Messages in this thread | | | Date | Tue, 10 May 2022 14:35:12 +0200 | From | Karel Zak <> | Subject | Re: [RFC PATCH] getting misc stats/attributes via xattr API |
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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@redhat.com> http://karelzak.blogspot.com
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