[RFC PATCH] getting misc stats/attributes via xattr API

[Karel Zak]

On Mon, May 09, 2022 at 02:48:15PM +0200, Christian Brauner wrote:
> One comment about this. We really need to have this interface support
> giving us mount options like "relatime" back in numeric form (I assume
> this will be possible.). It is royally annoying having to maintain a
> mapping table in userspace just to do:
> 
> relatime -> MS_RELATIME/MOUNT_ATTR_RELATIME
> ro	 -> MS_RDONLY/MOUNT_ATTR_RDONLY
> 
> A library shouldn't be required to use this interface. Conservative
> low-level software that keeps its shared library dependencies minimal
> will need to be able to use that interface without having to go to an
> external library that transforms text-based output to binary form (Which
> I'm very sure will need to happen if we go with a text-based
> interface.).

Sounds like David's fsinfo() :-)

We need an interface where the kernel returns a consistent mount table    
entry (more syscalls to get more key=value could be a way how to get
inconsistent data).                                              

IMHO all the attempts to make a trivial interface will be unsuccessful
because the mount table is complex (tree) and mixes strings, paths,
and flags. We will always end with a complex interface or complex
strings (like the last xatts attempt). There is no 3rd path to go ...

The best would be simplified fsinfo() where userspace defines
a request (wanted "keys"), and the kernel fills a buffer with data
separated by some header metadata struct. In this case, the kernel can
return strings and structs with binary data.  


I'd love something like:

ssize_t sz;
fsinfo_query query[] = {
    { .request = FSINFO_MOUNT_PATH },
    { .request = FSINFO_PROPAGATION },
    { .request = FSINFO_CHILDREN_IDS },
};

sz = fsinfo(dfd, "", AT_EMPTY_PATH,
                &query, ARRAY_SIZE(query),
                buf, sizeof(buf));

for (p = buf; p < buf + sz; ) {
{
    fsinfo_entry *e = (struct fsinfo_entry) p;
    char *data = p + sizeof(struct fsinfo_entry);

    switch(e->request) {
    case FSINFO_MOUNT_PATH:
        printf("mountpoint %s\n", data);
        break;
    case FSINFO_PROPAGATION:
        printf("propagation %x\n", (uintptr_t) data);
        break;
    case FSINFO_CHILDREN_IDS:
        fsinfo_child *x = (fsinfo_child *) data;
        for (i = 0; i < e->count; i++) {
            printf("child: %d\n", x[i].mnt_id);
        }
        break;
    ...
    }

    p += sizeof(struct fsinfo_entry) + e->len;
}



... my two cents :-)

    Karel

-- 
 Karel Zak  <kzak at redhat.com>
 http://karelzak.blogspot.com