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

[Andy Lutomirski <luto@amacapital.net>]

On Fri, Mar 13, 2015 at 3:31 PM,  <josh@joshtriplett.org> 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@joshtriplett.org> 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@joshtriplett.org>
>> > Signed-off-by: Thiago Macieira <thiago.macieira@intel.com>
>> > ---
>> >  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.

>
>> 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.

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@kernel.org> # assuming all bitnesses boot

--Andy

>
> (I'm wondering whose tree this series ought to go through, for that
> matter.)
>
> - Josh Triplett



-- 
Andy Lutomirski
AMA Capital Management, LLC