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tree-wide: statx() supports STATX_ATTR_MOUNT_ROOT since kernel 5.8

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Our baseline on kernel is 5.10, hence we can unconditionally use it.
This commit is contained in:
Yu Watanabe
2026-01-13 10:21:04 +09:00
parent ec21e5c58e
commit a98a6eb95c
5 changed files with 38 additions and 177 deletions
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src/basic/mountpoint-util.c
+3 -109
View File
@@ -198,7 +198,6 @@ bool file_handle_equal(const struct file_handle *a, const struct file_handle *b)
}
int is_mount_point_at(int fd, const char *filename, int flags) {
bool fd_is_self;
int r;
assert(fd >= 0 || fd == AT_FDCWD);
@@ -216,36 +215,13 @@ int is_mount_point_at(int fd, const char *filename, int flags) {
filename = "";
}
fd_is_self = true;
} else if (STR_IN_SET(filename, ".", "./"))
fd_is_self = true;
else {
} else if (!STR_IN_SET(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. */
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.
*
* If that fails, our second try is the name_to_handle_at() syscall, which tells us the mount id and
* an opaque file "handle". It is not supported everywhere though (kernel compile-time option, not
* all file systems are hooked up). If it works the mount id is usually good enough to tell us
* whether something is a mount point.
*
* 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.
*
* 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. */
struct statx sx = {}; /* explicitly initialize the struct to make msan silent. */
if (statx(fd, filename,
at_flags_normalize_nofollow(flags) |
@@ -255,93 +231,11 @@ int is_mount_point_at(int fd, const char *filename, int flags) {
&sx) < 0)
return -errno;
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))
return r;
if (!ERRNO_IS_NOT_SUPPORTED(r))
goto fallback_fdinfo;
/* 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 (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);
if (r < 0) {
if (is_name_to_handle_at_fatal_error(r))
return r;
if (!ERRNO_IS_NOT_SUPPORTED(r))
goto fallback_fdinfo;
if (nosupp)
/* Both the parent and the directory can't do name_to_handle_at() */
goto fallback_fdinfo;
/* The parent can't do name_to_handle_at() but the directory we are
* interested in can? If so, it must be a mount point. */
return 1;
}
/* The parent can do name_to_handle_at() but the directory we are interested in can't? If
* so, it must be a mount point. */
if (nosupp)
return 1;
/* If the file handle for the directory we are interested in and its parent are identical,
* we assume this is the root directory, which is a mount point. */
if (file_handle_equal(h_parent, h))
return 1;
return mount_id != mount_id_parent;
fallback_fdinfo:
r = fd_fdinfo_mnt_id(fd, filename, flags, &mount_id);
r = statx_warn_mount_root(&sx, LOG_DEBUG);
if (r < 0)
return r;
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);
if (r < 0)
return r;
if (mount_id != mount_id_parent)
return 1;
/* 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. */
struct stat a, b;
/* yay for fstatat() taking a different set of flags than the other _at() above */
if (fstatat(fd, filename, &a, at_flags_normalize_nofollow(flags)) < 0)
return -errno;
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. 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);
return FLAGS_SET(sx.stx_attributes, STATX_ATTR_MOUNT_ROOT);
}
/* flags can be AT_SYMLINK_FOLLOW or 0 */
src/basic/stat-util.c
+11
View File
@@ -572,3 +572,14 @@ mode_t inode_type_from_string(const char *s) {
return MODE_INVALID;
}
int statx_warn_mount_root(const struct statx *sx, int log_level) {
assert(sx);
/* The STATX_ATTR_MOUNT_ROOT flag is supported since kernel v5.8. */
if (!FLAGS_SET(sx->stx_attributes_mask, STATX_ATTR_MOUNT_ROOT))
return log_full_errno(log_level, SYNTHETIC_ERRNO(ENOSYS),
"statx() did not set STATX_ATTR_MOUNT_ROOT, running on an old kernel?");
return 0;
}
src/basic/stat-util.h
+2
View File
@@ -117,3 +117,5 @@ static inline bool inode_type_can_hardlink(mode_t m) {
* type). */
return IN_SET(m & S_IFMT, S_IFSOCK, S_IFLNK, S_IFREG, S_IFBLK, S_IFCHR, S_IFIFO);
}
int statx_warn_mount_root(const struct statx *sx, int log_level);