On Tue, Mar 03, 2020 at 10:26:21AM +0100, Miklos Szeredi wrote:

On Tue, Mar 3, 2020 at 10:13 AM David Howells [off-list ref] wrote:

quoted

Miklos Szeredi [off-list ref] wrote:

quoted

I'm doing a patch.   Let's see how it fares in the face of all these
preconceptions.

Don't forget the efficiency criterion.  One reason for going with fsinfo(2) is
that scanning /proc/mounts when there are a lot of mounts in the system is
slow (not to mention the global lock that is held during the read).

Now, going with sysfs files on top of procfs links might avoid the global
lock, and you can avoid rereading the options string if you export a change
notification, but you're going to end up injecting a whole lot of pathwalk
latency into the system.

Completely irrelevant.  Cached lookup is so much optimized, that you
won't be able to see any of it.

No, I don't think this is going to be a performance issue at all, but
if anything we could introduce a syscall

  ssize_t readfile(int dfd, const char *path, char *buf, size_t
bufsize, int flags);

that is basically the equivalent of open + read + close, or even a
vectored variant that reads multiple files.  But that's off topic
again, since I don't think there's going to be any performance issue
even with plain I/O syscalls.

quoted

On top of that, it isn't going to help with the case that I'm working towards
implementing where a container manager can monitor for mounts taking place
inside the container and supervise them.  What I'm proposing is that during
the action phase (eg. FSCONFIG_CMD_CREATE), fsconfig() would hand an fd
referring to the context under construction to the manager, which would then
be able to call fsinfo() to query it and fsconfig() to adjust it, reject it or
permit it.  Something like:

        fd = receive_context_to_supervise();
        struct fsinfo_params params = {
                .flags          = FSINFO_FLAGS_QUERY_FSCONTEXT,
                .request        = FSINFO_ATTR_SB_OPTIONS,
        };
        fsinfo(fd, NULL, &params, sizeof(params), buffer, sizeof(buffer));
        supervise_parameters(buffer);
        fsconfig(fd, FSCONFIG_SET_FLAG, "hard", NULL, 0);
        fsconfig(fd, FSCONFIG_SET_STRING, "vers", "4.2", 0);
        fsconfig(fd, FSCONFIG_CMD_SUPERVISE_CREATE, NULL, NULL, 0);
        struct fsinfo_params params = {
                .flags          = FSINFO_FLAGS_QUERY_FSCONTEXT,
                .request        = FSINFO_ATTR_SB_NOTIFICATIONS,
        };
        struct fsinfo_sb_notifications sbnotify;
        fsinfo(fd, NULL, &params, sizeof(params), &sbnotify, sizeof(sbnotify));
        watch_super(fd, "", AT_EMPTY_PATH, watch_fd, 0x03);
        fsconfig(fd, FSCONFIG_CMD_SUPERVISE_PERMIT, NULL, NULL, 0);
        close(fd);

However, the supervised mount may be happening in a completely different set
of namespaces, in which case the supervisor presumably wouldn't be able to see
the links in procfs and the relevant portions of sysfs.

It would be a "jump" link to the otherwise invisible directory.

More magic links to beam you around sounds like a bad idea. We had a
bunch of CVEs around them in containers and they were one of the major
reasons behind us pushing for openat2(). That's why it has a
RESOLVE_NO_MAGICLINKS flag.

Christian