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

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Date:   Thu, 7 Dec 2023 10:31:05 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
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 Wed, Dec 06, 2023 at 01:39:43PM -0800, Alexei Starovoitov wrote:


> All is ok until kCFI comes into picture.
> Here we probably need to teach arch_prepare_bpf_trampoline() to emit
> different __kcfi_typeid depending on kernel function proto,
> so that caller hash checking logic won't be tripped.
> I suspect that requires to reverse engineer an algorithm of computing kcfi from clang.
> other ideas?

I was going to try and extend bpf_struct_ops with a pointer, this
pointer will point to a struct of the right type with all ops filled out
as stubs.

Then I was going to have bpf_struct_ops_map_update_elem() pass a pointer
to the stub op (using moff) into bpf_struct_ops_prepare_trampoline() and
eventually arch_prepare_bpf_trampoline().

Additionally I was going to add BPF_TRAMP_F_INDIRECT.

Then when F_INDIRECT is set, have it generate the CFI preamble based on
the stub passed -- which will have the correct preamble for that method.

At least, that's what I'm thinking now, I've yet to try and implement
it.

> > > The other case:

> In the case of bpf_for_each_map_elem() the 'bloom_callback' is a subprog
> of bpf_callback_t type.
> So the kernel is doing:
>                 ret = callback_fn((u64)(long)map, (u64)(long)&key,
>                                   (u64)(long)val, (u64)(long)callback_ctx, 0);
> and that works on all archs including 32-bit.
> The kernel is doing conversion from native calling convention to bpf calling convention
> and for lucky archs like x86-64 the conversion is a true nop.
> It's a plain indirect call to JITed bpf prog.
> Note there is no interpreter support here. This works on archs with JITs only.
> No ftrace and no trampoline.
> 
> This case is easier to make work with kCFI.
> The JIT will use:
> cfi_bpf_hash:
>       .long   __kcfi_typeid___bpf_prog_runX  
> like your patch already does.
> And will use
> extern u64 __bpf_callback_fn(u64, u64, u64, u64, u64);
> cfi_bpf_subprog_hash:
>       .long   __kcfi_typeid___bpf_callback_fn
> to JIT all subprogs. See bpf_is_subprog().

Aaah!, yes it should be trivial to use another hash value when
is_subprog in emit_prologue().

> btw there are two patchsets in progress that will touch core bits of JITs.
> This one:
> https://patchwork.kernel.org/project/netdevbpf/cover/20231201190654.1233153-1-song@kernel.org/
> and this one:
> https://patchwork.kernel.org/project/netdevbpf/cover/20231011152725.95895-1-hffilwlqm@gmail.com/
> 
> so do you mind resending your current set with get_cfi_offset() change and
> I can land it into bpf-next, so we can fix one bug at a time,
> build on top, and avoid conflicts?

I can do.

> The more we dig the more it looks like that the follow up you planned to do
> on top of this set isn't going to happen soon.
> So should be ok going through bpf-next and then you can follow up with x86 things
> after merge window?

Yes, we can do that. Plans have changed on my side too -- I'm taking a 6
week break soon, so I'll do whatever I can before I'm out, and then
continue from whatever state I find when I get back.


Thanks for the details!