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;

+       }
+       return ctr;
+}
+
+SYSCALL_DEFINE4(listmnt, u64, mnt_id, u64 __user *, buf, size_t, bufsize,
+               unsigned int, flags)
+{
+       struct vfsmount *mnt;
+       struct path root;
+       long err;
+
+       if (flags)
+               return -EINVAL;
+
+       down_read(&namespace_sem);
+       mnt = lookup_mnt_in_ns(mnt_id, current->nsproxy->mnt_ns);
+       err = -ENOENT;
+       if (mnt) {
+               get_fs_root(current->fs, &root);
+               err = do_listmnt(mnt, buf, bufsize, &root);
+               path_put(&root);
+       }
+       up_read(&namespace_sem);
+
+       return err;
+}
+
+
 static void __init init_mount_tree(void)
 {
        struct vfsmount *mnt;

diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index 1099bd307fa7..5d776cdb6f18 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h

@@ -411,6 +411,8 @@ asmlinkage long sys_fstatfs64(unsigned int fd, size_t sz,
 asmlinkage long sys_statmnt(u64 mnt_id, u64 mask,
                            struct statmnt __user *buf, size_t bufsize,
                            unsigned int flags);
+asmlinkage long sys_listmnt(u64 mnt_id, u64 __user *buf, size_t bufsize,
+                           unsigned int flags);
 asmlinkage long sys_truncate(const char __user *path, long length);
 asmlinkage long sys_ftruncate(unsigned int fd, unsigned long length);
 #if BITS_PER_LONG == 32

diff --git a/include/uapi/asm-generic/unistd.h b/include/uapi/asm-generic/unistd.h
index 640997231ff6..a2b41370f603 100644
--- a/include/uapi/asm-generic/unistd.h
+++ b/include/uapi/asm-generic/unistd.h

@@ -826,8 +826,11 @@ __SYSCALL(__NR_fchmodat2, sys_fchmodat2)
 #define __NR_statmnt   454
 __SYSCALL(__NR_statmnt, sys_statmnt)

+#define __NR_listmnt   455
+__SYSCALL(__NR_listmnt, sys_listmnt)
+
 #undef __NR_syscalls
-#define __NR_syscalls 455
+#define __NR_syscalls 456

 /*
  * 32 bit systems traditionally used different
--

2.41.0