On Wed, 2021-02-10 at 21:03 +0000, Ken Moffat wrote:
> On Wed, Feb 10, 2021 at 08:49:56PM +0000, Ken Moffat wrote:
> > >
> > > Looks like I need to change the Frame pointer unwinder to the
> > > ORC unwinder to have the same config as you.
> > >
> >
> > The benefits of the ORC unwinder are mentioned at
> > https://www.kernel.org/doc/html/latest/x86/orc-unwinder.html
> >
> > It has been around for quite some time, but I probably picked it
> > up when it first appeared (test an -rc kernel, pick up new options
> > which might be useful). I guess that old configs from before its
> > introduction still default to the old unwinder.
> >
> In fact it caused trouble about 3 years ago, there are links to the
> -dev archive from around January 2018 when elfutils was still in
> BLFS, and at that time LFS had to use the frame pointer. So when
> libelf arrived in LFS I started to use it (or use it again, not sure
> which).
>
I managed to compile objtool with -g, to recompile apic.c to apic.o
(because it gets erased when objtool fails), and to run the objtool
command on it under gdb. The segfault is esay to understand:
Program received signal SIGSEGV, Segmentation fault.
0x0000000000412f71 in elf_rebuild_rela_reloc_section (sec=0xe22b10,
nr=16)
at elf.c:883
883 relocs[idx].r_info = GELF_R_INFO(reloc->sym-
>idx, reloc->type);
and the reloc struct is:
(gdb) p *reloc
$2 = {list = {next = 0xe23240, prev = 0xe23160}, hash = {next = 0x0,
pprev = 0xe23250}, {rela = {r_offset = 0, r_info = 0, r_addend =
0},
rel = {r_offset = 0, r_info = 0}}, sec = 0xe22b10, sym = 0x0,
offset = 48,
type = 2, addend = 467, idx = 0, jump_table_start = false}
So reloc->sym is zero, and reloc->sym->idx is a null dereference...
Now to understand why reloc->sym is zero is more complicated...
Pierre
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