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mountpoint-util: some tweaks for fd_is_mount_point()

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- Drop fstat() fallback path now that we assume fdinfo
  is available
- Use at_flags_normalize_nofollow()
- Accept empty path the same way as NULL
- Accept fd being AT_FDCWD and filename being "."
This commit is contained in:
Mike Yuan
2025-01-20 21:48:27 +01:00
parent e5a3127339
commit e2f97c790e
2 changed files with 68 additions and 56 deletions
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102
src/basic/mountpoint-util.c
View File

@@ -213,27 +213,35 @@ bool file_handle_equal(const struct file_handle *a, const struct file_handle *b)
}
int fd_is_mount_point(int fd, const char *filename, int flags) {
_cleanup_free_ struct file_handle *h = NULL, *h_parent = NULL;
int mount_id = -1, mount_id_parent = -1;
bool nosupp = false, check_st_dev = true;
STRUCT_STATX_DEFINE(sx);
struct stat a, b;
bool fd_is_self;
int r;
assert(fd >= 0);
assert(fd >= 0 || fd == AT_FDCWD);
assert((flags & ~AT_SYMLINK_FOLLOW) == 0);
if (!filename) {
/* If the file name is specified as NULL we'll see if the specified 'fd' is a mount
* point. That's only supported if the kernel supports statx(), or if the inode specified via
* 'fd' refers to a directory. Otherwise, we'll have to fail (ENOTDIR), because we have no
* kernel API to query the information we need. */
flags |= AT_EMPTY_PATH;
filename = "";
} else if (!filename_possibly_with_slash_suffix(filename))
/* Insist that the specified filename is actually a filename, and not a path, i.e. some inode further
* up or down the tree then immediately below the specified directory fd. */
return -EINVAL;
if (isempty(filename)) {
if (fd == AT_FDCWD)
filename = ".";
else {
/* If the file name is empty we'll see if the specified 'fd' is a mount point.
* That's only supported if the kernel supports statx(), or if the inode specified
* via 'fd' refers to a directory. Otherwise, we'll have to fail (ENOTDIR), because
* we have no kernel API to query the information we need. */
flags |= AT_EMPTY_PATH;
filename = "";
}
fd_is_self = true;
} else if (STR_IN_SET(filename, ".", "./"))
fd_is_self = true;
else {
/* Insist that the specified filename is actually a filename, and not a path, i.e. some inode
* further up or down the tree then immediately below the specified directory fd. */
if (!filename_possibly_with_slash_suffix(filename))
return -EINVAL;
fd_is_self = false;
}
/* First we will try statx()' STATX_ATTR_MOUNT_ROOT attribute, which is our ideal API, available
* since kernel 5.8.
@@ -246,18 +254,17 @@ int fd_is_mount_point(int fd, const char *filename, int flags) {
* If that didn't work we will try to read the mount id from /proc/self/fdinfo/<fd>. This is almost
* as good as name_to_handle_at(), however, does not return the opaque file handle. The opaque file
* handle is pretty useful to detect the root directory, which we should always consider a mount
* point. Hence we use this only as fallback. Exporting the mnt_id in fdinfo is a pretty recent
* kernel addition.
* point. Hence we use this only as fallback.
*
* As last fallback we do traditional fstat() based st_dev comparisons. This is how things were
* traditionally done, but unionfs breaks this since it exposes file systems with a variety of st_dev
* reported. Also, btrfs subvolumes have different st_dev, even though they aren't real mounts of
* their own. */
* Note that traditionally the check is done via fstat()-based st_dev comparisons. However, various
* file systems don't guarantee same st_dev across single fs anymore, e.g. unionfs exposes file systems
* with a variety of st_dev reported. Also, btrfs subvolumes have different st_dev, even though
* they aren't real mounts of their own. */
if (statx(fd,
filename,
(FLAGS_SET(flags, AT_SYMLINK_FOLLOW) ? 0 : AT_SYMLINK_NOFOLLOW) |
(flags & AT_EMPTY_PATH) |
STRUCT_STATX_DEFINE(sx);
if (statx(fd, filename,
at_flags_normalize_nofollow(flags) |
AT_NO_AUTOMOUNT | /* don't trigger automounts – mounts are a local concept, hence no need to trigger automounts to determine STATX_ATTR_MOUNT_ROOT */
AT_STATX_DONT_SYNC, /* don't go to the network for this – for similar reasons */
STATX_TYPE,
@@ -271,6 +278,10 @@ int fd_is_mount_point(int fd, const char *filename, int flags) {
} else if (FLAGS_SET(sx.stx_attributes_mask, STATX_ATTR_MOUNT_ROOT)) /* yay! */
return FLAGS_SET(sx.stx_attributes, STATX_ATTR_MOUNT_ROOT);
_cleanup_free_ struct file_handle *h = NULL, *h_parent = NULL;
int mount_id = -1, mount_id_parent = -1;
bool nosupp = false;
r = name_to_handle_at_try_fid(fd, filename, &h, &mount_id, flags);
if (r < 0) {
if (is_name_to_handle_at_fatal_error(r))
@@ -278,13 +289,12 @@ int fd_is_mount_point(int fd, const char *filename, int flags) {
if (!ERRNO_IS_NOT_SUPPORTED(r))
goto fallback_fdinfo;
/* This kernel or file system does not support name_to_handle_at(), hence let's see
* if the upper fs supports it (in which case it is a mount point), otherwise fall
* back to the traditional stat() logic */
/* This file system does not support name_to_handle_at(), hence let's see if the upper fs
* supports it (in which case it is a mount point), otherwise fall back to the fdinfo logic. */
nosupp = true;
}
if (isempty(filename))
if (fd_is_self)
r = name_to_handle_at_try_fid(fd, "..", &h_parent, &mount_id_parent, 0); /* can't work for non-directories 😢 */
else
r = name_to_handle_at_try_fid(fd, "", &h_parent, &mount_id_parent, AT_EMPTY_PATH);
@@ -316,12 +326,10 @@ int fd_is_mount_point(int fd, const char *filename, int flags) {
fallback_fdinfo:
r = fd_fdinfo_mnt_id(fd, filename, flags, &mount_id);
if (ERRNO_IS_NEG_NOT_SUPPORTED(r) || ERRNO_IS_NEG_PRIVILEGE(r))
goto fallback_fstat;
if (r < 0)
return r;
if (isempty(filename))
if (fd_is_self)
r = fd_fdinfo_mnt_id(fd, "..", 0, &mount_id_parent); /* can't work for non-directories 😢 */
else
r = fd_fdinfo_mnt_id(fd, "", AT_EMPTY_PATH, &mount_id_parent);
@@ -333,31 +341,27 @@ fallback_fdinfo:
/* Hmm, so, the mount ids are the same. This leaves one special case though for the root file
* system. For that, let's see if the parent directory has the same inode as we are interested
* in. Hence, let's also do fstat() checks now, too, but avoid the st_dev comparisons, since they
* aren't that useful on unionfs mounts. */
check_st_dev = false;
* in. */
struct stat a, b;
fallback_fstat:
/* yay for fstatat() taking a different set of flags than the other _at() above */
if (flags & AT_SYMLINK_FOLLOW)
flags &= ~AT_SYMLINK_FOLLOW;
else
flags |= AT_SYMLINK_NOFOLLOW;
if (fstatat(fd, filename, &a, flags) < 0)
if (fstatat(fd, filename, &a, at_flags_normalize_nofollow(flags)) < 0)
return -errno;
if (isempty(filename))
if (fd_is_self)
r = fstatat(fd, "..", &b, 0);
else
r = fstatat(fd, "", &b, AT_EMPTY_PATH);
if (r < 0)
return -errno;
/* A directory with same device and inode as its parent? Must be the root directory */
if (stat_inode_same(&a, &b))
return 1;
return check_st_dev && (a.st_dev != b.st_dev);
/* A directory with same device and inode as its parent must be the root directory. Otherwise
* not a mount point.
*
* NB: we avoid inode_same_at() here because it internally attempts name_to_handle_at_try_fid() first,
* which is redundant. */
return stat_inode_same(&a, &b);
}
/* flags can be AT_SYMLINK_FOLLOW or 0 */

