[PATCH bpf-next v2 5/9] bpf, verifier: improve signed ranges inference for BPF_AND

[Shung-Hsi Yu]

On Tue, Jul 23, 2024 at 02:36:18PM GMT, Shung-Hsi Yu wrote:
[...]
> > +1
> > Pls document the logic in the code.
> > commit log is good, but good chunk of it probably should be copied
> > as a comment.
> > 
> > I've applied the rest of the patches and removed 'test 3' selftest.
> > Pls respin this patch and a test.
> > More than one test would be nice too.
> 
> Ack. Will send send another series that:
> 
> 1. update current patch
>   - add code comment explanation how signed ranges are deduced in
>     scalar*_min_max_and()
>   - revert 229d6db14942 "selftests/bpf: Workaround strict bpf_lsm return
>     value check."
> 2. reintroduce Xu Kuohai's "test 3" into verifier_lsm.c
> 3. add a few tests for BPF_AND's signed range deduction
>    - should it be added to verifier_bounds*.c or verifier_and.c?
> 
>      I think former, because if we later add signed range deduction for
>      BPF_OR as well...

I was curious whether there would be imminent need for signed range
deduction for BPF_OR, though looks like there is _not_.

Looking at DAGCombiner::SimplifySelectCC() it does not do the
bitwise-OR variant of what we've encountered[1,2], that is

    fold (select_cc seteq (and x, y), 0, A, -1) -> (or (sra (shl x)) A)

In other words, transforming the following theoretial C code that
returns -EACCES when certain bit is unset, and -1 when certain bit is
set

    if (fmode & FMODE_WRITE)
        return -1;
    
    return -EACCESS;

into the following instructions

    r0  <<= 62
    r0 s>>= 63 /* set => r0 = -1, unset => r0 = 0 */
    r0  |= -13 /* set => r0 = (-1 | -13) = -1, unset => r0 = (0 | -13) = -13 = -EACCESS */
	exit       /* returns either -1 or -EACCESS */

So signed ranged deduction with BPF_OR is probably just a nice-to-have
for now.

1: https://github.com/llvm/llvm-project/blob/2b78303/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp#L27657-L27684
2: neither was the setne version transformed, i.e.
   fold (select_cc setne (and x, y), 0, A, 0) -> (and (sra (shl x)) A)
   
>      then test for signed range deducation of both
>      BPF_AND and BPF_OR can live in the same file, which would be nice
>      as signed range deduction of the two are somewhat symmetric