On 16 Dec 2020, at 15:08, Eelco Chaudron wrote:

On 15 Dec 2020, at 19:06, Maciej Fijalkowski wrote:

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On Tue, Dec 15, 2020 at 02:28:39PM +0100, Eelco Chaudron wrote:

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On 9 Dec 2020, at 13:07, Eelco Chaudron wrote:

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On 9 Dec 2020, at 12:10, Maciej Fijalkowski wrote:

<SNIP>

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+
+		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

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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?