22
src/test/test-mountpoint-util.c
View File

@@ -273,6 +273,7 @@ TEST(path_is_mount_point) {
TEST(fd_is_mount_point) {
_cleanup_(rm_rf_physical_and_freep) char *tmpdir = NULL;
_cleanup_free_ char *pwd = NULL;
_cleanup_close_ int fd = -EBADF;
int r;
@@ -281,11 +282,8 @@ TEST(fd_is_mount_point) {
/* Not allowed, since "/" is a path, not a plain filename */
assert_se(fd_is_mount_point(fd, "/", 0) == -EINVAL);
assert_se(fd_is_mount_point(fd, ".", 0) == -EINVAL);
assert_se(fd_is_mount_point(fd, "./", 0) == -EINVAL);
assert_se(fd_is_mount_point(fd, "..", 0) == -EINVAL);
assert_se(fd_is_mount_point(fd, "../", 0) == -EINVAL);
assert_se(fd_is_mount_point(fd, "", 0) == -EINVAL);
assert_se(fd_is_mount_point(fd, "/proc", 0) == -EINVAL);
assert_se(fd_is_mount_point(fd, "/proc/", 0) == -EINVAL);
assert_se(fd_is_mount_point(fd, "proc/sys", 0) == -EINVAL);
@@ -307,9 +305,20 @@ TEST(fd_is_mount_point) {
fd = open("/proc", O_RDONLY|O_CLOEXEC|O_DIRECTORY|O_NOCTTY);
assert_se(fd >= 0);
assert_se(fd_is_mount_point(fd, NULL, 0) > 0);
assert_se(fd_is_mount_point(fd, "", 0) == -EINVAL);
assert_se(fd_is_mount_point(fd, "version", 0) == 0);
ASSERT_OK_POSITIVE(fd_is_mount_point(fd, NULL, 0));
ASSERT_OK_POSITIVE(fd_is_mount_point(fd, "", 0));
ASSERT_OK_POSITIVE(fd_is_mount_point(fd, ".", 0));
ASSERT_OK_POSITIVE(fd_is_mount_point(fd, "./", 0));
ASSERT_OK_ZERO(fd_is_mount_point(fd, "version", 0));
ASSERT_OK(safe_getcwd(&pwd));
ASSERT_OK_ERRNO(fchdir(fd));
ASSERT_OK_POSITIVE(fd_is_mount_point(AT_FDCWD, NULL, 0));
ASSERT_OK_POSITIVE(fd_is_mount_point(AT_FDCWD, "", 0));
ASSERT_OK_POSITIVE(fd_is_mount_point(AT_FDCWD, "./", 0));
ASSERT_OK_ERRNO(chdir(pwd));
safe_close(fd);
fd = open("/proc/version", O_RDONLY|O_CLOEXEC|O_NOCTTY);
@@ -317,7 +326,6 @@ TEST(fd_is_mount_point) {
r = fd_is_mount_point(fd, NULL, 0);
assert_se(IN_SET(r, 0, -ENOTDIR)); /* on old kernels we can't determine if regular files are mount points if we have no directory fd */
assert_se(fd_is_mount_point(fd, "", 0) == -EINVAL);
if (!mount_new_api_supported())
return;