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

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Message-ID: <CAEf4BzaEOBtrSWZTx40AdT=SQY6Qaia405KWgU-NowaqNdmpkA@mail.gmail.com>
Date: Mon, 16 Dec 2024 15:24:01 -0800
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Eduard Zingerman <eddyz87@...il.com>
Cc: Daniel Xu <dxu@...uu.xyz>, andrii@...nel.org, ast@...nel.org, shuah@...nel.org, 
	daniel@...earbox.net, john.fastabend@...il.com, martin.lau@...ux.dev, 
	song@...nel.org, yonghong.song@...ux.dev, kpsingh@...nel.org, sdf@...ichev.me, 
	haoluo@...gle.com, jolsa@...nel.org, mykolal@...com, bpf@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org, 
	netdev@...r.kernel.org
Subject: Re: [PATCH bpf-next v5 4/5] bpf: verifier: Support eliding map lookup nullness

On Fri, Dec 13, 2024 at 7:13 PM Eduard Zingerman <eddyz87@...il.com> wrote:
>
> On Fri, 2024-12-13 at 19:44 -0700, Daniel Xu wrote:
>
> [...]
>
> > > > +       /* First handle precisely tracked STACK_ZERO, up to BPF_REG_SIZE */
> > > > +       stype = state->stack[spi].slot_type;
> > > > +       for (i = 0; i < BPF_REG_SIZE && stype[i] == STACK_ZERO; i++)
> > >
> > > it's Friday and I'm lazy, but please double-check that this works for
> > > both big-endian and little-endian :)
> >
> > Any tips? Are the existing tests running thru s390x hosts in CI
> > sufficient or should I add some tests writen in C (and not BPF
> > assembler)? I can never think about endianness correctly...
>
> I think that if test operates on a key like:
>
>       valid key 15
>              v
>       0000000f   <-- written to stack as a single u64 value
>       ^^^^^^^
>     stack zero marks
>
> and is executed (e.g. using __retval annotation),
> then CI passing for s390 should be enough.

+1, something like that where for big-endian it will be all zero while
for little endian it would be 0xf (and then make sure that the test
should *fail* by making sure that 0xf is not a valid index, so NULL
check is necessary)

>
> There is a guide on how to gen a s390 environment locally:
> https://docs.kernel.org/bpf/s390.html
> I used it recently to build a vmlinux for s390 with no or minimal
> issues. Used it to boot long time ago, but don't remember if there
> were any surprises.
>
> > > with Eduard's suggestion this also becomes interesting when you have
> > > 000mmm mix (as one example), because that gives you a small range, and
> > > all values might be valid keys for arrays
> >
> > Can you define what "small range" means? What range is there with 0's?
> > Any pointers would be helpful.
>
> I think Andrii means that each 'm' adds 8 bits of range.
> E.g. range for 0000_000m is 0-255, range for 0000_00mm is 0-65535, etc.

yes, exactly, thank you, Eduard!

>
> [...]
>