linux-kernel - Re: [PATCH v3 3/4] add statmount(2) syscall

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Message-ID: <CAHC9VhQxMHBiB--QuV-g6ffghdN-G4N0fX3i-v+z-nc9n1p49A@mail.gmail.com>
Date:   Wed, 4 Oct 2023 15:26:24 -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>,
        Matthew House <mattlloydhouse@...il.com>,
        Florian Weimer <fweimer@...hat.com>,
        Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH v3 3/4] add statmount(2) syscall

On Thu, Sep 28, 2023 at 9:03 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 the 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).  Pointers to strings are stored in u64 members to make
> the structure the same regardless of pointer size.  Strings are nul
> terminated.
>
> 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_32.tbl |   1 +
>  arch/x86/entry/syscalls/syscall_64.tbl |   1 +
>  fs/namespace.c                         | 283 +++++++++++++++++++++++++
>  fs/statfs.c                            |   1 +
>  include/linux/syscalls.h               |   5 +
>  include/uapi/asm-generic/unistd.h      |   5 +-
>  include/uapi/linux/mount.h             |  56 +++++
>  7 files changed, 351 insertions(+), 1 deletion(-)

...

> diff --git a/fs/namespace.c b/fs/namespace.c
> index c3a41200fe70..3326ba2b2810 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c

...

> +static int do_statmount(struct stmt_state *s)
> +{
> +       struct statmnt *sm = &s->sm;
> +       struct mount *m = real_mount(s->mnt);
> +       size_t copysize = min_t(size_t, s->bufsize, sizeof(*sm));
> +       int err;
> +
> +       err = security_sb_statfs(s->mnt->mnt_root);
> +       if (err)
> +               return err;

Thank you for adding the security_sb_statfs() call to this operation,
however I believe we want to place it *after* the capability check to
be consistent with other LSM calls.

> +       if (!capable(CAP_SYS_ADMIN) &&
> +           !is_path_reachable(m, m->mnt.mnt_root, &s->root))
> +               return -EPERM;
> +
> +       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_FS_TYPE, stmt_fs_type, &sm->fs_type);
> +       stmt_string(s, STMT_MNT_ROOT, stmt_mnt_root, &sm->mnt_root);
> +       stmt_string(s, STMT_MNT_POINT, stmt_mnt_point, &sm->mnt_point);
> +
> +       if (s->err)
> +               return s->err;
> +
> +       /* Return the number of bytes copied to the buffer */
> +       sm->size = copysize + s->pos;
> +
> +       if (copy_to_user(s->buf, sm, copysize))
> +               return -EFAULT;
> +
> +       return 0;
> +}

-- 
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