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

Andrii Nakryiko [off-list ref] writes:

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

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Andrii Nakryiko [off-list ref] writes:

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

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Andrii Nakryiko [off-list ref] writes:

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

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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 :)

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        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.

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        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 :)):

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

-Toke