[PATCH v9 03/10] open: O_EMPTYPATH: procfs-less file descriptor re-opening

Aleksa Sarai
2019-07-06T14:59:18Z

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@cyphar.com>
---
fs/fcntl.c | 2 +-
fs/namei.c | 27 +++++++++++++++++++++++++++
fs/open.c | 7 ++++++-
include/linux/fcntl.h | 2 +-
include/uapi/asm-generic/fcntl.h | 5 +++++
5 files changed, 40 insertions(+), 3 deletions(-)

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 4ec6168762db..4895717d2760 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -3605,6 +3605,31 @@ static int do_o_path(struct nameidata *nd, unsigned flags, struct file *file)
return error;
}

+static int do_o_emptypath(struct nameidata *nd, struct file *newfile)
+{
+ int error;
+ struct fd f;
+
+ /* We don't support AT_FDCWD since O_PATH cannot be set here. */
+ f = fdget_raw(nd->dfd);
+ if (!f.file)
+ return -EBADF;
+
+ /* Update opath_mask as though we went through trailing_symlink(). */
+ if (!(f.file->f_mode & (FMODE_READ | FMODE_PATH_READ)))
+ nd->opath_mask &= ~FMODE_PATH_READ;
+ if (!(f.file->f_mode & (FMODE_WRITE | FMODE_PATH_WRITE)))
+ nd->opath_mask &= ~FMODE_PATH_WRITE;
+
+ /* Obey the same restrictions as magic-links. */
+ error = may_open_magiclink(f.file->f_mode, nd->acc_mode);
+ if (!error)
+ error = vfs_open(&f.file->f_path, newfile);
+
+ fdput(f);
+ return error;
+}
+
static struct file *path_openat(struct nameidata *nd,
const struct open_flags *op, unsigned flags)
{
@@ -3620,6 +3645,8 @@ 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_o_emptypath(nd, file);
} else if (unlikely(file->f_flags & O_PATH)) {
error = do_o_path(nd, flags, file);
} else {
diff --git a/fs/open.c b/fs/open.c
index ab20eae39df7..bdca45528524 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -996,6 +996,8 @@ 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;
}
@@ -1057,14 +1059,17 @@ 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..307f7c414a51 100644
--- a/include/uapi/asm-generic/fcntl.h
+++ b/include/uapi/asm-generic/fcntl.h
@@ -89,6 +89,11 @@
#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)
--
2.22.0