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Date:   Tue, 1 Dec 2020 12:27:31 +0000
From:   Brendan Jackman <jackmanb@google.com>
To:     Yonghong Song <yhs@fb.com>
Cc:     bpf@vger.kernel.org, Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        KP Singh <kpsingh@chromium.org>,
        Florent Revest <revest@chromium.org>,
        linux-kernel@vger.kernel.org, Jann Horn <jannh@google.com>
Subject: Re: [PATCH v2 bpf-next 08/13] bpf: Add instructions for
 atomic_[cmp]xchg
Message-ID: <20201201122731.GE2114905@google.com>
References: <20201127175738.1085417-1-jackmanb@google.com>
 <20201127175738.1085417-9-jackmanb@google.com>
 <c6d1763a-9674-9af3-51c5-c1467332c22b@fb.com>
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On Fri, Nov 27, 2020 at 09:25:53PM -0800, Yonghong Song wrote:
> 
> 
> On 11/27/20 9:57 AM, Brendan Jackman wrote:
> > This adds two atomic opcodes, both of which include the BPF_FETCH
> > flag. XCHG without the BPF_FETCh flag would naturally encode
> 
> BPF_FETCH

Ack, thanks

> > atomic_set. This is not supported because it would be of limited
> > value to userspace (it doesn't imply any barriers). CMPXCHG without
> > BPF_FETCH woulud be an atomic compare-and-write. We don't have such
> > an operation in the kernel so it isn't provided to BPF either.
> > 
> > There are two significant design decisions made for the CMPXCHG
> > instruction:
> > 
> >   - To solve the issue that this operation fundamentally has 3
> >     operands, but we only have two register fields. Therefore the
> >     operand we compare against (the kernel's API calls it 'old') is
> >     hard-coded to be R0. x86 has similar design (and A64 doesn't
> >     have this problem).
> > 
> >     A potential alternative might be to encode the other operand's
> >     register number in the immediate field.
> > 
> >   - The kernel's atomic_cmpxchg returns the old value, while the C11
> >     userspace APIs return a boolean indicating the comparison
> >     result. Which should BPF do? A64 returns the old value. x86 returns
> >     the old value in the hard-coded register (and also sets a
> >     flag). That means return-old-value is easier to JIT.
> > 
> > Signed-off-by: Brendan Jackman <jackmanb@google.com>
> > ---
> >   arch/x86/net/bpf_jit_comp.c    |  8 ++++++++
> >   include/linux/filter.h         | 20 ++++++++++++++++++++
> >   include/uapi/linux/bpf.h       |  4 +++-
> >   kernel/bpf/core.c              | 20 ++++++++++++++++++++
> >   kernel/bpf/disasm.c            | 15 +++++++++++++++
> >   kernel/bpf/verifier.c          | 19 +++++++++++++++++--
> >   tools/include/linux/filter.h   | 20 ++++++++++++++++++++
> >   tools/include/uapi/linux/bpf.h |  4 +++-
> >   8 files changed, 106 insertions(+), 4 deletions(-)
> > 
> [...]
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index cd4c03b25573..c8311cc114ec 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -3601,10 +3601,13 @@ static int check_mem_access(struct bpf_verifier_env *env, int insn_idx, u32 regn
> >   static int check_atomic(struct bpf_verifier_env *env, int insn_idx, struct bpf_insn *insn)
> >   {
> >   	int err;
> > +	int load_reg;
> >   	switch (insn->imm) {
> >   	case BPF_ADD:
> >   	case BPF_ADD | BPF_FETCH:
> > +	case BPF_XCHG:
> > +	case BPF_CMPXCHG:
> >   		break;
> >   	default:
> >   		verbose(env, "BPF_ATOMIC uses invalid atomic opcode %02x\n", insn->imm);
> > @@ -3626,6 +3629,13 @@ static int check_atomic(struct bpf_verifier_env *env, int insn_idx, struct bpf_i
> >   	if (err)
> >   		return err;
> > +	if (insn->imm == BPF_CMPXCHG) {
> > +		/* check src3 operand */
> 
> better comment about what src3 means here?

Ack,  adding "Check comparison of R0 with memory location"