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From:   Alexei Starovoitov <alexei.starovoitov@gmail.com>
Date:   Tue, 18 Jul 2023 15:28:48 -0700
Message-ID: <CAADnVQK7+cUi2BvwNc+zdYj1SN1iMMe7Vgc5TkE+_MCATAuzUQ@mail.gmail.com>
Subject: Re: [PATCH bpf] bpf, arm64: Fix BTI type used for freplace attached functions
To:     Xu Kuohai <xukuohai@huaweicloud.com>
Cc:     Alexander Duyck <alexander.duyck@gmail.com>,
        bpf <bpf@vger.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
        LKML <linux-kernel@vger.kernel.org>
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On Sat, Jul 15, 2023 at 2:03=E2=80=AFAM Xu Kuohai <xukuohai@huaweicloud.com=
> wrote:
>
> On 7/14/2023 12:49 AM, Alexander Duyck wrote:
> > From: Alexander Duyck <alexanderduyck@fb.com>
> >
> > When running an freplace attached bpf program on an arm64 system w were
> > seeing the following issue:
> >    Unhandled 64-bit el1h sync exception on CPU47, ESR 0x000000003600000=
3 -- BTI
> >
> > After a bit of work to track it down I determined that what appeared to=
 be
> > happening is that the 'bti c' at the start of the program was somehow b=
eing
> > reached after a 'br' instruction. Further digging pointed me toward the
> > fact that the function was attached via freplace. This in turn led me t=
o
> > build_plt which I believe is invoking the long jump which is triggering
> > this error.
> >
> > To resolve it we can replace the 'bti c' with 'bti jc' and add a commen=
t
> > explaining why this has to be modified as such.
> >
> > Fixes: b2ad54e1533e ("bpf, arm64: Implement bpf_arch_text_poke() for ar=
m64")
> > Signed-off-by: Alexander Duyck <alexanderduyck@fb.com>
> > ---
> >   arch/arm64/net/bpf_jit_comp.c |    8 +++++++-
> >   1 file changed, 7 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/arm64/net/bpf_jit_comp.c b/arch/arm64/net/bpf_jit_com=
p.c
> > index 145b540ec34f..ec2174838f2a 100644
> > --- a/arch/arm64/net/bpf_jit_comp.c
> > +++ b/arch/arm64/net/bpf_jit_comp.c
> > @@ -322,7 +322,13 @@ static int build_prologue(struct jit_ctx *ctx, boo=
l ebpf_from_cbpf)
> >        *
> >        */
> >
> > -     emit_bti(A64_BTI_C, ctx);
> > +     /* bpf function may be invoked by 3 instruction types:
> > +      * 1. bl, attached via freplace to bpf prog via short jump
> > +      * 2. br, attached via freplace to bpf prog via long jump
> > +      * 3. blr, working as a function pointer, used by emit_call.
> > +      * So BTI_JC should used here to support both br and blr.
> > +      */
> > +     emit_bti(A64_BTI_JC, ctx);
>
> LGTM. Thanks for the fixes.
>
> Acked-by: Xu Kuohai <xukuohai@huawei.com>

Applied. Thanks