Re: [REGRESSION] x86/cpu fsgsbase breaks TLS in 32 bit rr tracees on a 64 bit system

[Kyle Huey]

On Mon, Aug 24, 2020 at 5:31 PM Andy Lutomirski <luto@xxxxxxxxxx> wrote:
>
> On Mon, Aug 24, 2020 at 4:52 PM H. Peter Anvin <hpa@xxxxxxxxx> wrote:
> >
> > On 2020-08-24 14:10, Andy Lutomirski wrote:
> > >
> > > PTRACE_READ_SEGMENT_DESCRIPTOR to read a segment descriptor.
> > >
> > > PTRACE_SET_FS / PTRACE_SET_GS: Sets FS or GS and updates the base accordingly.
> > >
> > > PTRACE_READ_SEGMENT_BASE: pass in a segment selector, get a base out.
> > > You would use this to populate the base fields.
> > >
> > > or perhaps a ptrace SETREGS variant that tries to preserve the old
> > > base semantics and magically sets the bases to match the selectors if
> > > the selectors are nonzero.
> > >
> > > Do any of these choices sound preferable to any of you?
> > >
> >
> > My suggestion would be to export the GDT and LDT as a (readonly or mostly
> > readonly) regset(s) rather than adding entirely new operations. We could allow
> > the LDT and the per-thread GDT entries to be written, subject to the same
> > limitations as the corresponding system calls.
> >
>
> That seems useful, although we'd want to do some extensive
> sanitization of the GDT.  But maybe it's obnoxious to ask Kyle and
> Robert to parse the GDT, LDT, and selector just to emulate the
> demented pre-5.9 ptrace() behavior.
>
> --Andy

We've already addressed the main issue on rr's side[0]. The only
outstanding issue is that if you record a trace with 32 bit programs
on a pre-5.9 64 bit kernel and then try to replay it on 5.9 it won't
work. If you hit this case rr will print an error message telling you
to boot your 5.9 kernel with nofsgsbase if you want to replay the
trace. I think that's probably sufficient. 32 bit is legacy stuff we
don't care that much about anyways, replaying traces on a different
kernel/machine has always been a bit dicey, and if you absolutely must
do it there is a workaround. I'm not inclined to do much work to
support the narrow remaining case.

- Kyle

[0] Namely, we're tracking fs/gsbase for 32 bit tracees on 64 bit
kernels where the fs/gsbase instructions work in new recordings now:
https://github.com/mozilla/rr/commit/c3292c75dbd8c9ce5256496108965c0442424eef