Messages in this thread | | | Date | Thu, 5 Oct 2023 12:23:29 +0800 | Subject | Re: [PATCH v3 4/4] add listmount(2) syscall | From | Ian Kent <> |
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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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