RE: FAILED unresolved symbol vfs_truncate on arm64 with LLVM

David Laight
2021-02-10T11:37:41Z

> > > 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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