As with openat2(2), this allows userspace to easily figure out what
flags and fields are supported by mount_setattr(2). As with clone3(2),
for fields which are not flag-based, we simply set every bit in the
field so that a naive bitwise-and would show that any value of the field
is valid.
The intended way of using this interface to get feature information
looks something like the following:
static bool mountattr_nosymfollow_supported;
static bool mountattr_idmap_supported;
int check_clone3_support(void)
{
int err;
struct mount_attr attr = {};
err = mount_attr(-EBADF, "", 0, &args, CHECK_FIELDS | sizeof(args));
assert(err < 0);
switch (errno) {
case EFAULT: case E2BIG:
/* Old kernel... */
check_support_the_old_way();
break;
case EEXTSYS_NOOP:
mountattr_nosymfollow_supported =
((attr.attr_clr | attr.attr_set) & MOUNT_ATTR_NOSYMFOLLOW);
mountattr_idmap_supported =
((attr.attr_clr | attr.attr_set) & MOUNT_ATTR_IDMAP) &&
(attr.userns_fd != 0);
break;
}
}
Signed-off-by: Aleksa Sarai <cyp...@cyphar.com>
---
fs/namespace.c | 17 +++++++++++++++++
1 file changed, 17 insertions(+)
diff --git a/fs/namespace.c b/fs/namespace.c
index 328087a4df8a..c7ae8d96b7b7 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -4771,6 +4771,7 @@ SYSCALL_DEFINE5(mount_setattr, int, dfd, const char
__user *, path,
size_t, usize)
{
int err;
+ bool check_fields;
struct path target;
struct mount_attr attr;
struct mount_kattr kattr;
@@ -4783,11 +4784,27 @@ SYSCALL_DEFINE5(mount_setattr, int, dfd, const char
__user *, path,
AT_NO_AUTOMOUNT))
return -EINVAL;
+ check_fields = usize & CHECK_FIELDS;
+ usize &= ~CHECK_FIELDS;
+
if (unlikely(usize > PAGE_SIZE))
return -E2BIG;
if (unlikely(usize < MOUNT_ATTR_SIZE_VER0))
return -EINVAL;
+ if (unlikely(check_fields)) {
+ memset(&attr, 0, sizeof(attr));
+ attr = (struct mount_attr) {
+ .attr_set = MOUNT_SETATTR_VALID_FLAGS,
+ .attr_clr = MOUNT_SETATTR_VALID_FLAGS,
+ .propagation = MOUNT_SETATTR_PROPAGATION_FLAGS,
+ .userns_fd = 0xFFFFFFFFFFFFFFFF,
+ };
+
+ err = copy_struct_to_user(uattr, usize, &attr, sizeof(attr),
NULL);
+ return err ?: -EEXTSYS_NOOP;
+ }
+
if (!may_mount())
return -EPERM;
--
2.46.1
