Re: [PATCH bpf 2/2] bpf, mips: limit to 33 tail calls

From: Paul Burton <paulburton@kernel.org>
To: Paul Chaignon <paul.chaignon@orange.com>
Cc: "Björn Töpel" <bjorn.topel@gmail.com>,
	"Mahshid Khezri" <khezri.mahshid@gmail.com>,
	paul.chaignon@gmail.com, bpf@vger.kernel.org,
	netdev@vger.kernel.org, "Alexei Starovoitov" <ast@kernel.org>,
	"Daniel Borkmann" <daniel@iogearbox.net>,
	"Martin KaFai Lau" <kafai@fb.com>,
	"Song Liu" <songliubraving@fb.com>, "Yonghong Song" <yhs@fb.com>,
	"Andrii Nakryiko" <andriin@fb.com>
Subject: Re: [PATCH bpf 2/2] bpf, mips: limit to 33 tail calls
Date: Tue, 10 Dec 2019 15:23:16 -0800	[thread overview]
Message-ID: <20191210232316.aastpgbirqp4yaoi@lantea.localdomain> (raw)
In-Reply-To: <b8eb2caac1c25453c539248e56ca22f74b5316af.1575916815.git.paul.chaignon@gmail.com>

Hi Paul,

On Mon, Dec 09, 2019 at 07:52:52PM +0100, Paul Chaignon 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 MIPS' JIT compiler to allow
> 33 tail calls.
> 
> Fixes: b6bd53f9c4e8 ("MIPS: Add missing file for eBPF JIT.")
> Reported-by: Mahshid Khezri <khezri.mahshid@gmail.com>
> Signed-off-by: Paul Chaignon <paul.chaignon@orange.com>

I'd be happy to take this through mips-fixes, but equally happy for it
to go through the BPF/net trees in which case:

  Acked-by: Paul Burton <paulburton@kernel.org>

Thanks,
    Paul

> ---
>  arch/mips/net/ebpf_jit.c | 9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/mips/net/ebpf_jit.c b/arch/mips/net/ebpf_jit.c
> index 46b76751f3a5..3ec69d9cbe88 100644
> --- a/arch/mips/net/ebpf_jit.c
> +++ b/arch/mips/net/ebpf_jit.c
> @@ -604,6 +604,7 @@ static void emit_const_to_reg(struct jit_ctx *ctx, int dst, u64 value)
>  static int emit_bpf_tail_call(struct jit_ctx *ctx, int this_idx)
>  {
>  	int off, b_off;
> +	int tcc_reg;
>  
>  	ctx->flags |= EBPF_SEEN_TC;
>  	/*
> @@ -616,14 +617,14 @@ static int emit_bpf_tail_call(struct jit_ctx *ctx, int this_idx)
>  	b_off = b_imm(this_idx + 1, ctx);
>  	emit_instr(ctx, bne, MIPS_R_AT, MIPS_R_ZERO, b_off);
>  	/*
> -	 * if (--TCC < 0)
> +	 * if (TCC-- < 0)
>  	 *     goto out;
>  	 */
>  	/* Delay slot */
> -	emit_instr(ctx, daddiu, MIPS_R_T5,
> -		   (ctx->flags & EBPF_TCC_IN_V1) ? MIPS_R_V1 : MIPS_R_S4, -1);
> +	tcc_reg = (ctx->flags & EBPF_TCC_IN_V1) ? MIPS_R_V1 : MIPS_R_S4;
> +	emit_instr(ctx, daddiu, MIPS_R_T5, tcc_reg, -1);
>  	b_off = b_imm(this_idx + 1, ctx);
> -	emit_instr(ctx, bltz, MIPS_R_T5, b_off);
> +	emit_instr(ctx, bltz, tcc_reg, b_off);
>  	/*
>  	 * prog = array->ptrs[index];
>  	 * if (prog == NULL)
> -- 
> 2.17.1
> 