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

[Alexander Lobakin <alobakin@xxxxxxxx>]

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@xxxxxxxxx>
> > > > Signed-off-by: Paul Chaignon <paul.chaignon@xxxxxxxxxx>
> > >
> > > 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@xxxxxxxxxx>
> > >
> > > 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