On Thu, 11 Apr 2019 07:13:03 +0100, Jiong Wang wrote:
> >> @@ -1150,17 +1150,17 @@ static int mark_reg_read(struct bpf_verifier_env *env,
> >> parent->var_off.value, parent->off);
> >> return -EFAULT;
> >> }
> >> - if (parent->live & REG_LIVE_READ)
> >> + if ((parent->live & REG_LIVE_READ) == flags)
> >> /* The parentage chain never changes and
> >> - * this parent was already marked as LIVE_READ.
> >> + * this parent was already marked with all read bits.
> >
> > Do we have to propagate all read bits? Read64 is strictly stronger
> > than read32, as long as read64 is set on the parent we should be good?
>
> We should be good, but I doubt there is value to differentiate on this in this
> kind of HOT function.
The entire if clause is an optimization. I'm saying you can maintain it
as more aggressive.
> >> @@ -6196,12 +6286,19 @@ static int propagate_liveness_reg(struct bpf_verifier_env *env,
> >> struct bpf_reg_state *reg,
> >> struct bpf_reg_state *parent_reg)
> >> {
> >> + u8 parent_bits = parent_reg->live & REG_LIVE_READ;
> >> + u8 bits = reg->live & REG_LIVE_READ;
> >> + u8 bits_diff = parent_bits ^ bits;
> >> + u8 bits_prop = bits_diff & bits;
> >> int err;
> >>
> >> - if (parent_reg->live & REG_LIVE_READ || !(reg->live & REG_LIVE_READ))
> >> + /* "reg" and "parent_reg" has the same read bits, or the bit doesn't
> >> + * belong to "reg".
> >> + */
> >> + if (!bits_diff || !bits_prop)
> >
> > bits_prop is a subset of bits_diff, no? !bits_prop is always true
> > if !bits_diff is true, no need to check both.
>
> Bits_prop is a subset of bits_diff WHEN it comes from “reg", we don’t want to
> do the propagation when the diff comes from “parent_reg”, so, we need to check
> both.
Not sure what you're saying, in this patch:
u8 bits_prop = bits_diff & bits;
Maybe you're talking about some patch down the line..
