Re: FAILED unresolved symbol vfs_truncate on arm64 with LLVM

Jiri Olsa
2021-02-09T21:10:14Z

On Tue, Feb 09, 2021 at 12:09:31PM -0800, Nick Desaulniers wrote:
> On Tue, Feb 9, 2021 at 11:06 AM Jiri Olsa <jolsa@redhat.com> wrote:
> >
> > On Tue, Feb 09, 2021 at 05:13:42PM +0100, Jiri Olsa wrote:
> > > On Tue, Feb 09, 2021 at 04:09:36PM +0100, Jiri Olsa wrote:
> > >
> > > SNIP
> > >
> > > > > > > > DW_AT_prototyped (true)
> > > > > > > > DW_AT_type (0x01cfdfe4 "long int")
> > > > > > > > DW_AT_external (true)
> > > > > > > >
> > > > > > >
> > > > > > > Ok, the problem appears to be not in DWARF, but in mcount_loc data.
> > > > > > > vfs_truncate's address is not recorded as ftrace-attachable, and thus
> > > > > > > pahole ignores it. I don't know why this happens and it's quite
> > > > > > > strange, given vfs_truncate is just a normal global function.
> > > > >
> > > > > right, I can't see it in mcount adresses.. but it begins with instructions
> > > > > that appears to be nops, which would suggest it's traceable
> > > > >
> > > > > 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
> > > > >
> > > > > > >
> > > > > > > I'd like to understand this issue before we try to fix it, but there
> > > > > > > is at least one improvement we can make: pahole should check ftrace
> > > > > > > addresses only for static functions, not the global ones (global ones
> > > > > > > should be always attachable, unless they are special, e.g., notrace
> > > > > > > and stuff). We can easily check that by looking at the corresponding
> > > > > > > symbol. But I'd like to verify that vfs_truncate is ftrace-attachable
> > > >
> > > > I'm still trying to build the kernel.. however ;-)
> > >
> > > I finally reproduced.. however arm's not using mcount_loc
> > > but some other special section.. so it's new mess for me
> >
> > so ftrace data actualy has vfs_truncate address but with extra 4 bytes:
> >
> > ffff80001031f434
> >
> > real vfs_truncate address:
> >
> > ffff80001031f430 g F .text 0000000000000168 vfs_truncate
> >
> > 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?)

I see, I think we can't take ftrace addresses as start of the function
because there could be extra instruction(s) related to the call before
it like here

we need to check ftrace address be within the function/symbol,
not exact start

jirka