netdev - [PATCH bpf-next 3/7] powerpc: bpf: Fix off-by-one in tail call count limiting

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Date:   Mon,  9 Aug 2021 11:34:33 +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 3/7] powerpc: bpf: Fix off-by-one in tail call count limiting

Before, the eBPF JITs 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-ppc and qemu-system-ppc64, respectively.

Signed-off-by: Johan Almbladh <johan.almbladh@...finetworks.com>
---
 arch/powerpc/net/bpf_jit_comp32.c | 4 ++--
 arch/powerpc/net/bpf_jit_comp64.c | 4 ++--
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/arch/powerpc/net/bpf_jit_comp32.c b/arch/powerpc/net/bpf_jit_comp32.c
index beb12cbc8c29..6d720728df09 100644
--- a/arch/powerpc/net/bpf_jit_comp32.c
+++ b/arch/powerpc/net/bpf_jit_comp32.c
@@ -221,13 +221,13 @@ static void bpf_jit_emit_tail_call(u32 *image, struct codegen_context *ctx, u32
 	PPC_BCC(COND_GE, out);
 
 	/*
-	 * if (tail_call_cnt > MAX_TAIL_CALL_CNT)
+	 * if (tail_call_cnt >= MAX_TAIL_CALL_CNT)
 	 *   goto out;
 	 */
 	EMIT(PPC_RAW_CMPLWI(_R0, MAX_TAIL_CALL_CNT));
 	/* tail_call_cnt++; */
 	EMIT(PPC_RAW_ADDIC(_R0, _R0, 1));
-	PPC_BCC(COND_GT, out);
+	PPC_BCC(COND_GE, out);
 
 	/* prog = array->ptrs[index]; */
 	EMIT(PPC_RAW_RLWINM(_R3, b2p_index, 2, 0, 29));
diff --git a/arch/powerpc/net/bpf_jit_comp64.c b/arch/powerpc/net/bpf_jit_comp64.c
index b87a63dba9c8..2f4d24ed90a4 100644
--- a/arch/powerpc/net/bpf_jit_comp64.c
+++ b/arch/powerpc/net/bpf_jit_comp64.c
@@ -227,12 +227,12 @@ static void bpf_jit_emit_tail_call(u32 *image, struct codegen_context *ctx, u32
 	PPC_BCC(COND_GE, out);
 
 	/*
-	 * if (tail_call_cnt > MAX_TAIL_CALL_CNT)
+	 * if (tail_call_cnt >= MAX_TAIL_CALL_CNT)
 	 *   goto out;
 	 */
 	PPC_BPF_LL(b2p[TMP_REG_1], 1, bpf_jit_stack_tailcallcnt(ctx));
 	EMIT(PPC_RAW_CMPLWI(b2p[TMP_REG_1], MAX_TAIL_CALL_CNT));
-	PPC_BCC(COND_GT, out);
+	PPC_BCC(COND_GE, out);
 
 	/*
 	 * tail_call_cnt++;
-- 
2.25.1