_EXCL, which encodes PIDFD_THREAD. */
- if (!IS_ERR(file))
- file->f_flags |= oflags & PIDFD_THREAD;
- return file;
+ if (WARN_ON_ONCE(oflags & PIDFD_AUTOKILL))
+ return ERR_PTR(-EINVAL);
+ /* Clear O_LARGEFILE as open_by_handle_at() forces it. */
+ return pidfs_dentry_open(path, oflags & ~O_LARGEFILE, current_cred());
}
static const struct export_operations pidfs_export_operations = {
@@ -1114,7 +1124,6 @@ static struct file_system_type pidfs_type = {
struct file *pidfs_alloc_file(struct pid *pid, unsigned int flags)
{
- struct file *pidfd_file;
struct path path __free(path_put) = {};
int ret;
@@ -1132,16 +1141,7 @@ struct file *pidfs_alloc_file(struct pid *pid, unsigned
int flags)
VFS_WARN_ON_ONCE(!pid->attr);
flags &= ~PIDFD_STALE;
- flags |= O_RDWR;
- pidfd_file = dentry_open(&path, flags, current_cred());
- /*
- * Raise PIDFD_THREAD and PIDFD_AUTOKILL explicitly as
- * do_dentry_open() strips O_EXCL and O_TRUNC.
- */
- if (!IS_ERR(pidfd_file))
- pidfd_file->f_flags |= (flags & (PIDFD_THREAD |
PIDFD_AUTOKILL));
-
- return pidfd_file;
+ return pidfs_dentry_open(&path, flags, current_cred());
}
void __init pidfs_init(void)
--
2.53.0
