[PATCH] x86: Terminate the kernel stacks for the unwinder

Always make sure RIP/EIP is 0 in the registers stored on the top
of the stack of a kernel thread. This makes sure the unwinder code
won't try a fallback but knows the stack has ended.

AK: this patch is a bit mysterious. in theory they should be terminated
anyways, but it seems to fix at least one crash. Anyways double termination
probably doesn't hurt.

Signed-off-by: Andi Kleen <ak@suse.de>

2 files changed, 10 insertions, 1 deletions

diff --git a/arch/i386/kernel/process.c b/arch/i386/kernel/process.c
index dad02a960e03..b0a07801d9df 100644
--- a/arch/i386/kernel/process.c
+++ b/arch/i386/kernel/process.c
@@ -328,6 +328,7 @@ extern void kernel_thread_helper(void);
 int kernel_thread(int (*fn)(void *), void * arg, unsigned long flags)
 {
         struct pt_regs regs;
+        int err;
 
         memset(&regs, 0, sizeof(regs));
 
@@ -342,7 +343,10 @@ int kernel_thread(int (*fn)(void *), void * arg, unsigned long flags)
         regs.eflags = X86_EFLAGS_IF | X86_EFLAGS_SF | X86_EFLAGS_PF | 0x2;
 
         /* Ok, create the new process.. */
-        return do_fork(flags | CLONE_VM | CLONE_UNTRACED, 0, &regs, 0, NULL, NULL);
+        err = do_fork(flags | CLONE_VM | CLONE_UNTRACED, 0, &regs, 0, NULL, NULL);
+        if (err == 0) /* terminate kernel stack */
+                task_pt_regs(current)->eip = 0;
+        return err;
 }
 EXPORT_SYMBOL(kernel_thread);
 
diff --git a/arch/x86_64/kernel/entry.S b/arch/x86_64/kernel/entry.S
index b8285cf1a9c3..ab9b2c4726f2 100644
--- a/arch/x86_64/kernel/entry.S
+++ b/arch/x86_64/kernel/entry.S
@@ -978,6 +978,11 @@ ENTRY(kernel_thread)
         call do_fork
         movq %rax,RAX(%rsp)
         xorl %edi,%edi
+        test %rax,%rax
+        jnz  1f
+        /* terminate stack in child */
+        movq %rdi,RIP(%rsp)
+1:
 
         /*
          * It isn't worth to check for reschedule here,