netdev - [PATCH bpf] bpf, riscv: Fix tail call count off by one in RV32 BPF JIT

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Message-Id: <20200421002804.5118-1-luke.r.nels@gmail.com>
Date:   Mon, 20 Apr 2020 17:28:04 -0700
From:   Luke Nelson <lukenels@...washington.edu>
To:     bpf@...r.kernel.org
Cc:     Luke Nelson <luke.r.nels@...il.com>, Xi Wang <xi.wang@...il.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Andrii Nakryiko <andriin@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        Björn Töpel <bjorn.topel@...il.com>,
        netdev@...r.kernel.org, linux-riscv@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH bpf] bpf, riscv: Fix tail call count off by one in RV32 BPF JIT

This patch fixes an off by one error in the RV32 JIT handling for BPF
tail call. Currently, the code decrements TCC before checking if it
is less than zero. This limits the maximum number of tail calls to 32
instead of 33 as in other JITs. The fix is to instead check the old
value of TCC before decrementing.

Fixes: 5f316b65e99f ("riscv, bpf: Add RV32G eBPF JIT")
Signed-off-by: Luke Nelson <luke.r.nels@...il.com>
---
 arch/riscv/net/bpf_jit_comp32.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/riscv/net/bpf_jit_comp32.c b/arch/riscv/net/bpf_jit_comp32.c
index 302934177760..11083d4d5f2d 100644
--- a/arch/riscv/net/bpf_jit_comp32.c
+++ b/arch/riscv/net/bpf_jit_comp32.c
@@ -770,12 +770,13 @@ static int emit_bpf_tail_call(int insn, struct rv_jit_context *ctx)
 	emit_bcc(BPF_JGE, lo(idx_reg), RV_REG_T1, off, ctx);
 
 	/*
-	 * if ((temp_tcc = tcc - 1) < 0)
+	 * temp_tcc = tcc - 1;
+	 * if (tcc < 0)
 	 *   goto out;
 	 */
 	emit(rv_addi(RV_REG_T1, RV_REG_TCC, -1), ctx);
 	off = (tc_ninsn - (ctx->ninsns - start_insn)) << 2;
-	emit_bcc(BPF_JSLT, RV_REG_T1, RV_REG_ZERO, off, ctx);
+	emit_bcc(BPF_JSLT, RV_REG_TCC, RV_REG_ZERO, off, ctx);
 
 	/*
 	 * prog = array->ptrs[index];
-- 
2.17.1