netdev - Re: [PATCH bpf 1/2] bpf, riscv: limit to 33 tail calls

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Date:   Mon, 9 Dec 2019 20:57:27 +0100
From:   Björn Töpel <bjorn.topel@...il.com>
To:     Paul Chaignon <paul.chaignon@...nge.com>
Cc:     Paul Burton <paulburton@...nel.org>,
        Mahshid Khezri <khezri.mahshid@...il.com>,
        paul.chaignon@...il.com, bpf <bpf@...r.kernel.org>,
        Netdev <netdev@...r.kernel.org>,
        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>,
        linux-riscv@...ts.infradead.org
Subject: Re: [PATCH bpf 1/2] bpf, riscv: limit to 33 tail calls

On Mon, 9 Dec 2019 at 19:52, Paul Chaignon <paul.chaignon@...nge.com> wrote:
>
> All BPF JIT compilers except RISC-V's and MIPS' enforce a 33-tail calls
> limit at runtime.  In addition, a test was recently added, in tailcalls2,
> to check this limit.
>
> This patch updates the tail call limit in RISC-V's JIT compiler to allow
> 33 tail calls.  I tested it using the above selftest on an emulated
> RISCV64.
>

33! ICK! ;-) Thanks for finding this!

Acked-by: Björn Töpel <bjorn.topel@...il.com>

> Fixes: 2353ecc6f91f ("bpf, riscv: add BPF JIT for RV64G")
> Reported-by: Mahshid Khezri <khezri.mahshid@...il.com>
> Signed-off-by: Paul Chaignon <paul.chaignon@...nge.com>
> ---
>  arch/riscv/net/bpf_jit_comp.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/arch/riscv/net/bpf_jit_comp.c b/arch/riscv/net/bpf_jit_comp.c
> index 5451ef3845f2..7fbf56aab661 100644
> --- a/arch/riscv/net/bpf_jit_comp.c
> +++ b/arch/riscv/net/bpf_jit_comp.c
> @@ -631,14 +631,14 @@ static int emit_bpf_tail_call(int insn, struct rv_jit_context *ctx)
>                 return -1;
>         emit(rv_bgeu(RV_REG_A2, RV_REG_T1, off >> 1), ctx);
>
> -       /* if (--TCC < 0)
> +       /* if (TCC-- < 0)
>          *     goto out;
>          */
>         emit(rv_addi(RV_REG_T1, tcc, -1), ctx);
>         off = (tc_ninsn - (ctx->ninsns - start_insn)) << 2;
>         if (is_13b_check(off, insn))
>                 return -1;
> -       emit(rv_blt(RV_REG_T1, RV_REG_ZERO, off >> 1), ctx);
> +       emit(rv_blt(tcc, RV_REG_ZERO, off >> 1), ctx);
>
>         /* prog = array->ptrs[index];
>          * if (!prog)
> --
> 2.17.1
>