netdev - RE: FAILED unresolved symbol vfs_truncate on arm64 with LLVM

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Date:   Wed, 10 Feb 2021 11:34:25 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Nathan Chancellor' <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>
CC:     Jiri Olsa <jolsa@...hat.com>,
        Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        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>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Veronika Kabatova <vkabatov@...hat.com>,
        Jiri Olsa <jolsa@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        Daniel Kiss <daniel.kiss@....com>
Subject: RE: FAILED unresolved symbol vfs_truncate on arm64 with LLVM

> > > vfs_truncate disasm:
> > >
> > >         ffff80001031f430 <vfs_truncate>:
> > >         ffff80001031f430: 5f 24 03 d5   hint    #34
> > >         ffff80001031f434: 1f 20 03 d5   nop
> > >         ffff80001031f438: 1f 20 03 d5   nop
> > >         ffff80001031f43c: 3f 23 03 d5   hint    #25
> > >
> > > thats why we don't match it in pahole.. I checked few other functions
> > > and some have the same problem and some match the function boundary
> > >
> > > those that match don't have that first hint instrucion, like:
> > >
> > >         ffff800010321e40 <do_faccessat>:
> > >         ffff800010321e40: 1f 20 03 d5   nop
> > >         ffff800010321e44: 1f 20 03 d5   nop
> > >         ffff800010321e48: 3f 23 03 d5   hint    #25
> > >
> > > any hints about hint instructions? ;-)
> >
> > aarch64 makes *some* newer instructions reuse the "hint" ie "nop"
> > encoding space to make software backwards compatible on older hardware
> > that doesn't support such instructions.  Is this BTI, perhaps? (The
> > function is perhaps the destination of an indirect call?)
> 
> It seems like it. The issue is not reproducible when
> CONFIG_ARM64_BTI_KERNEL is not set.

Is the compiler/linker doing something 'crazy'?

If a function address is taken then the BTI instruction is placed
before the function body and the symbol moved.
But non-indirect calls still jump to the original start of the function.
(In this case the first nop.)

This saves the execution time of the BTI instruction for non-indirect
calls.

	David

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