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libbpf: Add ability to fetch bpf_program's underlying instructions

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Add APIs providing read-only access to bpf_program BPF instructions ([0]).
This is useful for diagnostics purposes, but it also allows a cleaner
support for cloning BPF programs after libbpf did all the FD resolution
and CO-RE relocations, subprog instructions appending, etc. Currently,
cloning BPF program is possible only through hijacking a half-broken
bpf_program__set_prep() API, which doesn't really work well for anything
but most primitive programs. For instance, set_prep() API doesn't allow
adjusting BPF program load parameters which are necessary for loading
fentry/fexit BPF programs (the case where BPF program cloning is
a necessity if doing some sort of mass-attachment functionality).

Given bpf_program__set_prep() API is set to be deprecated, having
a cleaner alternative is a must. libbpf internally already keeps track
of linear array of struct bpf_insn, so it's not hard to expose it. The
only gotcha is that libbpf previously freed instructions array during
bpf_object load time, which would make this API much less useful overall,
because in between bpf_object__open() and bpf_object__load() a lot of
changes to instructions are done by libbpf.

So this patch makes libbpf hold onto prog->insns array even after BPF
program loading. I think this is a small price for added functionality
and improved introspection of BPF program code.

See retsnoop PR ([1]) for how it can be used in practice and code
savings compared to relying on bpf_program__set_prep().

  [0] Closes: https://github.com/libbpf/libbpf/issues/298
  [1] https://github.com/anakryiko/retsnoop/pull/1

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20211025224531.1088894-3-andrii@kernel.org
This commit is contained in:
Andrii Nakryiko
2021-10-25 15:45:29 -07:00
committed by Alexei Starovoitov
parent de5d0dcef6
commit 65a7fa2e4e
3 changed files with 46 additions and 4 deletions
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12
tools/lib/bpf/libbpf.c
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@@ -6653,8 +6653,6 @@ int bpf_program__load(struct bpf_program *prog, char *license, __u32 kern_ver)
out:
if (err)
pr_warn("failed to load program '%s'\n", prog->name);
zfree(&prog->insns);
prog->insns_cnt = 0;
return libbpf_err(err);
}
@@ -8143,6 +8141,16 @@ size_t bpf_program__size(const struct bpf_program *prog)
return prog->insns_cnt * BPF_INSN_SZ;
}
const struct bpf_insn *bpf_program__insns(const struct bpf_program *prog)
{
return prog->insns;
}
size_t bpf_program__insn_cnt(const struct bpf_program *prog)
{
return prog->insns_cnt;
}
int bpf_program__set_prep(struct bpf_program *prog, int nr_instances,
bpf_program_prep_t prep)
{