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

[Ian Kent]

On 14/9/23 14:00, Amir Goldstein wrote:
> On Wed, Sep 13, 2023 at 6:22 PM Miklos Szeredi <mszeredi@xxxxxxxxxx> 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@xxxxxxxxxx>
> > ---
> >   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