Re: [PATCH v5 bpf-next 20/23] bpf: enhance BPF_JEQ/BPF_JNE is_branch_taken logic

Alexei Starovoitov
2023-10-31T16:39:18Z

On Mon, Oct 30, 2023 at 11:16 PM Andrii Nakryiko
<andrii.nakryiko@gmail.com> wrote:
>
> On Mon, Oct 30, 2023 at 7:20 PM Alexei Starovoitov
> <alexei.starovoitov@gmail.com> wrote:
> >
> > On Fri, Oct 27, 2023 at 11:13:43AM -0700, Andrii Nakryiko wrote:
> > > Use 32-bit subranges to prune some 64-bit BPF_JEQ/BPF_JNE conditions
> > > that otherwise would be "inconclusive" (i.e., is_branch_taken() would
> > > return -1). This can happen, for example, when registers are initialized
> > > as 64-bit u64/s64, then compared for inequality as 32-bit subregisters,
> > > and then followed by 64-bit equality/inequality check. That 32-bit
> > > inequality can establish some pattern for lower 32 bits of a register
> > > (e.g., s< 0 condition determines whether the bit #31 is zero or not),
> > > while overall 64-bit value could be anything (according to a value range
> > > representation).
> > >
> > > This is not a fancy quirky special case, but actually a handling that's
> > > necessary to prevent correctness issue with BPF verifier's range
> > > tracking: set_range_min_max() assumes that register ranges are
> > > non-overlapping, and if that condition is not guaranteed by
> > > is_branch_taken() we can end up with invalid ranges, where min > max.
> >
> > This is_scalar_branch_taken() logic makes sense,
> > but if set_range_min_max() is delicate, it should have its own sanity
> > check for ranges.
> > Shouldn't be difficult to check for that dangerous overlap case.
>
> So let me clarify. As far as I'm concerned, is_branch_taken() is such
> a check for set_reg_min_max, and so duplicating such checks in
> set_reg_min_max() is just that a duplication of code and logic, and
> just a chance for more typos and subtle bugs.
>
> But the concern about invalid ranges is valid, so I don't know,
> perhaps we should just do a quick check after adjustment to validate
> that umin<=umax and so on? E.g., we can do that outside of
> reg_set_min_max(), to keep reg_set_min_max() non-failing. WDYT?

Sounds like a good option too.
Just trying to minimize breakage in the future.
Sanity check before or after should catch it.