lkml.org 
[lkml]   [2021]   [Jan]   [20]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
SubjectRE: gdbserver + fsgsbase kaputt
Date
> From: Andy Lutomirski <luto@kernel.org>
>
> On Tue, Jan 12, 2021 at 9:02 AM Metzger, Markus T
> <markus.t.metzger@intel.com> wrote:
> >
> > > [ 26.990644] getreg: gs_base = 0xf7f8e000
> > > [ 26.991694] getreg: GS=0x63, GSBASE=0xf7f8e000
> > > [ 26.993117] PTRACE_SETREGS
> > > [ 26.993813] putreg: change gsbase from 0xf7f8e000 to 0x0
> > > [ 26.995134] putreg: write GS=0x63; old GSBASE=0x0
> > > [ 26.996235] PTRACE_SETREGS done
> > >
> > > That's gdbserver reading GS and GSBASE and then telling the kernel to
> > > set GS to the same value and GSBASE to 0.
> > >
> > > I can come up with horrible kernel hacks to try to work around this,
> > > but gdbserver is really giving the kernel bad instructions here.
> >
> > I agree that this looks like a GDB bug rather than a kernel bug. GDB
> > should preserve the GS_BASE value if it doesn't intend to change it.
>
> Indeed. But we have this pesky no-userspace-regressions policy in the kernel.
>
> So the question I have is: is this enough of a regression that we need
> to hack around it in the kernel? The specific broken use case seems
> quite niche: 64-bit gdbserver targeting 32-bit userspace. It's taken
> two-and-a-half kernel releases for anyone to notice, because sensible
> people use plain gdb for local debugging and gdbserver for debugging
> VMs, embedded targets, and such.

IMHO I'd just fix GDB and leave it at that. The kernel changes exposed a bug
in gdbserver. Tom already submitted a fix.

I'm wondering why we're considering some ugly hacks in the kernel for what's
obviously a bug in user-space, yet ignore changes to GDB functionality (for changing
FS/GS from within GDB) we had been discussing a year ago.

Regards,
Markus.
Intel Deutschland GmbH
Registered Address: Am Campeon 10-12, 85579 Neubiberg, Germany
Tel: +49 89 99 8853-0, www.intel.de
Managing Directors: Christin Eisenschmid, Gary Kershaw
Chairperson of the Supervisory Board: Nicole Lau
Registered Office: Munich
Commercial Register: Amtsgericht Muenchen HRB 186928
\
 
 \ /
  Last update: 2021-01-20 18:19    [W:0.058 / U:0.224 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site