[RFC PATCH 3/3] add listmnt(2) syscall

[Amir Goldstein]

On Wed, Sep 13, 2023 at 6:22 PM Miklos Szeredi <mszeredi at redhat.com> wrote:
>
> 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 <mszeredi at redhat.com>
> ---
>  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.

Thanks,
Amir.






> +       }
> +       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
>