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

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.

Jori Koolstra committed 6 months ago

Commit 31cf44e1 parent 3e0be8c4 files · +10 −4Browse files

Unified · Split

4 files

fs/fcntl.c +1 −1
fs/open.c +4 −2
include/linux/fcntl.h +1 −1
include/uapi/asm-generic/fcntl.h +4 −0

fs/fcntl.c+1−1

@@ -1169,7 +1169,7 @@ static int __init fcntl_init(void) · 1168 unmodified lines
1169	1169	* Exceptions: O_NONBLOCK is a two bit define on parisc; O_NDELAY
1170	1170	* is defined as O_NONBLOCK on some platforms and not on others.
1171	1171	*/
1172		− BUILD_BUG_ON(20 - 1 /* for O_RDONLY being 0 */ !=
	1172	+ BUILD_BUG_ON(21 - 1 /* for O_RDONLY being 0 */ !=
1173	1173	HWEIGHT32(
1174	1174	(VALID_OPEN_FLAGS & ~(O_NONBLOCK \| O_NDELAY)) \|
1175	1175	__FMODE_EXEC));

fs/open.c+4−2

@@ -1158,7 +1158,7 @@ struct file kernel_file_open(const struct path path, int flags, · 1157 unmodified lines
1158	1158	EXPORT_SYMBOL_GPL(kernel_file_open);
1159	1159
1160	1160	#define WILL_CREATE(flags) (flags & (O_CREAT \| __O_TMPFILE))
1161		−#define O_PATH_FLAGS (O_DIRECTORY \| O_NOFOLLOW \| O_PATH \| O_CLOEXEC)
	1161	+#define O_PATH_FLAGS (O_DIRECTORY \| O_NOFOLLOW \| O_PATH \| O_CLOEXEC \| O_EMPTYPATH)
1162	1162
1163	1163	inline struct open_how build_open_how(int flags, umode_t mode)
1164	1164	{
@@ -1279,6 +1279,8 @@ inline int build_open_flags(const struct open_how how, struct open_flags op) · 114 unmodified lines
1279	1279	lookup_flags \|= LOOKUP_DIRECTORY;
1280	1280	if (!(flags & O_NOFOLLOW))
1281	1281	lookup_flags \|= LOOKUP_FOLLOW;
	1282	+ if (flags & O_EMPTYPATH)
	1283	+ lookup_flags \|= LOOKUP_EMPTY;
1282	1284
1283	1285	if (how->resolve & RESOLVE_NO_XDEV)
1284	1286	lookup_flags \|= LOOKUP_NO_XDEV;
@@ -1360,7 +1362,7 @@ static int do_sys_openat2(int dfd, const char __user *filename, · 75 unmodified lines
1360	1362	if (unlikely(err))
1361	1363	return err;
1362	1364
1363		− CLASS(filename, name)(filename);
	1365	+ CLASS(filename_flags, name)(filename, op.lookup_flags);
1364	1366	return FD_ADD(how->flags, do_file_open(dfd, name, &op));
1365	1367	}
1366	1368

include/linux/fcntl.h+1−1

@@ -10,7 +10,7 @@ · 9 unmodified lines
10	10	(O_RDONLY \| O_WRONLY \| O_RDWR \| O_CREAT \| O_EXCL \| O_NOCTTY \| O_TRUNC \| \
11	11	O_APPEND \| O_NDELAY \| O_NONBLOCK \| __O_SYNC \| O_DSYNC \| \
12	12	FASYNC \| O_DIRECT \| O_LARGEFILE \| O_DIRECTORY \| O_NOFOLLOW \| \
13		− O_NOATIME \| O_CLOEXEC \| O_PATH \| __O_TMPFILE)
	13	+ O_NOATIME \| O_CLOEXEC \| O_PATH \| __O_TMPFILE \| O_EMPTYPATH)
14	14
15	15	/* List of all valid flags for the how->resolve argument: */
16	16	#define VALID_RESOLVE_FLAGS \

include/uapi/asm-generic/fcntl.h+4−0

@@ -88,6 +88,10 @@ · 87 unmodified lines
88	88	#define __O_TMPFILE 020000000
89	89	#endif
90	90
	91	+#ifndef O_EMPTYPATH
	92	+#define O_EMPTYPATH (1 << 26) /* allow empty path */
	93	+#endif
	94	+
91	95	/* a horrid kludge trying to make sure that this will fail on old kernels */
92	96	#define O_TMPFILE (__O_TMPFILE \| O_DIRECTORY)
93	97