netdev - [PATCH bpf-next 4/7] s390: bpf: Fix off-by-one in tail call count limiting

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Date:   Mon,  9 Aug 2021 11:34:34 +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 4/7] s390: 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-s390x.

Signed-off-by: Johan Almbladh <johan.almbladh@...finetworks.com>
---
 arch/s390/net/bpf_jit_comp.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/s390/net/bpf_jit_comp.c b/arch/s390/net/bpf_jit_comp.c
index 88419263a89a..f6cdf13285ed 100644
--- a/arch/s390/net/bpf_jit_comp.c
+++ b/arch/s390/net/bpf_jit_comp.c
@@ -1363,7 +1363,7 @@ static noinline int bpf_jit_insn(struct bpf_jit *jit, struct bpf_prog *fp,
 				 jit->prg);
 
 		/*
-		 * if (tail_call_cnt++ > MAX_TAIL_CALL_CNT)
+		 * if (tail_call_cnt++ >= MAX_TAIL_CALL_CNT)
 		 *         goto out;
 		 */
 
@@ -1377,8 +1377,8 @@ static noinline int bpf_jit_insn(struct bpf_jit *jit, struct bpf_prog *fp,
 		EMIT6_DISP_LH(0xeb000000, 0x00fa, REG_W1, REG_W0, REG_15, off);
 		/* clij %w1,MAX_TAIL_CALL_CNT,0x2,out */
 		patch_2_clij = jit->prg;
-		EMIT6_PCREL_RIEC(0xec000000, 0x007f, REG_W1, MAX_TAIL_CALL_CNT,
-				 2, jit->prg);
+		EMIT6_PCREL_RIEC(0xec000000, 0x007f, REG_W1,
+				 MAX_TAIL_CALL_CNT - 1, 2, jit->prg);
 
 		/*
 		 * prog = array->ptrs[index];
-- 
2.25.1