Re: [PATCH bpf-next v4 07/11] bpf: Add instructions for atomic_[cmp]xchg

[Brendan Jackman]

Seems I never replied to this, thanks for the reviews!

On Mon, Dec 07, 2020 at 10:37:32PM -0800, John Fastabend wrote:
> 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
> > 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.
> 
> Just a nit as it looks like perhaps we get one more spin here. Would
> be nice to be explicit about what the code does here. The above reads
> like it could go either way. Just an extra line "So we use ...' would
> be enough.

Ack, adding the note.

> > Signed-off-by: Brendan Jackman <jackmanb@xxxxxxxxxx>
> > ---
> 
> One question below.
> 
> >  arch/x86/net/bpf_jit_comp.c    |  8 ++++++++
> >  include/linux/filter.h         | 22 ++++++++++++++++++++++
> >  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   | 22 ++++++++++++++++++++++
> >  tools/include/uapi/linux/bpf.h |  4 +++-
> >  8 files changed, 110 insertions(+), 4 deletions(-)
> > 
> 
> [...]
> 
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index f8c4e809297d..f5f4460b3e4e 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -3608,11 +3608,14 @@ 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 load_reg;
> >  	int err;
> >  
> >  	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);
> > @@ -3634,6 +3637,13 @@ 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 comparison of R0 with memory location */
> > +		err = check_reg_arg(env, BPF_REG_0, SRC_OP);
> > +		if (err)
> > +			return err;
> > +	}
> > +
> 
> I need to think a bit more about it, but do we need to update is_reg64()
> at all for these?

I don't think so - this all falls into the same
`if (class == BPF_STX)` case as the existing BPF_STX_XADD instruction.

> >  	if (is_pointer_value(env, insn->src_reg)) {
> >  		verbose(env, "R%d leaks addr into mem\n", insn->src_reg);
> >  		return -EACCES;
> > @@ -3664,8 +3674,13 @@ static int check_atomic(struct bpf_verifier_env *env, int insn_idx, struct bpf_i
> >  	if (!(insn->imm & BPF_FETCH))
> >  		return 0;
> >  
> > -	/* check and record load of old value into src reg  */
> > -	err = check_reg_arg(env, insn->src_reg, DST_OP);
> > +	if (insn->imm == BPF_CMPXCHG)
> > +		load_reg = BPF_REG_0;
> > +	else
> > +		load_reg = insn->src_reg;
> > +
> > +	/* check and record load of old value */
> > +	err = check_reg_arg(env, load_reg, DST_OP);
> >  	if (err)
> >  		return err;