Messages in this thread Patch in this message | | | From | Roberto Sassu <> | Subject | [RFC][PATCH v2 7/7] selftests/bpf: Change return value in test_libbpf_get_fd_by_id_opts.c | Date | Wed, 7 Dec 2022 18:24:34 +0100 |
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From: Roberto Sassu <roberto.sassu@huawei.com>
In the no_alu32 version, the eBPF assembly looks like:
0: b7 00 00 00 00 00 00 00 r0 = 0 1: 79 12 00 00 00 00 00 00 r2 = *(u64 *)(r1 + 0) 2: 18 03 00 00 00 00 00 00 00 00 00 00 00 00 00 00 r3 = 0 ll 4: 5d 32 04 00 00 00 00 00 if r2 != r3 goto +4 <LBB0_2> 5: 79 10 08 00 00 00 00 00 r0 = *(u64 *)(r1 + 8) 6: 67 00 00 00 3e 00 00 00 r0 <<= 62 7: c7 00 00 00 3f 00 00 00 r0 s>>= 63 8: 57 00 00 00 f3 ff ff ff r0 &= -13
Unfortunately, ANDing of negative numbers is not yet supported in the verifier. As a consequence, current bounds are lost in the AND operation, resulting in estimating a positive return value, even if there isn't.
For now, return -EPERM instead of -EACCES, to remove the AND operation and let the verifier know that the return value is legitimate (negative or zero).
Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com> --- .../selftests/bpf/progs/test_libbpf_get_fd_by_id_opts.c | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-)
diff --git a/tools/testing/selftests/bpf/progs/test_libbpf_get_fd_by_id_opts.c b/tools/testing/selftests/bpf/progs/test_libbpf_get_fd_by_id_opts.c index f5ac5f3e8919..a143dbbd5573 100644 --- a/tools/testing/selftests/bpf/progs/test_libbpf_get_fd_by_id_opts.c +++ b/tools/testing/selftests/bpf/progs/test_libbpf_get_fd_by_id_opts.c @@ -29,8 +29,13 @@ int BPF_PROG(check_access, struct bpf_map *map, fmode_t fmode) if (map != (struct bpf_map *)&data_input) return 0; + /* + * Prefer -EPERM to -EACCES to avoid ANDing negative numbers in the + * no_alu32 version, which results in the current register bounds to + * be lost. + */ if (fmode & FMODE_WRITE) - return -EACCES; + return -EPERM; return 0; } -- 2.25.1
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