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

[Andy Lutomirski]

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