bpf: Fix off-by-one in tail call count limiting

Before, the interpreter allowed up to MAX_TAIL_CALL_CNT + 1 tail calls.
Now precisely MAX_TAIL_CALL_CNT is allowed, which is in line with the
behavior of the x86 JITs.

Signed-off-by: Johan Almbladh <johan.almbladh@anyfinetworks.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20210728164741.350370-1-johan.almbladh@anyfinetworks.com

diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index b1a5fc0..fe807b2 100644 (file)
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -1562,7 +1562,7 @@ select_insn:
 
                if (unlikely(index >= array->map.max_entries))
                        goto out;
-               if (unlikely(tail_call_cnt > MAX_TAIL_CALL_CNT))
+               if (unlikely(tail_call_cnt >= MAX_TAIL_CALL_CNT))
                        goto out;
 
                tail_call_cnt++;