Re: [PATCH 2/6] x86: Opt into HAVE_COPY_THREAD_TLS, for both 32-bit and 64-bit

[josh]

On Fri, Mar 13, 2015 at 03:45:16PM -0700, Andy Lutomirski wrote:
> On Fri, Mar 13, 2015 at 3:43 PM,  <josh@xxxxxxxxxxxxxxxx> wrote:
> > On Fri, Mar 13, 2015 at 03:38:31PM -0700, Andy Lutomirski wrote:
> >> On Fri, Mar 13, 2015 at 3:31 PM,  <josh@xxxxxxxxxxxxxxxx> wrote:
> >> > On Fri, Mar 13, 2015 at 03:01:16PM -0700, Andy Lutomirski wrote:
> >> >> On Thu, Mar 12, 2015 at 6:40 PM, Josh Triplett <josh@xxxxxxxxxxxxxxxx> wrote:
> >> >> > For 32-bit userspace on a 64-bit kernel, this requires modifying
> >> >> > stub32_clone to actually swap the appropriate arguments to match
> >> >> > CONFIG_CLONE_BACKWARDS, rather than just leaving the C argument for tls
> >> >> > broken.
> >> >> >
> >> >> > Signed-off-by: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
> >> >> > Signed-off-by: Thiago Macieira <thiago.macieira@xxxxxxxxx>
> >> >> > ---
> >> >> >  arch/x86/Kconfig             | 1 +
> >> >> >  arch/x86/ia32/ia32entry.S    | 2 +-
> >> >> >  arch/x86/kernel/process_32.c | 6 +++---
> >> >> >  arch/x86/kernel/process_64.c | 8 ++++----
> >> >> >  4 files changed, 9 insertions(+), 8 deletions(-)
> >> >> >
> >> >> > diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> >> >> > index b7d31ca..4960b0d 100644
> >> >> > --- a/arch/x86/Kconfig
> >> >> > +++ b/arch/x86/Kconfig
> >> >> > @@ -124,6 +124,7 @@ config X86
> >> >> >         select MODULES_USE_ELF_REL if X86_32
> >> >> >         select MODULES_USE_ELF_RELA if X86_64
> >> >> >         select CLONE_BACKWARDS if X86_32
> >> >> > +       select HAVE_COPY_THREAD_TLS
> >> >> >         select ARCH_USE_BUILTIN_BSWAP
> >> >> >         select ARCH_USE_QUEUE_RWLOCK
> >> >> >         select OLD_SIGSUSPEND3 if X86_32 || IA32_EMULATION
> >> >> > diff --git a/arch/x86/ia32/ia32entry.S b/arch/x86/ia32/ia32entry.S
> >> >> > index 156ebca..0286735 100644
> >> >> > --- a/arch/x86/ia32/ia32entry.S
> >> >> > +++ b/arch/x86/ia32/ia32entry.S
> >> >> > @@ -487,7 +487,7 @@ GLOBAL(\label)
> >> >> >         ALIGN
> >> >> >  GLOBAL(stub32_clone)
> >> >> >         leaq sys_clone(%rip),%rax
> >> >> > -       mov     %r8, %rcx
> >> >> > +       xchg %r8, %rcx
> >> >> >         jmp  ia32_ptregs_common
> >> >>
> >> >> Do I understand correct that whatever function this is a stub for just
> >> >> takes its arguments in the wrong order?  If so, can we just fix it
> >> >> instead of using xchg here?
> >> >
> >> > 32-bit x86 and 64-bit x86 take the arguments to clone in a different
> >> > order, and stub32_clone fixes up the argument order then calls the
> >> > 64-bit sys_clone.
> >> >
> >> > I'd love to see *all* the 32-on-64 compat stubs for clone rewritten in C
> >> > under CONFIG_COMPAT.  However, doing so would require encoding the
> >> > knowledge for each 64-bit architecture for how its corresponding 32-bit
> >> > architecture accepts arguments to clone, which is information that the
> >> > current CONFIG_CLONE_BACKWARDS{1,2,3} don't include; it would then
> >> > require cleaning up all the architecture-specific assembly stubs for
> >> > 32-bit clone entry points.
> >> >
> >> > In the meantime, doing that *just* for 32-bit x86 on 64-bit x86 doesn't
> >> > seem worth it, since it would require adding a new C entry point for
> >> > compat_sys_clone under arch/x86 somewhere.
> >> >
> >> > One cleanup at a time. :)
> >>
> >> Fine w/ me.
> >
> > Thanks.
> >
> >> >
> >> >> In general, I much prefer C code to new asm where it makes sense to
> >> >> make this tradeoff.
> >> >
> >> > Agreed completely.  However, this is at least conservation-of-asm, or
> >> > reduction if you consider the pt_regs argument-grabbing hack to be
> >> > asm-esque code.
> >> >
> >> >> Other than that, this is a huge improvement.  You'll have minor
> >> >> conflicts against -tip, though.
> >> >
> >> > Right, I've seen your current changes there.  Should be a trivial merge
> >> > though.
> >> >
> >> > Would you mind providing an ack for the series, or at least for the
> >> > first two patches?
> >>
> >> I can give you an ok-in-principle on the first two.  I'd need to stare
> >> at the awful code for a bit to understand the @!*&! clone variants to
> >> really ack them convincingly.
> >
> > I'd definitely appreciate the staring. :)
> >
> >> OTOH, it would be rather surprising if you messed it up in a way that
> >> still boots on all three variants (native 32-bit, native 64-bit, and
> >> compat).
> >>
> >> So, for the first two patches:
> >>
> >> Acked-by: Andy Lutomirski <luto@xxxxxxxxxx> # assuming all bitnesses boot
> >
> > I did test all three, not just with booting but with a thread-local
> > storage test.
> 
> And it's fairly clear that no one ever tested clone-based TLS in 32
> bits from a 64-bit ELF binary, because it was broken until very
> recently :-/

I'm not sure *anyone* other than exploit-seekers test 32-bit system
calls from a 64-bit binary. :)

> This stuff is too magical and too poorly documented for my tastes.

Agreed.  That was my reaction when I figured out what was happening with
CLONE_SETTLS and pt_regs, and my goal with the first two patches in this
series was precisely to make it *less* magical. :)

- Josh Triplett
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