From 7541c87c9b7a7e07c84481f37f2c19063b44469b Mon Sep 17 00:00:00 2001 From: Ilya Leoshkevich Date: Mon, 28 Oct 2019 13:29:02 +0100 Subject: bpf: Allow narrow loads of bpf_sysctl fields with offset > 0 "ctx:file_pos sysctl:read read ok narrow" works on s390 by accident: it reads the wrong byte, which happens to have the expected value of 0. Improve the test by seeking to the 4th byte and expecting 4 instead of 0. This makes the latent problem apparent: the test attempts to read the first byte of bpf_sysctl.file_pos, assuming this is the least-significant byte, which is not the case on big-endian machines: a non-zero offset is needed. The point of the test is to verify narrow loads, so we cannot cheat our way out by simply using BPF_W. The existence of the test means that such loads have to be supported, most likely because llvm can generate them. Fix the test by adding a big-endian variant, which uses an offset to access the least-significant byte of bpf_sysctl.file_pos. This reveals the final problem: verifier rejects accesses to bpf_sysctl fields with offset > 0. Such accesses are already allowed for a wide range of structs: __sk_buff, bpf_sock_addr and sk_msg_md to name a few. Extend this support to bpf_sysctl by using bpf_ctx_range instead of offsetof when matching field offsets. Fixes: 7b146cebe30c ("bpf: Sysctl hook") Fixes: e1550bfe0de4 ("bpf: Add file_pos field to bpf_sysctl ctx") Fixes: 9a1027e52535 ("selftests/bpf: Test file_pos field in bpf_sysctl ctx") Signed-off-by: Ilya Leoshkevich Signed-off-by: Alexei Starovoitov Acked-by: Andrey Ignatov Acked-by: Andrii Nakryiko Link: https://lore.kernel.org/bpf/20191028122902.9763-1-iii@linux.ibm.com --- tools/testing/selftests/bpf/test_sysctl.c | 8 +++++++- 1 file changed, 7 insertions(+), 1 deletion(-) (limited to 'tools/testing/selftests/bpf/test_sysctl.c') diff --git a/tools/testing/selftests/bpf/test_sysctl.c b/tools/testing/selftests/bpf/test_sysctl.c index a320e3844b17..7c6e5b173f33 100644 --- a/tools/testing/selftests/bpf/test_sysctl.c +++ b/tools/testing/selftests/bpf/test_sysctl.c @@ -161,9 +161,14 @@ static struct sysctl_test tests[] = { .descr = "ctx:file_pos sysctl:read read ok narrow", .insns = { /* If (file_pos == X) */ +#if __BYTE_ORDER == __LITTLE_ENDIAN BPF_LDX_MEM(BPF_B, BPF_REG_7, BPF_REG_1, offsetof(struct bpf_sysctl, file_pos)), - BPF_JMP_IMM(BPF_JNE, BPF_REG_7, 0, 2), +#else + BPF_LDX_MEM(BPF_B, BPF_REG_7, BPF_REG_1, + offsetof(struct bpf_sysctl, file_pos) + 3), +#endif + BPF_JMP_IMM(BPF_JNE, BPF_REG_7, 4, 2), /* return ALLOW; */ BPF_MOV64_IMM(BPF_REG_0, 1), @@ -176,6 +181,7 @@ static struct sysctl_test tests[] = { .attach_type = BPF_CGROUP_SYSCTL, .sysctl = "kernel/ostype", .open_flags = O_RDONLY, + .seek = 4, .result = SUCCESS, }, { -- cgit v1.2.3-59-g8ed1b