From: Runqing Yang rainkin1993@gmail.com
[ Upstream commit d252a4a499a07bec21c65873f605c3a1ef52ffed ]
Background: Libbpf automatically replaces calls to BPF bpf_probe_read_{kernel,user} [_str]() helpers with bpf_probe_read[_str](), if libbpf detects that kernel doesn't support new APIs. Specifically, libbpf invokes the probe_kern_probe_read_kernel function to load a small eBPF program into the kernel in which bpf_probe_read_kernel API is invoked and lets the kernel checks whether the new API is valid. If the loading fails, libbpf considers the new API invalid and replaces it with the old API.
static int probe_kern_probe_read_kernel(void) { struct bpf_insn insns[] = { BPF_MOV64_REG(BPF_REG_1, BPF_REG_10), /* r1 = r10 (fp) */ BPF_ALU64_IMM(BPF_ADD, BPF_REG_1, -8), /* r1 += -8 */ BPF_MOV64_IMM(BPF_REG_2, 8), /* r2 = 8 */ BPF_MOV64_IMM(BPF_REG_3, 0), /* r3 = 0 */ BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0, BPF_FUNC_probe_read_kernel), BPF_EXIT_INSN(), }; int fd, insn_cnt = ARRAY_SIZE(insns);
fd = bpf_prog_load(BPF_PROG_TYPE_KPROBE, NULL, "GPL", insns, insn_cnt, NULL); return probe_fd(fd); }
Bug: On older kernel versions [0], the kernel checks whether the version number provided in the bpf syscall, matches the LINUX_VERSION_CODE. If not matched, the bpf syscall fails. eBPF However, the probe_kern_probe_read_kernel code does not set the kernel version number provided to the bpf syscall, which causes the loading process alwasys fails for old versions. It means that libbpf will replace the new API with the old one even the kernel supports the new one.
Solution: After a discussion in [1], the solution is using BPF_PROG_TYPE_TRACEPOINT program type instead of BPF_PROG_TYPE_KPROBE because kernel does not enfoce version check for tracepoint programs. I test the patch in old kernels (4.18 and 4.19) and it works well.
[0] https://elixir.bootlin.com/linux/v4.19/source/kernel/bpf/syscall.c#L1360 [1] Closes: https://github.com/libbpf/libbpf/issues/473
Signed-off-by: Runqing Yang rainkin1993@gmail.com Signed-off-by: Andrii Nakryiko andrii@kernel.org Link: https://lore.kernel.org/bpf/20220409144928.27499-1-rainkin1993@gmail.com Signed-off-by: Sasha Levin sashal@kernel.org --- tools/lib/bpf/libbpf.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c index 94a6a8543cbc..41515a770e3a 100644 --- a/tools/lib/bpf/libbpf.c +++ b/tools/lib/bpf/libbpf.c @@ -4564,7 +4564,7 @@ static int probe_kern_probe_read_kernel(void) }; int fd, insn_cnt = ARRAY_SIZE(insns);
- fd = bpf_prog_load(BPF_PROG_TYPE_KPROBE, NULL, "GPL", insns, insn_cnt, NULL); + fd = bpf_prog_load(BPF_PROG_TYPE_TRACEPOINT, NULL, "GPL", insns, insn_cnt, NULL); return probe_fd(fd); }