I feel like the answer to this is a pointer to the relevant code, but since I can't find it... The basic question is: how does the kernel part of a bpf program identify its maps (e.g., map_k1, map_k2, map_k3) in a way that the user space program (e.g., map_u1, map_u2, map_u3) identifies the same maps and put data in to the right places? I know bcc and libbpf (from the kernel tree) are supposed to take care of this magic for us, but I'd like to better understand how it works. It's not like there's type checking across the user/kernel space boundary, but the only information passed in the bpf(2) system call is the map schema. I'm surprised there's no "map ID" that gets passed on both sides to make sure they're talking about the same map. Looking through samples/bpf/xdp_*, the kernel side of each program identifies each map by a struct bpf_map_def, which is not a kernel structure but a (random?) data type from tools/lib/bpf/libbpf.h. Somehow that gets translated (by the JIT?) into the kernel 'struct bpf_map' data structure (which contains a critical ID field) but then the identifier that the userspace gets back after the bpf(BPF_CREATE_MAP) system call is a file descriptor. Does it basically come down to declaring the maps in the same order? Any help here would be appreciated - TIA! - Rob .
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