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Date: Thu, 10 Jun 2021 10:52:37 -0700
From: Alexei Starovoitov <>
To: Kees Cook <>
Cc: Yonghong Song <>, Dmitry Vyukov <>, 
	Kurt Manucredo <>,, 
	Andrii Nakryiko <>, Alexei Starovoitov <>, bpf <>, 
	Daniel Borkmann <>, "David S. Miller" <>, 
	Jesper Dangaard Brouer <>, John Fastabend <>, 
	Martin KaFai Lau <>, KP Singh <>, Jakub Kicinski <>, 
	LKML <>, Network Development <>, 
	Song Liu <>, syzkaller-bugs <>,, Nick Desaulniers <>, 
	Clang-Built-Linux ML <>,, 
	Shuah Khan <>, Greg Kroah-Hartman <>, 
	Kernel Hardening <>, 
	kasan-dev <>
Subject: Re: [PATCH v4] bpf: core: fix shift-out-of-bounds in ___bpf_prog_run

On Thu, Jun 10, 2021 at 10:06 AM Kees Cook <> wrote:
> > > I guess the main question: what should happen if a bpf program writer
> > > does _not_ use compiler nor check_shl_overflow()?
> I think the BPF runtime needs to make such actions defined, instead of
> doing a blind shift. It needs to check the size of the shift explicitly
> when handling the shift instruction.

Such ideas were brought up in the past and rejected.
We're not going to sacrifice performance to make behavior a bit more
'defined'. CPUs are doing it deterministically. It's the C standard
that needs fixing.

> Sure, but the point of UBSAN is to find and alert about undefined
> behavior, so we still need to fix this.

No. The undefined behavior of C standard doesn't need "fixing" most of the time.

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