Re: [PATCH 5/7] bpf: Add BPF_FETCH field / create atomic_fetch_add instruction

[Brendan Jackman <jackmanb@google.com>]

On Mon, Nov 23, 2020 at 10:52:57PM -0800, Alexei Starovoitov wrote:
> On Mon, Nov 23, 2020 at 05:32:00PM +0000, Brendan Jackman wrote:
> > @@ -3644,8 +3649,21 @@ static int check_atomic(struct bpf_verifier_env *env, int insn_idx, struct bpf_i
> >  		return err;
> >  
> >  	/* check whether we can write into the same memory */
> > -	return check_mem_access(env, insn_idx, insn->dst_reg, insn->off,
> > -				BPF_SIZE(insn->code), BPF_WRITE, -1, true);
> > +	err = check_mem_access(env, insn_idx, insn->dst_reg, insn->off,
> > +			       BPF_SIZE(insn->code), BPF_WRITE, -1, true);
> > +	if (err)
> > +		return err;
> > +
> > +	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 (err)
> > +		return err;
> > +	regs[insn->src_reg].type = SCALAR_VALUE;
> 
> check_reg_arg() will call mark_reg_unknown() which will set type to SCALAR_VALUE.
> What is the point of another assignment?

Yep, this is just an oversight - thanks, will remove.