Re: [PATCH bpf-next v5 4/5] bpf: verifier: Support eliding map lookup nullness

[Eduard Zingerman <eddyz87@gmail.com>]

On Thu, 2024-12-19 at 17:40 -0700, Daniel Xu wrote:

[...]

> > Ok, thinking a bit more, the best test I can come up with is:
> > 
> >   u8 vals[8];
> >   vals[0] = 0;
> >   ...
> >   vals[6] = 0;
> >   vals[7] = 0xf;
> >   p = bpf_map_lookup_elem(... vals ...);
> >   *p = 42;
> > 
> > For LE vals as u32 should be 0x0f;
> > For BE vals as u32 should be 0xf000_0000.
> > Hence, it is not safe to remove null check for this program.
> > What would verifier think about the value of such key?
> > As far as I understand, there would be stack zero for for vals[0-6]
> > and u8 stack spill for vals[7].
> 
> Right. By checking that spill size is same as key size, we stay endian
> neutral, as constant values are tracked in native endianness.
> 
> However, if we were to start interpreting combinations of STACK_ZERO,
> STACK_MISC, and STACK_SPILL, the verifier would have to be endian aware
> (IIUC). Which makes it a somewhat interesting problem but also requires
> some thought to correctly handle the state space.

Right.

> > You were going to add a check for the spill size, which should help here.
> > So, a negative test like above that checks that verifier complains
> > that 'p' should be checked for nullness first?
> > 
> > If anyone has better test in mind, please speak-up.
> 
> I think this case reduces down to a spill_size != key_size test. As long
> as the sizes match, we don't have to worry about endianness.

Agree.