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btrfs-util: Make nested subvolume operations work unpriv

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BTRFS_IOC_SEARCH is only available to root in the
initial userns. This means we fail to recursively 
snapshot even if a subvolume has no nested subvolumes
at the moment.

Let's fix this by using the newer btrfs ioctls which
do work even if we don't have CAP_SYS_ADMIN in the initial
userns.
This commit is contained in:
Daan De Meyer
2026-05-12 13:03:06 +02:00
committed by Daan De Meyer
parent dc6b6e9d64
commit 60d6f70ca4
1 changed files with 46 additions and 104 deletions
Download Patch File Download Diff File Expand all files Collapse all files
src/shared/btrfs-util.c
+46 -104
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@@ -863,19 +863,6 @@ int btrfs_qgroup_unassign(int fd, uint64_t child, uint64_t parent) {
}
static int subvol_remove_children(int fd, const char *subvolume, uint64_t subvol_id, BtrfsRemoveFlags flags) {
struct btrfs_ioctl_search_args args = {
.key.tree_id = BTRFS_ROOT_TREE_OBJECTID,
.key.min_objectid = BTRFS_FIRST_FREE_OBJECTID,
.key.max_objectid = BTRFS_LAST_FREE_OBJECTID,
.key.min_type = BTRFS_ROOT_BACKREF_KEY,
.key.max_type = BTRFS_ROOT_BACKREF_KEY,
.key.min_transid = 0,
.key.max_transid = UINT64_MAX,
};
struct btrfs_ioctl_vol_args vol_args = {};
_cleanup_close_ int subvol_fd = -EBADF;
bool made_writable = false;
@@ -923,43 +910,29 @@ static int subvol_remove_children(int fd, const char *subvolume, uint64_t subvol
if (!(flags & BTRFS_REMOVE_RECURSIVE) || errno != ENOTEMPTY)
return -errno;
/* OK, the subvolume is not empty, let's look for child
* subvolumes, and remove them, first */
/* OK, the subvolume is not empty, let's look for child subvolumes, and remove them, first.
* BTRFS_IOC_GET_SUBVOL_ROOTREF and BTRFS_IOC_INO_LOOKUP_USER (kernel 4.18+) enumerate child
* subvolumes without requiring CAP_SYS_ADMIN in the initial user namespace, unlike the older
* BTRFS_IOC_TREE_SEARCH ioctl. */
args.key.min_offset = args.key.max_offset = subvol_id;
struct btrfs_ioctl_get_subvol_rootref_args rootref_args = {};
while (btrfs_ioctl_search_args_compare(&args) <= 0) {
struct btrfs_ioctl_search_header sh;
const void *body;
args.key.nr_items = 256;
if (ioctl(fd, BTRFS_IOC_TREE_SEARCH, &args) < 0)
for (;;) {
/* BTRFS_IOC_GET_SUBVOL_ROOTREF returns up to BTRFS_MAX_ROOTREF_BUFFER_NUM entries per
* call. If more are available, it returns -EOVERFLOW with num_items filled in and
* min_treeid advanced so we can resume on the next iteration. */
int ioctl_ret = ioctl(subvol_fd, BTRFS_IOC_GET_SUBVOL_ROOTREF, &rootref_args);
if (ioctl_ret < 0 && errno != EOVERFLOW)
return -errno;
if (args.key.nr_items <= 0)
break;
for (uint8_t i = 0; i < rootref_args.num_items; i++) {
uint64_t child_subvol_id = rootref_args.rootref[i].treeid;
FOREACH_BTRFS_IOCTL_SEARCH_HEADER(sh, body, args) {
_cleanup_free_ char *p = NULL;
