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SubjectRe: [PATCH v3 4/4] add listmount(2) syscall
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On 5/10/23 12:01, Miklos Szeredi wrote:
> On Wed, 4 Oct 2023 at 21:38, Paul Moore <paul@paul-moore.com> wrote:
>> On Thu, Sep 28, 2023 at 9:04 AM Miklos Szeredi <mszeredi@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 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@redhat.com>
>>> ---
>>> arch/x86/entry/syscalls/syscall_32.tbl | 1 +
>>> arch/x86/entry/syscalls/syscall_64.tbl | 1 +
>>> fs/namespace.c | 69 ++++++++++++++++++++++++++
>>> include/linux/syscalls.h | 3 ++
>>> include/uapi/asm-generic/unistd.h | 5 +-
>>> include/uapi/linux/mount.h | 3 ++
>>> 6 files changed, 81 insertions(+), 1 deletion(-)
>> ...
>>
>>> diff --git a/fs/namespace.c b/fs/namespace.c
>>> index 3326ba2b2810..050e2d2af110 100644
>>> --- a/fs/namespace.c
>>> +++ b/fs/namespace.c
>>> @@ -4970,6 +4970,75 @@ SYSCALL_DEFINE4(statmount, const struct __mount_arg __user *, req,
>>> return ret;
>>> }
>>>
>>> +static long do_listmount(struct vfsmount *mnt, u64 __user *buf, size_t bufsize,
>>> + const struct path *root, unsigned int flags)
>>> +{
>>> + struct mount *r, *m = real_mount(mnt);
>>> + struct path rootmnt = {
>>> + .mnt = root->mnt,
>>> + .dentry = root->mnt->mnt_root
>>> + };
>>> + long ctr = 0;
>>> + bool reachable_only = true;
>>> + int err;
>>> +
>>> + err = security_sb_statfs(mnt->mnt_root);
>>> + if (err)
>>> + return err;
>>> +
>>> + if (flags & LISTMOUNT_UNREACHABLE) {
>>> + if (!capable(CAP_SYS_ADMIN))
>>> + return -EPERM;
>>> + reachable_only = false;
>>> + }
>>> +
>>> + if (reachable_only && !is_path_reachable(m, mnt->mnt_root, &rootmnt))
>>> + return capable(CAP_SYS_ADMIN) ? 0 : -EPERM;
>>> +
>>> + list_for_each_entry(r, &m->mnt_mounts, mnt_child) {
>>> + if (reachable_only &&
>>> + !is_path_reachable(r, r->mnt.mnt_root, root))
>>> + continue;
>> I believe we would want to move the security_sb_statfs() call from
>> above to down here; something like this I think ...
>>
>> err = security_sb_statfs(r->mnt.mnt_root);
>> if (err)
>> /* if we can't access the mount, pretend it doesn't exist */
>> continue;
> Hmm. Why is this specific to listing mounts (i.e. why doesn't readdir
> have a similar filter)?
>
> Also why hasn't this come up with regards to the proc interfaces that
> list mounts?

The proc interfaces essentially use <mount namespace>->list to provide

the mounts that can be seen so it's filtered by mount namespace of the

task that's doing the open().


See fs/namespace.c:mnt_list_next() and just below the m_start(), m_next(),

etc.


Ian

>
> I just want to understand the big picture here.
>
> Thanks,
> Miklos

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