[PATCH v12 07/12] open: O_EMPTYPATH: procfs-less file descriptor re-opening

From: Aleksa Sarai
Date: Wed Sep 04 2019 - 16:22:21 EST

Userspace has made use of /proc/self/fd very liberally to allow for
descriptors to be re-opened. There are a wide variety of uses for this
feature, but it has always required constructing a pathname and could
not be done without procfs mounted. The obvious solution for this is to
extend openat(2) to have an AT_EMPTY_PATH-equivalent -- O_EMPTYPATH.

Now that descriptor re-opening has been made safe through the new
magic-link resolution restrictions, we can replicate these restrictions
for O_EMPTYPATH. In particular, we only allow "upgrading" the file
descriptor if the corresponding FMODE_PATH_* bit is set (or the
FMODE_{READ,WRITE} cases for non-O_PATH file descriptors).

When doing openat(O_EMPTYPATH|O_PATH), O_PATH takes precedence and
O_EMPTYPATH is ignored. Very few users ever have a need to O_PATH
re-open an existing file descriptor, and so accommodating them at the
expense of further complicating O_PATH makes little sense. Ultimately,
if users ask for this we can always add RESOLVE_EMPTY_PATH to
resolveat(2) in the future.

Signed-off-by: Aleksa Sarai <cyphar@xxxxxxxxxx>
---
arch/alpha/include/uapi/asm/fcntl.h | 1 +
arch/parisc/include/uapi/asm/fcntl.h | 39 ++++++++++++++--------------
arch/sparc/include/uapi/asm/fcntl.h | 1 +
fs/fcntl.c | 2 +-
fs/namei.c | 20 ++++++++++++++
fs/open.c | 7 ++++-
include/linux/fcntl.h | 2 +-
include/uapi/asm-generic/fcntl.h | 4 +++
8 files changed, 54 insertions(+), 22 deletions(-)

diff --git a/arch/alpha/include/uapi/asm/fcntl.h b/arch/alpha/include/uapi/asm/fcntl.h
index 50bdc8e8a271..1f879bade68b 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_EMPTYPATH 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 03ce20e5ad7d..5d709058a76f 100644
--- a/arch/parisc/include/uapi/asm/fcntl.h
+++ b/arch/parisc/include/uapi/asm/fcntl.h
@@ -2,26 +2,27 @@
#ifndef _PARISC_FCNTL_H
#define _PARISC_FCNTL_H

-#define O_APPEND 000000010
-#define O_BLKSEEK 000000100 /* HPUX only */
-#define O_CREAT 000000400 /* not fcntl */
-#define O_EXCL 000002000 /* not fcntl */
-#define O_LARGEFILE 000004000
-#define __O_SYNC 000100000
+#define O_APPEND 0000000010
+#define O_BLKSEEK 0000000100 /* HPUX only */
+#define O_CREAT 0000000400 /* not fcntl */
+#define O_EXCL 0000002000 /* not fcntl */
+#define O_LARGEFILE 0000004000
+#define __O_SYNC 0000100000
#define O_SYNC (__O_SYNC|O_DSYNC)
-#define O_NONBLOCK 000200004 /* HPUX has separate NDELAY & NONBLOCK */
-#define O_NOCTTY 000400000 /* not fcntl */
-#define O_DSYNC 001000000 /* HPUX only */
-#define O_RSYNC 002000000 /* HPUX only */
-#define O_NOATIME 004000000
-#define O_CLOEXEC 010000000 /* set close_on_exec */
-
-#define O_DIRECTORY 000010000 /* must be a directory */
-#define O_NOFOLLOW 000000200 /* don't follow links */
-#define O_INVISIBLE 004000000 /* invisible I/O, for DMAPI/XDSM */
-
-#define O_PATH 020000000
-#define __O_TMPFILE 040000000
+#define O_NONBLOCK 0000200004 /* HPUX has separate NDELAY & NONBLOCK */
+#define O_NOCTTY 0000400000 /* not fcntl */
+#define O_DSYNC 0001000000 /* HPUX only */
+#define O_RSYNC 0002000000 /* HPUX only */
+#define O_NOATIME 0004000000
+#define O_CLOEXEC 0010000000 /* set close_on_exec */
+
+#define O_DIRECTORY 0000010000 /* must be a directory */
+#define O_NOFOLLOW 0000000200 /* don't follow links */
+#define O_INVISIBLE 0004000000 /* invisible I/O, for DMAPI/XDSM */
+
+#define O_PATH 0020000000
+#define __O_TMPFILE 0040000000
+#define O_EMPTYPATH 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..dc86c9eaf950 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_EMPTYPATH 0x4000000

