netdev - Re: [PATCH v2 2/2] x86/cfi,bpf: Fix BPF JIT call

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Date: Mon, 4 Dec 2023 10:58:48 -0800
From: Sami Tolvanen <samitolvanen@...gle.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Jiri Olsa <olsajiri@...il.com>, Alexei Starovoitov <alexei.starovoitov@...il.com>, 
	Song Liu <song@...nel.org>, Song Liu <songliubraving@...a.com>, 
	Paul Walmsley <paul.walmsley@...ive.com>, Palmer Dabbelt <palmer@...belt.com>, 
	Albert Ou <aou@...s.berkeley.edu>, Thomas Gleixner <tglx@...utronix.de>, 
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, X86 ML <x86@...nel.org>, 
	"H. Peter Anvin" <hpa@...or.com>, "David S. Miller" <davem@...emloft.net>, David Ahern <dsahern@...nel.org>, 
	Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, 
	Andrii Nakryiko <andrii@...nel.org>, Martin KaFai Lau <martin.lau@...ux.dev>, 
	Yonghong Song <yonghong.song@...ux.dev>, John Fastabend <john.fastabend@...il.com>, 
	KP Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...gle.com>, Hao Luo <haoluo@...gle.com>, 
	Arnd Bergmann <arnd@...db.de>, Kees Cook <keescook@...omium.org>, 
	Nathan Chancellor <nathan@...nel.org>, Nick Desaulniers <ndesaulniers@...gle.com>, 
	linux-riscv <linux-riscv@...ts.infradead.org>, LKML <linux-kernel@...r.kernel.org>, 
	Network Development <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>, 
	linux-arch <linux-arch@...r.kernel.org>, clang-built-linux <llvm@...ts.linux.dev>, 
	Josh Poimboeuf <jpoimboe@...nel.org>, Joao Moreira <joao@...rdrivepizza.com>, 
	Mark Rutland <mark.rutland@....com>
Subject: Re: [PATCH v2 2/2] x86/cfi,bpf: Fix BPF JIT call

On Mon, Dec 4, 2023 at 10:34 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> So afaict this is used through bpf_for_each_map_elem(), where the
> argument still is properly callback_fn. However, in the desriptor
> bpf_for_each_map_elem_proto the argument gets described as:
> ARG_PTR_TO_FUNC, which in turn has a comment like:
>
>   ARG_PTR_TO_FUNC,        /* pointer to a bpf program function */
>
> Which to me sounds like there is definite type punning involved. The
> call in bpf_for_each_array_elem() is a regular C indirect call, which
> gets adorned with the kCFI magic.
>
> But I doubt the BPF function that gets used gets the correct matching
> bits on.
>
> TL;DR, I think this is a pre-existing problem with kCFI + eBPF and not
> caused by my patches.

It is a pre-existing problem, I ran into the same failures when I
looked into this briefly last year:

https://github.com/ClangBuiltLinux/linux/issues/1727

In addition to bpf_for_each_array_elem, a few other callers also use
the same function pointer type that doesn't match cfi_bpf_hash.

Sami