On 14/9/23 14:00, Amir Goldstein wrote:

On Wed, Sep 13, 2023 at 6:22 PM Miklos Szeredi [off-list ref] wrote:

quoted

Add way to query the children of a particular mount.  This is a more
flexible way to iterate the mount tree than having to parse the complete
/proc/self/mountinfo.

Lookup the mount by the old (32bit) or new (64bit) mount ID.  If a mount
needs to be queried based on path, then statx(2) can be used to first query
the mount ID belonging to the path.

Return an array of new (64bit) mount ID's.  Without privileges only mounts
are listed which are reachable from the task's root.

Signed-off-by: Miklos Szeredi <redacted>
---
  arch/x86/entry/syscalls/syscall_64.tbl |  1 +
  fs/namespace.c                         | 51 ++++++++++++++++++++++++++
  include/linux/syscalls.h               |  2 +
  include/uapi/asm-generic/unistd.h      |  5 ++-
  4 files changed, 58 insertions(+), 1 deletion(-)

diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl
index 6d807c30cd16..0d9a47b0ce9b 100644
--- a/arch/x86/entry/syscalls/syscall_64.tbl
+++ b/arch/x86/entry/syscalls/syscall_64.tbl

@@ -376,6 +376,7 @@
  452    common  fchmodat2               sys_fchmodat2
  453    64      map_shadow_stack        sys_map_shadow_stack
  454    common  statmnt                 sys_statmnt
+455    common  listmnt                 sys_listmnt

  #
  # Due to a historical design error, certain syscalls are numbered differently

diff --git a/fs/namespace.c b/fs/namespace.c
index 088a52043bba..5362b1ffb26f 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c

@@ -4988,6 +4988,57 @@ SYSCALL_DEFINE5(statmnt, u64, mnt_id,
         return err;
  }

+static long do_listmnt(struct vfsmount *mnt, u64 __user *buf, size_t bufsize,
+                     const struct path *root)
+{
+       struct mount *r, *m = real_mount(mnt);
+       struct path rootmnt = { .mnt = root->mnt, .dentry = root->mnt->mnt_root };
+       long ctr = 0;
+
+       if (!capable(CAP_SYS_ADMIN) &&
+           !is_path_reachable(m, mnt->mnt_root, &rootmnt))
+               return -EPERM;
+
+       list_for_each_entry(r, &m->mnt_mounts, mnt_child) {
+               if (!capable(CAP_SYS_ADMIN) &&
+                   !is_path_reachable(r, r->mnt.mnt_root, root))
+                       continue;
+
+               if (ctr >= bufsize)
+                       return -EOVERFLOW;
+               if (put_user(r->mnt_id_unique, buf + ctr))
+                       return -EFAULT;
+               ctr++;
+               if (ctr < 0)
+                       return -ERANGE;

I think it'd be good for userspace to be able to query required
bufsize with NULL buf, listattr style, rather than having to
guess and re-guess on EOVERFLOW.

Agreed, I also think that would be useful.


Ian