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

From: Alexander Lobakin <alobakin@dlink.ru>
To: Paul Chaignon <paul.chaignon@orange.com>
Cc: "Paul Burton" <paulburton@kernel.org>,
	"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: Wed, 18 Dec 2019 13:02:41 +0300	[thread overview]
Message-ID: <4090532015bc8b26e89ba90f9f881d7f@dlink.ru> (raw)
In-Reply-To: <20191218095825.GA15840@Omicron>

Paul Chaignon wrote 18.12.2019 12:58:
> On Wed, Dec 18, 2019 at 12:32:53PM +0300, Alexander Lobakin wrote:
>> Paul Burton wrote 11.12.2019 02:23:
>> > 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.
>> 
>> Hi Paul,
>> 
>> You've restored MIPS cBPF in mips-fixes tree, doesn't it require any
>> changes to limit tail calls to 33? This series includes only eBPF as
>> there was no MIPS cBPF at the moment of writing.
> 
> cBPF doesn't support tail calls or even the call instruction for
> helpers.

Aww, didn't know that. Thanks!

> Paul C.
> 
>> 
>> > > 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
>> > >
>> 
>> Regards,
>> ᚷ ᛖ ᚢ ᚦ ᚠ ᚱ

Regards,
ᚷ ᛖ ᚢ ᚦ ᚠ ᚱ