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

Andrii Nakryiko
2023-10-31T06:16:40Z

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?