On Tue, 2023-11-21 at 10:14 -0800, Andrii Nakryiko wrote:
> On Tue, Nov 21, 2023 at 8:20 AM Eduard Zingerman <eddyz87@xxxxxxxxx> wrote:
> >
> > On Mon, 2023-11-20 at 16:22 -0800, Andrii Nakryiko wrote:
> > > Similar to special handling of STACK_ZERO, when reading 1/2/4 bytes from
> > > stack from slot that has register spilled into it and that register has
> > > a constant value zero, preserve that zero and mark spilled register as
> > > precise for that. This makes spilled const zero register and STACK_ZERO
> > > cases equivalent in their behavior.
> > >
> > > Signed-off-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
> > > ---
> >
> > Acked-by: Eduard Zingerman <eddyz87@xxxxxxxxx>
> >
> > [...]
> >
> > > + if (spill_cnt == size &&
> > > + tnum_is_const(reg->var_off) && reg->var_off.value == 0) {
> > > + __mark_reg_const_zero(&state->regs[dst_regno]);
> > > + /* this IS register fill, so keep insn_flags */
> > > + } else if (zero_cnt == size) {
> > > + /* similarly to mark_reg_stack_read(), preserve zeroes */
> > > + __mark_reg_const_zero(&state->regs[dst_regno]);
> > > + insn_flags = 0; /* not restoring original register state */
> >
> > nit: In case if there would be v3, could you please
> > leave a comment here, something like below:
> >
> > when check_stack_write_fixed_off() puts STACK_ZERO marks
> > for writes, not aligned on register boundary, it marks source
> > registers precise. Thus, additional precision propagation is
> > necessary in this case and insn_flags could be cleared.
> >
> > or something along these lines?
> >
>
> Hm... this seems misleading, precision propagation of original
> register on write is orthogonal to this. The real reason why we clear
> insn_flags is because there is no *register* fill here. There might
> not be any spilled register at all here, or a completely irrelevant
> spilled register. This instruction is basically `r1 = 0;`, though
> obfuscated as stack dereference. So from the backtracking side of
> things, there is no stack access here.
Hm, I mostly agree. This comment would be relevant only before patch #8
in this series.
