Re: [PATCH bpf v2] bpf: verifier: prevent userspace memory access

[Alexei Starovoitov]

On Thu, Mar 21, 2024 at 4:05 AM Alexei Starovoitov
<alexei.starovoitov@xxxxxxxxx> wrote:
>
> On Thu, Mar 21, 2024 at 3:11 AM Puranjay Mohan <puranjay12@xxxxxxxxx> wrote:
> >
> > diff --git a/arch/s390/net/bpf_jit_comp.c b/arch/s390/net/bpf_jit_comp.c
> > index e613eebfd349..e61a51a5b4be 100644
> > --- a/arch/s390/net/bpf_jit_comp.c
> > +++ b/arch/s390/net/bpf_jit_comp.c
> > @@ -2691,3 +2691,8 @@ bool bpf_jit_supports_subprog_tailcalls(void)
> >  {
> >         return true;
> >  }
> > +
> > +u64 bpf_arch_uaddress_limit(void)
> > +{
> > +       return -ENOTSUPP;
> > +}
>
> Looks good and should work, but s390 CI is still not happy.
> Ideas?
> sock tests were not failing before. So something is going on.

I think I have an explanation.
-ENOTSUPP and u64... and later:
u64 uaddress_limit = bpf_arch_uaddress_limit()
if (uaddress_limit < 0)

I bet the compiler simply removes this check since unsigned cannot
be negative.
Odd that there is no compiler warning.

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