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

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Date:   Wed, 22 Nov 2023 16:51:23 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     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>,
        Song Liu <song@...nel.org>,
        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>, Jiri Olsa <jolsa@...nel.org>,
        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 2/2] x86/cfi,bpf: Fix BPF JIT call

On Wed, Nov 22, 2023 at 4:41 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
>
> +/*
> + * Emit the various CFI preambles, see the large comment about FineIBT
> + * in arch/x86/kernel/alternative.c

.. and in cfi.h ..
which will have a copy-paste from your other email?

>                 prog->bpf_func = (void *)image + ctx.prog_offset;
>                 prog->jited = 1;
>                 prog->jited_len = proglen - ctx.prog_offset; // XXX?

Just drop XXX.

> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -1431,6 +1431,9 @@ struct bpf_prog_aux {
>         struct bpf_kfunc_desc_tab *kfunc_tab;
>         struct bpf_kfunc_btf_tab *kfunc_btf_tab;
>         u32 size_poke_tab;
> +#ifdef CONFIG_FINEIBT
> +       struct bpf_ksym ksym_prefix;
> +#endif
>         struct bpf_ksym ksym;
>         const struct bpf_prog_ops *ops;
>         struct bpf_map **used_maps;
> --- a/kernel/bpf/core.c
> +++ b/kernel/bpf/core.c
> @@ -683,6 +683,23 @@ void bpf_prog_kallsyms_add(struct bpf_pr
>         fp->aux->ksym.prog = true;
>
>         bpf_ksym_add(&fp->aux->ksym);
> +
> +#ifdef CONFIG_FINEIBT
> +       /*
> +        * When FineIBT, code in the __cfi_foo() symbols can get executed
> +        * and hence unwinder needs help.
> +        */

I like the idea!

> +       if (cfi_mode != CFI_FINEIBT)
> +               return;

The cfi_mode var needs to be global along with enum ?
Or some new helper function from arch/x86 ?

> +
> +       snprintf(fp->aux->ksym_prefix.name, KSYM_NAME_LEN,
> +                "__cfi_%s", fp->aux->ksym.name);
> +
> +       prog->aux->ksym_prefix.start = (unsigned long) prog->bpf_func - 16;
> +       prog->aux->ksym_prefix.end   = (unsigned long) prog->bpf_func;
> +
> +       bpf_ksym_add(&fp->aux->ksym_prefix);
> +#endif
>  }
>
>  void bpf_prog_kallsyms_del(struct bpf_prog *fp)

and handle deletion of ksym_prefix here.

I think it's shaping up nicely.
Pls resend both patches as a set and cc bpf @ vger.
BPF CI will pick it up and test on arm64, x86-64, s390 with gcc and clang.
We don't do CONFIG_*IBT testing automatically, but
I can manually try that after the holidays.