btrfs_ioctl_search_args_set(&args, &sh);
if (sh.type != BTRFS_ROOT_BACKREF_KEY)
continue;
if (sh.offset != subvol_id)
continue;
const struct btrfs_root_ref *ref = body;
p = memdup_suffix0((char*) ref + sizeof(struct btrfs_root_ref), le64toh(ref->name_len));
if (!p)
return -ENOMEM;
struct btrfs_ioctl_ino_lookup_args ino_args = {
.treeid = subvol_id,
.objectid = htole64(ref->dirid),
struct btrfs_ioctl_ino_lookup_user_args lookup_args = {
.dirid = rootref_args.rootref[i].dirid,
.treeid = child_subvol_id,
};
if (ioctl(fd, BTRFS_IOC_INO_LOOKUP, &ino_args) < 0)
if (ioctl(subvol_fd, BTRFS_IOC_INO_LOOKUP_USER, &lookup_args) < 0)
return -errno;
if (!made_writable) {
@@ -970,29 +943,26 @@ static int subvol_remove_children(int fd, const char *subvolume, uint64_t subvol
made_writable = true;
}
if (isempty(ino_args.name))
/* Subvolume is in the top-level
* directory of the subvolume. */
r = subvol_remove_children(subvol_fd, p, sh.objectid, flags);
if (isempty(lookup_args.path))
/* Subvolume is in the top-level directory of the subvolume. */
r = subvol_remove_children(subvol_fd, lookup_args.name, child_subvol_id, flags);
else {
_cleanup_close_ int child_fd = -EBADF;
/* Subvolume is somewhere further down,
* hence we need to open the
/* Subvolume is somewhere further down, hence we need to open the
* containing directory first */
child_fd = openat(subvol_fd, ino_args.name, O_RDONLY|O_NOCTTY|O_CLOEXEC|O_DIRECTORY|O_NOFOLLOW);
child_fd = openat(subvol_fd, lookup_args.path, O_RDONLY|O_NOCTTY|O_CLOEXEC|O_DIRECTORY|O_NOFOLLOW);
if (child_fd < 0)
return -errno;
r = subvol_remove_children(child_fd, p, sh.objectid, flags);
r = subvol_remove_children(child_fd, lookup_args.name, child_subvol_id, flags);
}
if (r < 0)
return r;
}
/* Increase search key by one, to read the next item, if we can. */
if (!btrfs_ioctl_search_args_inc(&args))
if (ioctl_ret >= 0)
break;
}
@@ -1223,19 +1193,6 @@ static int subvol_snapshot_children(
uint64_t old_subvol_id,
BtrfsSnapshotFlags flags) {
struct btrfs_ioctl_search_args args = {
.key.tree_id = BTRFS_ROOT_TREE_OBJECTID,
.key.min_objectid = BTRFS_FIRST_FREE_OBJECTID,
.key.max_objectid = BTRFS_LAST_FREE_OBJECTID,
.key.min_type = BTRFS_ROOT_BACKREF_KEY,
.key.max_type = BTRFS_ROOT_BACKREF_KEY,
.key.min_transid = 0,
.key.max_transid = UINT64_MAX,
};
struct btrfs_ioctl_vol_args_v2 vol_args = {
.flags = flags & BTRFS_SNAPSHOT_READ_ONLY ? BTRFS_SUBVOL_RDONLY : 0,
.fd = old_fd,
@@ -1293,50 +1250,36 @@ static int subvol_snapshot_children(
return flags & BTRFS_SNAPSHOT_LOCK_BSD ? TAKE_FD(subvolume_fd) : 0;
}
args.key.min_offset = args.key.max_offset = old_subvol_id;
/* Enumerate child subvolumes via BTRFS_IOC_GET_SUBVOL_ROOTREF + BTRFS_IOC_INO_LOOKUP_USER
* (kernel 4.18+), neither of which requires CAP_SYS_ADMIN, unlike BTRFS_IOC_TREE_SEARCH. */
while (btrfs_ioctl_search_args_compare(&args) <= 0) {
struct btrfs_ioctl_search_header sh;
const void *body;
struct btrfs_ioctl_get_subvol_rootref_args rootref_args = {};
