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

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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
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