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Subject[PATCH v3] Add new open(2) flag - O_EMPTY_PATH
Date
This patch adds a new flag O_EMPTY_PATH that allows openat and open
system calls to open a file referenced by fd if the path is empty,
and it is very similar to the FreeBSD O_EMPTY_PATH flag. This can be
beneficial in some cases since it would avoid having to grant /proc
access to things like samba containers for reopening files to change
flags in a race-free way.

Signed-off-by: Ameer Hamza <ahamza@ixsystems.com>

---
Change in v3:
resolve O_EMPTY_PATH conflict with __FMODE_NONOTIFY for sparc.

Change in v2:
add nonconflicting values for O_EMPTY_PATH on architectures
where default conflicts with existing flags.
---
---
arch/alpha/include/uapi/asm/fcntl.h | 1 +
arch/parisc/include/uapi/asm/fcntl.h | 1 +
arch/sparc/include/uapi/asm/fcntl.h | 1 +
fs/fcntl.c | 2 +-
fs/namei.c | 4 ++--
fs/open.c | 2 +-
include/linux/fcntl.h | 2 +-
include/uapi/asm-generic/fcntl.h | 4 ++++
tools/include/uapi/asm-generic/fcntl.h | 4 ++++
9 files changed, 16 insertions(+), 5 deletions(-)

diff --git a/arch/alpha/include/uapi/asm/fcntl.h b/arch/alpha/include/uapi/asm/fcntl.h
index 50bdc8e8a271..ea08341bb9fe 100644
--- a/arch/alpha/include/uapi/asm/fcntl.h
+++ b/arch/alpha/include/uapi/asm/fcntl.h
@@ -34,6 +34,7 @@

#define O_PATH 040000000
#define __O_TMPFILE 0100000000
+#define O_EMPTY_PATH 0200000000

#define F_GETLK 7
#define F_SETLK 8
diff --git a/arch/parisc/include/uapi/asm/fcntl.h b/arch/parisc/include/uapi/asm/fcntl.h
index 03dee816cb13..e6144823ee5b 100644
--- a/arch/parisc/include/uapi/asm/fcntl.h
+++ b/arch/parisc/include/uapi/asm/fcntl.h
@@ -19,6 +19,7 @@

#define O_PATH 020000000
#define __O_TMPFILE 040000000
+#define O_EMPTY_PATH 0100000000

#define F_GETLK64 8
#define F_SETLK64 9
diff --git a/arch/sparc/include/uapi/asm/fcntl.h b/arch/sparc/include/uapi/asm/fcntl.h
index 67dae75e5274..ed99e4e4a717 100644
--- a/arch/sparc/include/uapi/asm/fcntl.h
+++ b/arch/sparc/include/uapi/asm/fcntl.h
@@ -37,6 +37,7 @@

#define O_PATH 0x1000000
#define __O_TMPFILE 0x2000000
+#define O_EMPTY_PATH 0x8000000

#define F_GETOWN 5 /* for sockets. */
#define F_SETOWN 6 /* for sockets. */
diff --git a/fs/fcntl.c b/fs/fcntl.c
index 146c9ab0cd4b..7aac650e16e2 100644
--- a/fs/fcntl.c
+++ b/fs/fcntl.c
@@ -1027,7 +1027,7 @@ 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(21 - 1 /* for O_RDONLY being 0 */ !=
+ BUILD_BUG_ON(22 - 1 /* for O_RDONLY being 0 */ !=
HWEIGHT32(
(VALID_OPEN_FLAGS & ~(O_NONBLOCK | O_NDELAY)) |
__FMODE_EXEC | __FMODE_NONOTIFY));
diff --git a/fs/namei.c b/fs/namei.c
index 309ae6fc8c99..2b2735af6d03 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -192,7 +192,7 @@ getname_flags(const char __user *filename, int flags, int *empty)
if (unlikely(!len)) {
if (empty)
*empty = 1;
- if (!(flags & LOOKUP_EMPTY)) {
+ if (!(flags & (LOOKUP_EMPTY | O_EMPTY_PATH))) {
putname(result);
return ERR_PTR(-ENOENT);
}
@@ -2347,7 +2347,7 @@ static const char *path_init(struct nameidata *nd, unsigned flags)
if ((flags & (LOOKUP_RCU | LOOKUP_CACHED)) == LOOKUP_CACHED)
return ERR_PTR(-EAGAIN);

- if (!*s)
+ if (!*s && unlikely(!(flags & O_EMPTY_PATH)))
flags &= ~LOOKUP_RCU;
if (flags & LOOKUP_RCU)
rcu_read_lock();
diff --git a/fs/open.c b/fs/open.c
index 82c1a28b3308..b4ec054a418f 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -1301,7 +1301,7 @@ static long do_sys_openat2(int dfd, const char __user *filename,
if (fd)
return fd;

- tmp = getname(filename);
+ tmp = getname_flags(filename, how->flags & O_EMPTY_PATH, NULL);
if (IS_ERR(tmp))
return PTR_ERR(tmp);

diff --git a/include/linux/fcntl.h b/include/linux/fcntl.h
index a332e79b3207..bf8467bb0bd2 100644
--- a/include/linux/fcntl.h
+++ b/include/linux/fcntl.h
@@ -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_EMPTY_PATH)

/* List of all valid flags for the how->resolve argument: */
#define VALID_RESOLVE_FLAGS \
diff --git a/include/uapi/asm-generic/fcntl.h b/include/uapi/asm-generic/fcntl.h
index 1ecdb911add8..a03f4275517b 100644
--- a/include/uapi/asm-generic/fcntl.h
+++ b/include/uapi/asm-generic/fcntl.h
@@ -89,6 +89,10 @@
#define __O_TMPFILE 020000000
#endif

+#ifndef O_EMPTY_PATH
+#define O_EMPTY_PATH 040000000
+#endif
+
/* a horrid kludge trying to make sure that this will fail on old kernels */
#define O_TMPFILE (__O_TMPFILE | O_DIRECTORY)
#define O_TMPFILE_MASK (__O_TMPFILE | O_DIRECTORY | O_CREAT)
diff --git a/tools/include/uapi/asm-generic/fcntl.h b/tools/include/uapi/asm-generic/fcntl.h
index b02c8e0f4057..f32a81604296 100644
--- a/tools/include/uapi/asm-generic/fcntl.h
+++ b/tools/include/uapi/asm-generic/fcntl.h
@@ -89,6 +89,10 @@
#define __O_TMPFILE 020000000
#endif

+#ifndef O_EMPTY_PATH
+#define O_EMPTY_PATH 040000000
+#endif
+
/* a horrid kludge trying to make sure that this will fail on old kernels */
#define O_TMPFILE (__O_TMPFILE | O_DIRECTORY)
#define O_TMPFILE_MASK (__O_TMPFILE | O_DIRECTORY | O_CREAT)
--
2.25.1
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