Re: [PATCH RFC 3/6] Use %gs as the PDA base-segment in the kernel.

Jeremy Fitzhardinge
2006-08-27T16:36:56Z

Andi Kleen wrote:
>>
>> - /* Clear %fs and %gs. */
>> - asm volatile ("movl %0, %%fs; movl %0, %%gs" : : "r" (0));
>> + /* Clear %fs. */
>> + asm volatile ("mov %0, %%fs" : : "r" (0));
>> +
>> + /* Set %gs for this CPU's PDA */
>> + asm volatile ("mov %0, %%gs" : : "r" (__KERNEL_PDA));
>>
>
> I would add memory clobbers here to make sure the dependency on read/write pda
> is right.
>

Yep. And the "m" args in the pda asm isn't quite right for rmw PDA ops
(not that there are any at the moment).

>> +1: movw GS(%esp), %gs
>>
>
> movl is recommended in 32bit mode
>

OK. I thought the assembler objected to me about it.

>> --- a/arch/i386/kernel/signal.c
>> +++ b/arch/i386/kernel/signal.c
>> @@ -128,7 +128,7 @@ restore_sigcontext(struct pt_regs *regs,
>> X86_EFLAGS_TF | X86_EFLAGS_SF | X86_EFLAGS_ZF | \
>> X86_EFLAGS_AF | X86_EFLAGS_PF | X86_EFLAGS_CF)
>>
>> - GET_SEG(gs);
>> + COPY_SEG(gs);
>> GET_SEG(fs);
>> COPY_SEG(es);
>> COPY_SEG(ds);
>> @@ -244,9 +244,7 @@ setup_sigcontext(struct sigcontext __use
>> {
>> int tmp, err = 0;
>>
>> - tmp = 0;
>> - savesegment(gs, tmp);
>> - err |= __put_user(tmp, (unsigned int __user *)&sc->gs);
>> + err |= __put_user(regs->xgs, (unsigned int __user *)&sc->gs);
>> savesegment(fs, tmp);
>> err |= __put_user(tmp, (unsigned int __user *)&sc->fs);
>>
>
> Hmm, changing it for the sc looks a bit bogus. If everything
> is right nothing should change for user space, but this changes something.
>

The sigcontext contains the userspace register state at the time of the
signal. Since userspace %gs is stored in the on-stack pt_regs, that
should be where it fetches it from to fill out the sigcontext, rather
than the kernel's internal value of %gs - in other words, it should be
the same as ds and es. Or am I missing something?

>> @@ -306,7 +306,7 @@ static void do_sys_vm86(struct kernel_vm
>> tsk->thread.screen_bitmap = info->screen_bitmap;
>> if (info->flags & VM86_SCREEN_BITMAP)
>> mark_screen_rdonly(tsk->mm);
>> - __asm__ __volatile__("xorl %eax,%eax; movl %eax,%fs; movl %eax,%gs\n\t");
>> + __asm__ __volatile__("movl %0,%%fs\n\t" : : "r" (0));
>>
>
> This is actually a useful bug fix on its own.
>

Yep. But there seems to be some other very dubious code in there as
well (the asm("mov %%eax,%0" : "=r" (eax)) sequence). I was wondering
about what it all does...

J