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

[Daniel Xu]

On Thu, Dec 19, 2024 at 04:49:13PM -0800, Alexei Starovoitov wrote:
> On Thu, Dec 19, 2024 at 4:43 PM Eduard Zingerman <eddyz87@xxxxxxxxx> wrote:
> >
> > 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.
> 
> Earlier I suggested to generalize this zero/misc/spill counting
> into a helper and reuse here and in check_stack_read_fixed_off().
> 
> We do very similar checks there with a similar purpose.

Looked again, didn't see any obvious way to share code that doesn't make
it more confusing. Let me post v6 without this particular refactor. If I
missed something I'll fix it up in v7.

> 
> It sounds there are ideas to make this particular feature smarter
> than what we have in check_stack_read_fixed_off().
> Let's not overdo it.
> Even if a common helper is not possible, keep things consistent.
> The simpler the better.

Fair enough. We can keep it simple. 

Thanks,
Daniel