Re: [PATCH bpf] bpf, arm64: Fix BTI type used for freplace attached functions

[Alexei Starovoitov]

On Sat, Jul 15, 2023 at 2:03 AM Xu Kuohai <xukuohai@xxxxxxxxxxxxxxx> wrote:
>
> On 7/14/2023 12:49 AM, Alexander Duyck wrote:
> > From: Alexander Duyck <alexanderduyck@xxxxxx>
> >
> > 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 0x0000000036000003 -- 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 being
> > reached after a 'br' instruction. Further digging pointed me toward the
> > fact that the function was attached via freplace. This in turn led me to
> > 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 comment
> > explaining why this has to be modified as such.
> >
> > Fixes: b2ad54e1533e ("bpf, arm64: Implement bpf_arch_text_poke() for arm64")
> > Signed-off-by: Alexander Duyck <alexanderduyck@xxxxxx>
> > ---
> >   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_comp.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, bool 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@xxxxxxxxxx>

Applied. Thanks