Re: Make kernel DWARF unwinder work with ksalr.

[Mark Wielaard <mark at klomp dot org>]

Hi Torsten,

On Mon, 2018-04-23 at 22:20 +0200, Torsten Polle wrote:
> Am 22.04.2018 um 23:09 schrieb Mark Wielaard <mark@klomp.org>:
> > On Sun, 2018-04-22 at 10:52 +0200, Torsten Polle wrote:
> > > The only difference is that you leave out the alignment of s-
> > > >static_addr.
> > > s->static_addr is equal to „_stext“, which is 0xc9600358 on my test instance.
> > > While s->sec_load_offset is aligned to 0xc1000000 in a particular run. This
> > > results in a misaligned offset. While on X86 and ARM64, the offset is aligned.
> > > Without the alignment, I get strange results.
> > > 
> > > Arguably I choose a hard coded value 0x1fffff. Actually it should be based on
> > > CONFIG_PHYSICAL_ALIGN on X86, but on ARM64 it is hard coded as well.
> > 
> > On my system it is 0x200000 and it looks like sec_load_offset is
> > already aligned to that, which explains why it seems to work for me.
> 
> sec_load_offset is aligned to 0x200000. The problem is the
> „misalignment" of static_addr, which results in an offset that is not
> aligned. The problem is also difficult to spot. As a lot of results
> show nice call stacks.

OK, so the sec_load_offset which we set at translation time from the
on-disk addresses is "good". But the static_addr of the kernel/_stext
section that is set at runtime based on the _stext symbol gotten from
/proc/kallsyms is off/wrong/unaligned.

That is surprising.
You are seeing this on i386 and arm64, but not x86_64?

Could you on those systems grep _stext /proc/kallsyms to see what that
reports as _stext address?

For me (on x86_64, 3.10.0-862.el7.x86_64) it is:
ffffffff83e00000 T _stext

Thanks,

Mark