netdev - [PATCH bpf-next 2/7] arm64: bpf: Fix off-by-one in tail call count limiting

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Date:   Mon,  9 Aug 2021 11:34:32 +0200
From:   Johan Almbladh <johan.almbladh@...finetworks.com>
To:     ast@...nel.org, daniel@...earbox.net, andrii@...nel.org
Cc:     kafai@...com, songliubraving@...com, yhs@...com,
        john.fastabend@...il.com, kpsingh@...nel.org,
        netdev@...r.kernel.org, bpf@...r.kernel.org,
        illusionist.neo@...il.com, zlim.lnx@...il.com,
        paulburton@...nel.org, naveen.n.rao@...ux.ibm.com,
        sandipan@...ux.ibm.com, luke.r.nels@...il.com, bjorn@...nel.org,
        iii@...ux.ibm.com, hca@...ux.ibm.com, gor@...ux.ibm.com,
        davem@...emloft.net, udknight@...il.com,
        Johan Almbladh <johan.almbladh@...finetworks.com>
Subject: [PATCH bpf-next 2/7] arm64: bpf: Fix off-by-one in tail call count limiting

Before, the eBPF JIT allowed up to MAX_TAIL_CALL_CNT + 1 tail calls.
Now, precisely MAX_TAIL_CALL_CNT is allowed, which is in line with the
behaviour of the interpreter. Verified with the test_bpf test suite
on qemu-system-aarch64.

Signed-off-by: Johan Almbladh <johan.almbladh@...finetworks.com>
---
 arch/arm64/net/bpf_jit_comp.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/net/bpf_jit_comp.c b/arch/arm64/net/bpf_jit_comp.c
index 41c23f474ea6..cda53e33bbec 100644
--- a/arch/arm64/net/bpf_jit_comp.c
+++ b/arch/arm64/net/bpf_jit_comp.c
@@ -286,11 +286,11 @@ static int emit_bpf_tail_call(struct jit_ctx *ctx)
 	emit(A64_CMP(0, r3, tmp), ctx);
 	emit(A64_B_(A64_COND_CS, jmp_offset), ctx);
 
-	/* if (tail_call_cnt > MAX_TAIL_CALL_CNT)
+	/* if (tail_call_cnt >= MAX_TAIL_CALL_CNT)
 	 *     goto out;
 	 * tail_call_cnt++;
 	 */
-	emit_a64_mov_i64(tmp, MAX_TAIL_CALL_CNT, ctx);
+	emit_a64_mov_i64(tmp, MAX_TAIL_CALL_CNT - 1, ctx);
 	emit(A64_CMP(1, tcc, tmp), ctx);
 	emit(A64_B_(A64_COND_HI, jmp_offset), ctx);
 	emit(A64_ADD_I(1, tcc, tcc, 1), ctx);
-- 
2.25.1