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From: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Date: Thu, 21 Mar 2024 04:31:29 -0700
Message-ID: <CAADnVQLHrmkJ5p2gEUJkf_CRxq9gv8rcSuBm5GeZ_nUJxQOE0Q@mail.gmail.com>
Subject: Re: [PATCH bpf v2] bpf: verifier: prevent userspace memory access
To: Puranjay Mohan <puranjay12@gmail.com>, Ilya Leoshkevich <iii@linux.ibm.com>
Cc: "David S. Miller" <davem@davemloft.net>, David Ahern <dsahern@kernel.org>, 
	Alexei Starovoitov <ast@kernel.org>, Daniel Borkmann <daniel@iogearbox.net>, 
	Andrii Nakryiko <andrii@kernel.org>, Martin KaFai Lau <martin.lau@linux.dev>, 
	Eduard Zingerman <eddyz87@gmail.com>, Song Liu <song@kernel.org>, 
	Yonghong Song <yonghong.song@linux.dev>, John Fastabend <john.fastabend@gmail.com>, 
	KP Singh <kpsingh@kernel.org>, Stanislav Fomichev <sdf@google.com>, Hao Luo <haoluo@google.com>, 
	Jiri Olsa <jolsa@kernel.org>, Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>, 
	Borislav Petkov <bp@alien8.de>, Dave Hansen <dave.hansen@linux.intel.com>, X86 ML <x86@kernel.org>, 
	"H. Peter Anvin" <hpa@zytor.com>, Jean-Philippe Brucker <jean-philippe@linaro.org>, 
	Network Development <netdev@vger.kernel.org>, bpf <bpf@vger.kernel.org>, 
	LKML <linux-kernel@vger.kernel.org>
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On Thu, Mar 21, 2024 at 4:05=E2=80=AFAM Alexei Starovoitov
<alexei.starovoitov@gmail.com> wrote:
>
> On Thu, Mar 21, 2024 at 3:11=E2=80=AFAM Puranjay Mohan <puranjay12@gmail.=
com> 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 =3D 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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