linux-kernel - Re: [PATCH bpf-next v6 3/4] bpf, arm64: Impelment bpf_arch_text

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Message-ID: <YsfqFp+IkhvXPoDl@myrica>
Date:   Fri, 8 Jul 2022 09:25:58 +0100
From:   Jean-Philippe Brucker <jean-philippe@...aro.org>
To:     Xu Kuohai <xukuohai@...wei.com>
Cc:     bpf@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        Mark Rutland <mark.rutland@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Zi Shen Lim <zlim.lnx@...il.com>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        "David S . Miller" <davem@...emloft.net>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        David Ahern <dsahern@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H . Peter Anvin" <hpa@...or.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        Russell King <rmk+kernel@...linux.org.uk>,
        James Morse <james.morse@....com>,
        Hou Tao <houtao1@...wei.com>,
        Jason Wang <wangborong@...rlc.com>
Subject: Re: [PATCH bpf-next v6 3/4] bpf, arm64: Impelment
 bpf_arch_text_poke() for arm64

On Fri, Jul 08, 2022 at 10:41:46AM +0800, Xu Kuohai wrote:
> >> +/* generated prologue:
> >> + *      bti c // if CONFIG_ARM64_BTI_KERNEL
> >> + *      mov x9, lr
> >> + *      nop  // POKE_OFFSET
> >> + *      paciasp // if CONFIG_ARM64_PTR_AUTH_KERNEL
> > 
> > Any reason for the change regarding BTI and pointer auth?  We used to put
> > 'bti c' at the function entry if (BTI && !PA), or 'paciasp' if (BTI && PA),
> > because 'paciasp' is an implicit BTI.
> > 
> 
> Assuming paciasp is the first instruction if (BTI && PA), when a
> trampoline with BPF_TRAMP_F_CALL_ORIG flag attached, we'll encounter the
> following scenario.
> 
> bpf_prog:
>         paciasp // LR1
>         mov x9, lr
>         bl <trampoline> ----> trampoline:
>                                       ....
>                                       mov x10, <entry_for_CALL_ORIG>
>                                       blr x10
>                                         |
> CALL_ORIG_entry:                        |
>         bti c        <------------------|
>         stp x29, lr, [sp, #- 16]!
>         ...
>         autiasp // LR2
>         ret
> 
> Because LR1 and LR2 are not equal, the autiasp will fail!
> 
> To make this scenario work properly, the first instruction should be
> 'bti c'.

Right my mistake, this layout is also what GCC generates for normal kernel
functions when (BTI && PA), so it makes sense to use the same

Thanks,
Jean