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; }