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

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Date: Fri, 8 Dec 2023 13:04:22 -0800
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Jiri Olsa <olsajiri@...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>, Sami Tolvanen <samitolvanen@...gle.com>, 
	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 Fri, Dec 8, 2023 at 12:56 PM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Fri, Dec 08, 2023 at 12:45:51PM -0800, Alexei Starovoitov wrote:
>
> > I mean we don't need to store a pointer to a func in stubs.
> > Can it be, roughly:
> >
> > extern void bpf_tcp_ca_cong_avoid(struct sock *sk, u32 ack, u32 acked);
> > KCFI_MACRO(hash_of_cong_avoid, bpf_tcp_ca_cong_avoid);
> > u32 __array_of_kcfi_hash[] = {hash_of_cong_avoid, hash_of_set_state,...};
> >       .bpf_ops_stubs = __array_of_kcfi_hash,
>
> But then how do I index this array? The bpf_ops_stubs thing having the
> same layout at the target struct made it easy and we could use 'moff'
> for both.
>
> That also remains working if someone adds a member to the struct or
> moves some members around.

I was thinking to just moff/2 assuming u32 array will have the same order,
but I missed the fact that init() and release() in tcp_congestion_ops
come after some variables.
And you're right it's more fragile when things change in tcp_congestion_ops.
Storing u32 hash as (void *) into function pointers is just ugly.
Let's go with your initial approach.