From: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
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@xxxxxxxxxx>
---
.../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
