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Date:   Mon, 18 Jan 2021 17:48:55 +0100
From:   Maciej Fijalkowski <maciej.fijalkowski@intel.com>
To:     Eelco Chaudron <echaudro@redhat.com>
Cc:     Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Lorenzo Bianconi <lorenzo@kernel.org>, bpf@vger.kernel.org,
        netdev@vger.kernel.org, brouer@redhat.com, bjorn@kernel.org,
        toke@redhat.com, john.fastabend@gmail.com
Subject: Re: [PATCH v5 bpf-next 13/14] bpf: add new frame_length field to the
 XDP ctx
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On Fri, Jan 15, 2021 at 05:36:23PM +0100, Eelco Chaudron wrote:
> 
> 
> On 16 Dec 2020, at 15:08, Eelco Chaudron wrote:
> 
> > On 15 Dec 2020, at 19:06, Maciej Fijalkowski wrote:
> > 
> > > On Tue, Dec 15, 2020 at 02:28:39PM +0100, Eelco Chaudron wrote:
> > > > 
> > > > 
> > > > On 9 Dec 2020, at 13:07, Eelco Chaudron wrote:
> > > > 
> > > > > On 9 Dec 2020, at 12:10, Maciej Fijalkowski wrote:
> > > > 
> > > > <SNIP>
> > > > 
> > > > > > > > > +
> > > > > > > > > +		ctx_reg = (si->src_reg == si->dst_reg) ? scratch_reg - 1 :
> > > > > > > > > si->src_reg;
> > > > > > > > > +		while (dst_reg == ctx_reg || scratch_reg == ctx_reg)
> > > > > > > > > +			ctx_reg--;
> > > > > > > > > +
> > > > > > > > > +		/* Save scratch registers */
> > > > > > > > > +		if (ctx_reg != si->src_reg) {
> > > > > > > > > +			*insn++ = BPF_STX_MEM(BPF_DW, si->src_reg, ctx_reg,
> > > > > > > > > +					      offsetof(struct xdp_buff,
> > > > > > > > > +						       tmp_reg[1]));
> > > > > > > > > +
> > > > > > > > > +			*insn++ = BPF_MOV64_REG(ctx_reg, si->src_reg);
> > > > > > > > > +		}
> > > > > > > > > +
> > > > > > > > > +		*insn++ = BPF_STX_MEM(BPF_DW, ctx_reg, scratch_reg,
> > > > > > > > > +				      offsetof(struct xdp_buff, tmp_reg[0]));
> > > > > > > > 
> > > > > > > > Why don't you push regs to stack, use it and then pop it
> > > > > > > > back? That way
> > > > > > > > I
> > > > > > > > suppose you could avoid polluting xdp_buff with tmp_reg[2].
> > > > > > > 
> > > > > > > There is no “real” stack in eBPF, only a read-only frame
> > > > > > > pointer, and as we
> > > > > > > are replacing a single instruction, we have no info on what we
> > > > > > > can use as
> > > > > > > scratch space.
> > > > > > 
> > > > > > Uhm, what? You use R10 for stack operations. Verifier tracks the
> > > > > > stack
> > > > > > depth used by programs and then it is passed down to JIT so that
> > > > > > native
> > > > > > asm will create a properly sized stack frame.
> > > > > > 
> > > > > > From the top of my head I would let know
> > > > > > xdp_convert_ctx_access of a
> > > > > > current stack depth and use it for R10 stores, so your
> > > > > > scratch space
> > > > > > would
> > > > > > be R10 + (stack depth + 8), R10 + (stack_depth + 16).
> > > > > 
> > > > > Other instances do exactly the same, i.e. put some scratch
> > > > > registers in
> > > > > the underlying data structure, so I reused this approach. From the
> > > > > current information in the callback, I was not able to
> > > > > determine the
> > > > > current stack_depth. With "real" stack above, I meant having
> > > > > a pop/push
> > > > > like instruction.
> > > > > 
> > > > > I do not know the verifier code well enough, but are you
> > > > > suggesting I
> > > > > can get the current stack_depth from the verifier in the
> > > > > xdp_convert_ctx_access() callback? If so any pointers?
> > > > 
> > > > Maciej any feedback on the above, i.e. getting the stack_depth in
> > > > xdp_convert_ctx_access()?
> > > 
> > > Sorry. I'll try to get my head around it. If i recall correctly stack
> > > depth is tracked per subprogram whereas convert_ctx_accesses is
> > > iterating
> > > through *all* insns (so a prog that is not chunked onto subprogs),
> > > but
> > > maybe we could dig up the subprog based on insn idx.
> > > 
> > > But at first, you mentioned that you took the approach from other
> > > instances, can you point me to them?
> > 
> > Quick search found the following two (sure there is one more with two
> > regs):
> > 
> > https://elixir.bootlin.com/linux/v5.10.1/source/kernel/bpf/cgroup.c#L1718
> > https://elixir.bootlin.com/linux/v5.10.1/source/net/core/filter.c#L8977
> > 
> > > I'd also like to hear from Daniel/Alexei/John and others their
> > > thoughts.
> > 
> > Please keep me in the loop…
> 
> Any thoughts/update on the above so I can move this patchset forward?

Cc: John, Jesper, Bjorn

I didn't spend time thinking about it, but I still am against xdp_buff
extension for the purpose that code within this patch has.

Daniel/Alexei/John/Jesper/Bjorn,

any objections for not having the scratch registers but rather use the
stack and update the stack depth to calculate the frame length?

This seems not trivial so I really would like to have an input from better
BPF developers than me :)

> 
>