Behavior of bpf_obj_get


Adam Drescher
 

I am seeing unexpected behavior from bpf_obj_get, although this is
likely due to my inexperience with BPF.

In a loader program, I create a pinned map at
"sys/fs/bpf/test/xdp_stats_map". In a separate statistics program, I
access the pinned map via file descriptor -- I got the file descriptor
from a call to bpf_obj_get and provided the pathname above.

In a polling loop, I call bpf_obj_get on the same pathname and compare
this value to the original. However, instead of getting the same file
descriptor, the file descriptor returned by bpf_obj_get increments by
1 each invocation (so it returns 4, 5, 6, 7, ...). I am not doing
anything externally to reload the map or change the file descriptor.
Why is this happening? Looking at samples/bpf/fds_example.c as example
usage of bpf_obj_get, I would expect to get the same file descriptor
back. Any ideas?

Relevant code (filename has already been populated elsewhere):

#ifndef PATH_MAX
#define PATH_MAX 4096
#endif
char filename[PATH_MAX];

static void stats_poll(int map_fd, __u32 map_type, int interval)
{
int fd;
while (1) {
fd = bpf_obj_get(filename);
printf("filename: %s\n", filename);
printf("bpf_obj_get: %d - map_fd: %d\n", fd, map_fd);
sleep(interval);
}
}

Relevant output:
- BPF map (bpf_map_type:6) id:42 name:xdp_stats_map key_size:4
value_size:16 max_entries:5
filename: /sys/fs/bpf/test/xdp_stats_map
bpf_obj_get: 4 - map_fd: 3

filename: /sys/fs/bpf/test/xdp_stats_map
bpf_obj_get: 5 - map_fd: 3

filename: /sys/fs/bpf/test/xdp_stats_map
bpf_obj_get: 6 - map_fd: 3

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