| Message ID | 20220409144928.27499-1-rainkin1993@gmail.com (mailing list archive) |
|---|---|
| State | Accepted |
| Commit | d252a4a499a07bec21c65873f605c3a1ef52ffed |
| Delegated to: | BPF |
| Headers | show |
| Series | [bpf-next] libbpf: Fix a bug that checking bpf_probe_read_kernel API fails in old kernels | expand |
Hello: This patch was applied to bpf/bpf-next.git (master) by Andrii Nakryiko <andrii@kernel.org>: On Sat, 9 Apr 2022 22:49:28 +0800 you wrote: > 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. > > [...] Here is the summary with links: - [bpf-next] libbpf: Fix a bug that checking bpf_probe_read_kernel API fails in old kernels https://git.kernel.org/bpf/bpf-next/c/d252a4a499a0 You are awesome, thank you!
diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c index 465b7c0996f1..bf4f7ac54ebf 100644 --- a/tools/lib/bpf/libbpf.c +++ b/tools/lib/bpf/libbpf.c @@ -4594,7 +4594,7 @@ 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); }
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] https://github.com/libbpf/libbpf/issues/473 Signed-off-by: Runqing Yang <rainkin1993@gmail.com> --- tools/lib/bpf/libbpf.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)