linux-kernel - Re: [RFC PATCH 2/3] add statmnt(2) syscall

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Date:   Thu, 14 Sep 2023 16:39:43 -0400
From:   Paul Moore <paul@...l-moore.com>
To:     Miklos Szeredi <mszeredi@...hat.com>
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-api@...r.kernel.org, linux-man@...r.kernel.org,
        linux-security-module@...r.kernel.org, Karel Zak <kzak@...hat.com>,
        Ian Kent <raven@...maw.net>,
        David Howells <dhowells@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <christian@...uner.io>,
        Amir Goldstein <amir73il@...il.com>
Subject: Re: [RFC PATCH 2/3] add statmnt(2) syscall

On Wed, Sep 13, 2023 at 11:23 AM Miklos Szeredi <mszeredi@...hat.com> wrote:
>
> Add a way to query attributes of a single mount instead of having to parse
> the complete /proc/$PID/mountinfo, which might be huge.
>
> 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.
>
> Design is based on a suggestion by Linus:
>
>   "So I'd suggest something that is very much like "statfsat()", which gets
>    a buffer and a length, and returns an extended "struct statfs" *AND*
>    just a string description at the end."
>
> The interface closely mimics that of statx.
>
> Handle ASCII attributes by appending after the end of the structure (as per
> above suggestion).  Allow querying multiple string attributes with
> individual offset/length for each.  String are nul terminated (termination
> isn't counted in length).
>
> Mount options are also delimited with nul characters.  Unlike proc, special
> characters are not quoted.
>
> Link: https://lore.kernel.org/all/CAHk-=wh5YifP7hzKSbwJj94+DZ2czjrZsczy6GBimiogZws=rg@mail.gmail.com/
> Signed-off-by: Miklos Szeredi <mszeredi@...hat.com>
> ---
>  arch/x86/entry/syscalls/syscall_64.tbl |   1 +
>  fs/internal.h                          |   5 +
>  fs/namespace.c                         | 312 ++++++++++++++++++++++++-
>  fs/proc_namespace.c                    |  19 +-
>  fs/statfs.c                            |   1 +
>  include/linux/syscalls.h               |   3 +
>  include/uapi/asm-generic/unistd.h      |   5 +-
>  include/uapi/linux/mount.h             |  36 +++
>  8 files changed, 373 insertions(+), 9 deletions(-)

...

> diff --git a/fs/namespace.c b/fs/namespace.c
> index de47c5f66e17..088a52043bba 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c

...

> +static int do_statmnt(struct stmt_state *s)
> +{
> +       struct statmnt *sm = &s->sm;
> +       struct mount *m = real_mount(s->mnt);
> +
> +       if (!capable(CAP_SYS_ADMIN) &&
> +           !is_path_reachable(m, m->mnt.mnt_root, &s->root))
> +               return -EPERM;

I realize statmnt() is different from fstatfs(), but from an access
control perspective they look a lot alike to me which is why I think
we should probably have a security_sb_statfs() call here.  Same thing
for the listmnt() syscall in patch 3/3.

> +       stmt_numeric(s, STMT_SB_BASIC, stmt_sb_basic);
> +       stmt_numeric(s, STMT_MNT_BASIC, stmt_mnt_basic);
> +       stmt_numeric(s, STMT_PROPAGATE_FROM, stmt_propagate_from);
> +       stmt_string(s, STMT_MNT_ROOT, stmt_mnt_root, &sm->mnt_root);
> +       stmt_string(s, STMT_MOUNTPOINT, stmt_mountpoint, &sm->mountpoint);
> +       stmt_string(s, STMT_FS_TYPE, stmt_fs_type, &sm->fs_type);
> +       stmt_string(s, STMT_SB_OPTS, stmt_sb_opts, &sm->sb_opts);
> +
> +       if (s->err)
> +               return s->err;
> +
> +       if (copy_to_user(s->buf, sm, min_t(size_t, s->bufsize, sizeof(*sm))))
> +               return -EFAULT;
> +
> +       return 0;
> +}

-- 
paul-moore.com