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vfs: add O_EMPTYPATH to openat(2)/openat2(2)

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To get an operable version of an O_PATH file descriptor, it is possible
to use openat(fd, ".", O_DIRECTORY) for directories, but other files
currently require going through open("/proc/<pid>/fd/<nr>"), which
depends on a functioning procfs.

This patch adds the O_EMPTYPATH flag to openat(2)/openat2(2). If passed,
LOOKUP_EMPTY is set at path resolution time.

Note: This implies that you cannot rely anymore on disabling procfs from
being mounted (e.g. inside a container without procfs mounted and with
CAP_SYS_ADMIN dropped) to prevent O_PATH fds from being re-opened
read-write.

Signed-off-by: Jori Koolstra <jkoolstra@xs4all.nl>
Link: https://patch.msgid.link/20260424114611.1678641-2-jkoolstra@xs4all.nl
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
This commit is contained in:
Jori Koolstra
2026-04-24 13:46:02 +02:00
committed by Christian Brauner
parent 3e0be8ccff
commit 31cf44efa6
4 changed files with 10 additions and 4 deletions
Download Patch File Download Diff File Expand all files Collapse all files

2
fs/fcntl.c
View File

@@ -1169,7 +1169,7 @@ static int __init fcntl_init(void)
* Exceptions: O_NONBLOCK is a two bit define on parisc; O_NDELAY
* is defined as O_NONBLOCK on some platforms and not on others.
*/
BUILD_BUG_ON(20 - 1 /* for O_RDONLY being 0 */ !=
BUILD_BUG_ON(21 - 1 /* for O_RDONLY being 0 */ !=
HWEIGHT32(
(VALID_OPEN_FLAGS & ~(O_NONBLOCK | O_NDELAY)) |
__FMODE_EXEC));

6
fs/open.c
View File

@@ -1158,7 +1158,7 @@ struct file *kernel_file_open(const struct path *path, int flags,
EXPORT_SYMBOL_GPL(kernel_file_open);
#define WILL_CREATE(flags) (flags & (O_CREAT | __O_TMPFILE))
#define O_PATH_FLAGS (O_DIRECTORY | O_NOFOLLOW | O_PATH | O_CLOEXEC)
#define O_PATH_FLAGS (O_DIRECTORY | O_NOFOLLOW | O_PATH | O_CLOEXEC | O_EMPTYPATH)
inline struct open_how build_open_how(int flags, umode_t mode)
{
@@ -1279,6 +1279,8 @@ inline int build_open_flags(const struct open_how *how, struct open_flags *op)
lookup_flags |= LOOKUP_DIRECTORY;
if (!(flags & O_NOFOLLOW))
lookup_flags |= LOOKUP_FOLLOW;
if (flags & O_EMPTYPATH)
lookup_flags |= LOOKUP_EMPTY;
if (how->resolve & RESOLVE_NO_XDEV)
lookup_flags |= LOOKUP_NO_XDEV;
@@ -1360,7 +1362,7 @@ static int do_sys_openat2(int dfd, const char __user *filename,
if (unlikely(err))
return err;
CLASS(filename, name)(filename);
CLASS(filename_flags, name)(filename, op.lookup_flags);
return FD_ADD(how->flags, do_file_open(dfd, name, &op));
}

2
include/linux/fcntl.h
View File

@@ -10,7 +10,7 @@
(O_RDONLY | O_WRONLY | O_RDWR | O_CREAT | O_EXCL | O_NOCTTY | O_TRUNC | \
O_APPEND | O_NDELAY | O_NONBLOCK | __O_SYNC | O_DSYNC | \
FASYNC | O_DIRECT | O_LARGEFILE | O_DIRECTORY | O_NOFOLLOW | \
O_NOATIME | O_CLOEXEC | O_PATH | __O_TMPFILE)
O_NOATIME | O_CLOEXEC | O_PATH | __O_TMPFILE | O_EMPTYPATH)
/* List of all valid flags for the how->resolve argument: */
#define VALID_RESOLVE_FLAGS \

4
include/uapi/asm-generic/fcntl.h
View File

@@ -88,6 +88,10 @@
#define __O_TMPFILE 020000000
#endif
#ifndef O_EMPTYPATH
#define O_EMPTYPATH (1 << 26) /* allow empty path */
#endif
/* a horrid kludge trying to make sure that this will fail on old kernels */
#define O_TMPFILE (__O_TMPFILE | O_DIRECTORY)