args.key.nr_items = 256;
if (ioctl(old_fd, BTRFS_IOC_TREE_SEARCH, &args) < 0)
for (;;) {
/* BTRFS_IOC_GET_SUBVOL_ROOTREF returns up to BTRFS_MAX_ROOTREF_BUFFER_NUM entries per
* call. If more are available, it returns -EOVERFLOW with num_items filled in and
* min_treeid advanced so we can resume on the next iteration. */
int ioctl_ret = ioctl(old_fd, BTRFS_IOC_GET_SUBVOL_ROOTREF, &rootref_args);
if (ioctl_ret < 0 && errno != EOVERFLOW)
return -errno;
if (args.key.nr_items <= 0)
break;
FOREACH_BTRFS_IOCTL_SEARCH_HEADER(sh, body, args) {
_cleanup_free_ char *p = NULL, *c = NULL, *np = NULL;
for (uint8_t i = 0; i < rootref_args.num_items; i++) {
_cleanup_free_ char *c = NULL, *np = NULL;
_cleanup_close_ int old_child_fd = -EBADF, new_child_fd = -EBADF;
btrfs_ioctl_search_args_set(&args, &sh);
if (sh.type != BTRFS_ROOT_BACKREF_KEY)
continue;
/* Avoid finding the source subvolume a second time */
if (sh.offset != old_subvol_id)
continue;
uint64_t child_subvol_id = rootref_args.rootref[i].treeid;
/* Avoid running into loops if the new subvolume is below the old one. */
if (sh.objectid == new_subvol_id)
if (child_subvol_id == new_subvol_id)
continue;
const struct btrfs_root_ref *ref = body;
p = memdup_suffix0((char*) ref + sizeof(struct btrfs_root_ref), le64toh(ref->name_len));
if (!p)
return -ENOMEM;
struct btrfs_ioctl_ino_lookup_args ino_args = {
.treeid = old_subvol_id,
.objectid = htole64(ref->dirid),
struct btrfs_ioctl_ino_lookup_user_args lookup_args = {
.dirid = rootref_args.rootref[i].dirid,
.treeid = child_subvol_id,
};
if (ioctl(old_fd, BTRFS_IOC_INO_LOOKUP, &ino_args) < 0)
if (ioctl(old_fd, BTRFS_IOC_INO_LOOKUP_USER, &lookup_args) < 0)
return -errno;
c = path_join(ino_args.name, p);
c = path_join(lookup_args.path, lookup_args.name);
if (!c)
return -ENOMEM;
@@ -1344,7 +1287,7 @@ static int subvol_snapshot_children(
if (old_child_fd < 0)
return -errno;
np = path_join(subvolume, ino_args.name);
np = path_join(subvolume, lookup_args.path);
if (!np)
return -ENOMEM;
@@ -1369,7 +1312,7 @@ static int subvol_snapshot_children(
/* When btrfs clones the subvolumes, child subvolumes appear as empty
* directories. Remove them, so that we can create a new snapshot in their place */
if (unlinkat(new_child_fd, p, AT_REMOVEDIR) < 0) {
if (unlinkat(new_child_fd, lookup_args.name, AT_REMOVEDIR) < 0) {
int k = -errno;
if (flags & BTRFS_SNAPSHOT_READ_ONLY)
@@ -1378,7 +1321,7 @@ static int subvol_snapshot_children(
return k;
}
r = subvol_snapshot_children(old_child_fd, new_child_fd, p, sh.objectid,
r = subvol_snapshot_children(old_child_fd, new_child_fd, lookup_args.name, child_subvol_id,
flags & ~(BTRFS_SNAPSHOT_FALLBACK_COPY|BTRFS_SNAPSHOT_LOCK_BSD));
/* Restore the readonly flag */
@@ -1394,8 +1337,7 @@ static int subvol_snapshot_children(
return r;
}
/* Increase search key by one, to read the next item, if we can. */
if (!btrfs_ioctl_search_args_inc(&args))
if (ioctl_ret >= 0)
break;
}