[PATCH AUTOSEL 4.14 31/59] bpf: Fix off-by-one in tail call count limiting

[Sasha Levin]

From: Johan Almbladh <johan.almbladh@xxxxxxxxxxxxxxxxx>

[ Upstream commit b61a28cf11d61f512172e673b8f8c4a6c789b425 ]

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@xxxxxxxxxxxxxxxxx>
Signed-off-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
Acked-by: Yonghong Song <yhs@xxxxxx>
Link: https://lore.kernel.org/bpf/20210728164741.350370-1-johan.almbladh@xxxxxxxxxxxxxxxxx
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 kernel/bpf/core.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index e7211b0fa27c..1d19f4fa7f44 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -1095,7 +1095,7 @@ static unsigned int ___bpf_prog_run(u64 *regs, const struct bpf_insn *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++;
-- 
2.30.2