#define F_GETOWN 5 /* for sockets. */
#define F_SETOWN 6 /* for sockets. */
diff --git a/fs/fcntl.c b/fs/fcntl.c
index 3d40771e8e7c..4cf05a2fd162 100644
--- a/fs/fcntl.c
+++ b/fs/fcntl.c
@@ -1031,7 +1031,7 @@ 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 54d57dad0f91..e39b573fcc4d 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -3571,6 +3571,24 @@ static int trailing_magiclink(struct nameidata *nd, int acc_mode,
return may_open_magiclink(upgrade_mask, acc_mode);
}

+static int do_emptypath(struct nameidata *nd, const struct open_flags *op,
+ struct file *file)
+{
+ int error;
+ /* We don't support AT_FDCWD (since O_PATH is disallowed here). */
+ struct fd f = fdget_raw(nd->dfd);
+
+ if (!f.file)
+ return -EBADF;
+
+ /* Apply trailing_magiclink()-like restrictions. */
+ error = may_open_magiclink(f.file->f_mode, op->acc_mode);
+ if (!error)
+ error = vfs_open(&f.file->f_path, file);
+ fdput(f);
+ return error;
+}
+
static struct file *path_openat(struct nameidata *nd,
const struct open_flags *op, unsigned flags)
{
@@ -3583,6 +3601,8 @@ static struct file *path_openat(struct nameidata *nd,

if (unlikely(file->f_flags & __O_TMPFILE)) {
error = do_tmpfile(nd, flags, op, file);
+ } else if (unlikely(file->f_flags & O_EMPTYPATH)) {
+ error = do_emptypath(nd, op, file);
} else if (unlikely(file->f_flags & O_PATH)) {
/* Inlined path_lookupat() with a trailing_magiclink() check. */
fmode_t opath_mask = op->opath_mask;
diff --git a/fs/open.c b/fs/open.c
index 806a75d685e1..310b896eecf0 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -1015,6 +1015,8 @@ static inline int build_open_flags(int flags, umode_t mode, struct open_flags *o
lookup_flags |= LOOKUP_DIRECTORY;
if (!(flags & O_NOFOLLOW))
lookup_flags |= LOOKUP_FOLLOW;
+ if (flags & O_EMPTYPATH)
+ lookup_flags |= LOOKUP_EMPTY;
op->lookup_flags = lookup_flags;
return 0;
}
@@ -1076,14 +1078,17 @@ long do_sys_open(int dfd, const char __user *filename, int flags, umode_t mode)
{
struct open_flags op;
int fd = build_open_flags(flags, mode, &op);
+ int empty = 0;
struct filename *tmp;

if (fd)
return fd;

- tmp = getname(filename);
+ tmp = getname_flags(filename, op.lookup_flags, &empty);
if (IS_ERR(tmp))
return PTR_ERR(tmp);
+ if (!empty)
+ op.open_flag &= ~O_EMPTYPATH;

fd = get_unused_fd_flags(flags);
if (fd >= 0) {
diff --git a/include/linux/fcntl.h b/include/linux/fcntl.h
index d019df946cb2..2868ae6c8fc1 100644
--- a/include/linux/fcntl.h
+++ b/include/linux/fcntl.h
@@ -9,7 +9,7 @@
(O_RDONLY | O_WRONLY | O_RDWR | O_CREAT | O_EXCL | O_NOCTTY | O_TRUNC | \
O_APPEND | O_NDELAY | O_NONBLOCK | O_NDELAY | __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)

#ifndef force_o_largefile
#define force_o_largefile() (!IS_ENABLED(CONFIG_ARCH_32BIT_OFF_T))
diff --git a/include/uapi/asm-generic/fcntl.h b/include/uapi/asm-generic/fcntl.h
index 9dc0bf0c5a6e..ae6862f69cc2 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_EMPTYPATH
+#define O_EMPTYPATH 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)
--
2.23.0