Re: [PATCH bpf-next 1/3] libbpf: Store map pin path in struct bpf_map

On Tue, Oct 22, 2019 at 12:06 PM Toke Høiland-Jørgensen [off-list ref] wrote:

Andrii Nakryiko [off-list ref] writes:

quoted

On Tue, Oct 22, 2019 at 11:45 AM Toke Høiland-Jørgensen [off-list ref] wrote:

quoted

Andrii Nakryiko [off-list ref] writes:

quoted

On Tue, Oct 22, 2019 at 11:13 AM Toke Høiland-Jørgensen [off-list ref] wrote:

quoted

Andrii Nakryiko [off-list ref] writes:

quoted

On Tue, Oct 22, 2019 at 9:08 AM Toke Høiland-Jørgensen [off-list ref] wrote:

quoted

From: Toke Høiland-Jørgensen <redacted>

When pinning a map, store the pin path in struct bpf_map so it can be
re-used later for un-pinning. This simplifies the later addition of per-map
pin paths.

Signed-off-by: Toke Høiland-Jørgensen <redacted>
---
 tools/lib/bpf/libbpf.c |   19 ++++++++++---------
 1 file changed, 10 insertions(+), 9 deletions(-)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index cccfd9355134..b4fdd8ee3bbd 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c

@@ -226,6 +226,7 @@ struct bpf_map {
        void *priv;
        bpf_map_clear_priv_t clear_priv;
        enum libbpf_map_type libbpf_type;
+       char *pin_path;
 };

 struct bpf_secdata {

@@ -1929,6 +1930,7 @@ int bpf_map__reuse_fd(struct bpf_map *map, int fd)
        if (err)
                goto err_close_new_fd;
        free(map->name);
+       zfree(&map->pin_path);

While you are touching this function, can you please also fix error
handling in it? We should store -errno locally on error, before we
call close() which might change errno.

Didn't actually look much at the surrounding function, TBH. I do expect
that I will need to go poke into this for the follow-on "automatic reuse
of pinned maps" series anyway. But sure, I can do a bit of cleanup in a
standalone patch first :)

quoted

quoted

        map->fd = new_fd;
        map->name = new_name;

@@ -4022,6 +4024,7 @@ int bpf_map__pin(struct bpf_map *map, const char *path)
                return -errno;
        }

+       map->pin_path = strdup(path);

if (!map->pin_path) {
    err = -errno;
    goto err_close_new_fd;
}

Right.

quoted

quoted

        pr_debug("pinned map '%s'\n", path);

        return 0;

@@ -4031,6 +4034,9 @@ int bpf_map__unpin(struct bpf_map *map, const char *path)
 {
        int err;

+       if (!path)
+               path = map->pin_path;

This semantics is kind of weird. Given we now remember pin_path,
should we instead check that user-provided path is actually correct
and matches what we stored? Alternatively, bpf_map__unpin() w/o path
argument looks like a cleaner API.

Yeah, I guess the function without a path argument would make the most
sense. However, we can't really change the API of bpf_map__unpin()
(unless you're proposing a symbol-versioned new version?). Dunno if it's
worth it to include a new, somewhat oddly-named, function to achieve
this? For the internal libbpf uses at least it's easy enough for the
caller to just go bpf_map__unpin(map, map->pin_path), so I could also
just drop this change? WDYT?

I'd probably do strcmp(map->pin_path, path), if path is specified.
This will support existing use cases, will allow NULL if we don't want
to bother remembering pin_path, will prevent weird use case of pinning
to one path, but unpinning another one.

So something like

if (path && map->pin_path && strcmp(path, map->pin_path))

can we unpin not pinned map? sounds like an error condition?

See my other comment about programs exiting. For an example, see the XDP
tutorial (just pretend for a moment that that TODO comment was actually
there :)):

replied about re-pinning/sharing, that should come from map definition
anyways and should be handled by map sharing/reuse code, so I think we
should be good there.

https://github.com/xdp-project/xdp-tutorial/blob/master/basic04-pinning-maps/xdp_loader.c#L135

For the clean up use case, if we pinned map, we have its pin_path
stored, and can unpin. For rare case where we want unconditionally
"unpin" map, why app can't just delete the file, if app is so smart as
to know pin path of some other app's map ;)

-Toke