Currently stackmaps can't be iterated over. The keys are obtained
through other maps and look ups have to be performed. In new usecases,
its useful to be able to iterate over the stackmap independently.
Implement bpf_get_next_key to make this possible.

More details of use case:
Currently iterating over stack maps is done like so, for example
in BCC tools offcputime script:
1. Obtain keys from the 'counts' hash map.
2. Look up stackmap with each of the keys obtained from step 1.

This makes the iteration of stackmap dependent on the counts map.
In a new tool I'm working on called BPFd [1], it gives a huge speed
up when I dump entire maps before transmitting them to BCC tools
on a different machine [2]. This patch makes it possible to dump
stackmaps independent of other maps.

Tested on x86 and arm64 machines.

[1] https://lwn.net/Articles/744522/
[2] https://github.com/joelagnel/bpfd/issues/8

Cc: Alexei Starovoitov <a...@kernel.org>
Cc: Daniel Borkmann <dan...@iogearbox.net>
Cc: Brendan Gregg <bgr...@netflix.com>
Cc: Brenden Blanco <bbla...@gmail.com>
Signed-off-by: Joel Fernandes <joe...@google.com>
---
 kernel/bpf/stackmap.c | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c
index a15bc636cc98..b0bf7d009f76 100644
--- a/kernel/bpf/stackmap.c
+++ b/kernel/bpf/stackmap.c
@@ -228,7 +228,23 @@ int bpf_stackmap_copy(struct bpf_map *map, void *key, void 
*value)
 
 static int stack_map_get_next_key(struct bpf_map *map, void *key, void 
*next_key)
 {
-       return -EINVAL;
+       struct bpf_stack_map *smap = container_of(map, struct bpf_stack_map, 
map);
+       u32 id = 0;
+
+       if (key) {
+               id = *(u32 *)key;
+               if (unlikely(id >= smap->n_buckets))
+                       return -ENOENT;
+               id++;
+       }
+
+       for ( ; id < smap->n_buckets; id++) {
+               if (READ_ONCE(smap->buckets[id])) {
+                       *(u32 *)next_key = id;
+                       return 0;
+               }
+       }
+       return -ENOENT;
 }
 
 static int stack_map_update_elem(struct bpf_map *map, void *key, void *value,
-- 
2.16.0.rc1.238.g530d649a79-goog

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