Re: [uml-devel] [PATCH] Early x86_64 crash

[Blaisorblade <blaisorblade@yahoo.it>]

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On Thursday 29 September 2005 18:16, Jeff Dike wrote:
> If you are running UML/x86_64 and are seeing it crash before starting init,
> I'd like you to do the following:
I assume you've been able to reproduce this crash and see this patch fix it, 
right?

I'm going to take it for -bs3, meanwhile, as soon as you answer on the 
attachment (I'd like it at least to compile).
> 	- make sure that you have CONFIG_FRAME_POINTER disabled
> 	- enable it and see that UML boots
> 	- apply the patch below, disable CONFIG_FRAME_POINTER again, and
> see if UML still boots
> 	- report results back to me

> 				Jeff

> @@ -22,11 +32,14 @@
>
>  	__asm__("movq %0, %%rax ; syscall": : "g" (__NR_getpid));
Why there's no "%rax" in the register clobber list, here and at the end?
>  	__asm__("movq %%rax, %%rdi ; movq %0, %%rax ; movq %1, %%rsi ;"
> -		"syscall": : "g" (__NR_kill), "g" (SIGUSR1));
> -	/* Two popqs to restore the stack to the state just before entering
> -	 * the handler, one pops the return address, the other pops the frame
> -	 * pointer.
> +		"syscall": : "g" (__NR_kill), "g" (SIGUSR1) :
> +                "%rdi", "%rax", "%rsi");
Here you're correctly informing GCC that your asm uses these regs.
> +	/* sys_sigreturn expects that the stack pointer will be 8 bytes into
> +	 * the signal frame.  So, we use the ucontext pointer, which we know
> +	 * already, to get the signal frame pointer, and add 8 to that.
>  	 */
> -	__asm__("popq %%rax ; popq %%rax ; movq %0, %%rax ; syscall" : : "g"
> -		(__NR_rt_sigreturn));
> +        __asm__("movq %0, %%rsp": :
> +                "g" ((unsigned long) container_of(uc, struct rt_sigframe,
> +						  uc) + 8));
> +	__asm__("movq %0, %%rax ; syscall" : : "g" (__NR_rt_sigreturn));
>  }
I've attached the addon with additional clobber, and with fixed tabs (i.e. 
hardtabs). Not compiled it (no time to reboot right now).
-- 
Inform me of my mistakes, so I can keep imitating Homer Simpson's "Doh!".
Paolo Giarrusso, aka Blaisorblade (Skype ID "PaoloGiarrusso", ICQ 215621894)
http://www.user-mode-linux.org/~blaisorblade

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Index: linux-2.6.13/arch/um/sys-x86_64/stub_segv.c
===================================================================
--- linux-2.6.13.orig/arch/um/sys-x86_64/stub_segv.c
+++ linux-2.6.13/arch/um/sys-x86_64/stub_segv.c
@@ -25,20 +25,20 @@ stub_segv_handler(int sig)
 {
 	struct ucontext *uc;
 
-	__asm__("movq %%rdx, %0" : "=g" (uc) :);
+	__asm__("movq %%rdx, %0" : "=g" (uc) : );
         GET_FAULTINFO_FROM_SC(*((struct faultinfo *) UML_CONFIG_STUB_DATA),
                               &uc->uc_mcontext);
 
-	__asm__("movq %0, %%rax ; syscall": : "g" (__NR_getpid));
+	__asm__("movq %0, %%rax ; syscall": : "g" (__NR_getpid) : "%rax");
 	__asm__("movq %%rax, %%rdi ; movq %0, %%rax ; movq %1, %%rsi ;"
 		"syscall": : "g" (__NR_kill), "g" (SIGUSR1) :
-                "%rdi", "%rax", "%rsi");
+		"%rdi", "%rax", "%rsi");
 	/* sys_sigreturn expects that the stack pointer will be 8 bytes into
 	 * the signal frame.  So, we use the ucontext pointer, which we know
 	 * already, to get the signal frame pointer, and add 8 to that.
 	 */
-        __asm__("movq %0, %%rsp": :
-                "g" ((unsigned long) container_of(uc, struct rt_sigframe,
+	__asm__("movq %0, %%rsp": :
+		"g" ((unsigned long) container_of(uc, struct rt_sigframe,
 						  uc) + 8));
-	__asm__("movq %0, %%rax ; syscall" : : "g" (__NR_rt_sigreturn));
+	__asm__("movq %0, %%rax ; syscall" : : "g" (__NR_rt_sigreturn) : "%rax");
 }