linux-kernel - Re: [PATCH bpf-next v3 07/14] bpf: Add BPF_FETCH field / create atomic_fetch

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Date:   Fri, 4 Dec 2020 09:12:21 +0000
From:   Brendan Jackman <jackmanb@...gle.com>
To:     Yonghong Song <yhs@...com>
Cc:     bpf@...r.kernel.org, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        KP Singh <kpsingh@...omium.org>,
        Florent Revest <revest@...omium.org>,
        linux-kernel@...r.kernel.org, Jann Horn <jannh@...gle.com>
Subject: Re: [PATCH bpf-next v3 07/14] bpf: Add BPF_FETCH field / create
 atomic_fetch_add instruction

On Thu, Dec 03, 2020 at 09:27:04PM -0800, Yonghong Song wrote:
> On 12/3/20 8:02 AM, Brendan Jackman wrote:
[...]
> > diff --git a/kernel/bpf/disasm.c b/kernel/bpf/disasm.c
> > index 37c8d6e9b4cc..3ee2246a52ef 100644
> > --- a/kernel/bpf/disasm.c
> > +++ b/kernel/bpf/disasm.c
> > @@ -160,6 +160,13 @@ void print_bpf_insn(const struct bpf_insn_cbs *cbs,
> >   				bpf_ldst_string[BPF_SIZE(insn->code) >> 3],
> >   				insn->dst_reg, insn->off,
> >   				insn->src_reg);
> > +		} else if (BPF_MODE(insn->code) == BPF_ATOMIC &&
> > +			   insn->imm == (BPF_ADD | BPF_FETCH)) {
> > +			verbose(cbs->private_data, "(%02x) r%d = atomic%s_fetch_add(*(%s *)(r%d %+d), r%d)\n",
> 
> We should not do dereference here (withough first *), right? since the input
> is actually an address. something like below?
>    r2 = atomic[64]_fetch_add((u64/u32 *)(r3 +40), r2)

Ah yep  - thanks!

[...]
> > diff --git a/tools/include/linux/filter.h b/tools/include/linux/filter.h
> > index 95ff51d97f25..ac7701678e1a 100644
> > --- a/tools/include/linux/filter.h
> > +++ b/tools/include/linux/filter.h
> > @@ -180,6 +180,16 @@
> >   		.imm   = BPF_ADD })
> >   #define BPF_STX_XADD BPF_ATOMIC_ADD /* alias */
> > +/* Atomic memory add with fetch, src_reg = atomic_fetch_add(*(dst_reg + off), src_reg); */
> 
> Maybe src_reg = atomic_fetch_add(dst_reg + off, src_reg)?

Yep - and the same for the bitwise ops in the later patch.