[uml-devel] [PATCH] Early x86_64 crash

[Jeff Dike <jdike@addtoit.com>]

If you are running UML/x86_64 and are seeing it crash before starting init,
I'd like you to do the following:
	- 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

Index: test/arch/um/sys-x86_64/stub_segv.c
===================================================================
--- test.orig/arch/um/sys-x86_64/stub_segv.c	2005-09-29 12:19:07.000000000 -0400
+++ test/arch/um/sys-x86_64/stub_segv.c	2005-09-29 12:21:03.000000000 -0400
@@ -11,6 +11,16 @@
 #include "sysdep/sigcontext.h"
 #include "sysdep/faultinfo.h"
 
+/* Copied from sys-x86_64/signal.c - Can't find an equivalent definition
+ * in the libc headers anywhere.
+ */
+struct rt_sigframe
+{
+        char *pretcode;
+        struct ucontext uc;
+        struct siginfo info;
+};
+
 void __attribute__ ((__section__ (".__syscall_stub")))
 stub_segv_handler(int sig)
 {
@@ -22,11 +32,14 @@
 
 	__asm__("movq %0, %%rax ; syscall": : "g" (__NR_getpid));
 	__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");
+	/* 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));
 }


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