x86/boot/32: Fix the end of the stack for idle tasks

The frame at the end of each idle task stack is inconsistent with real
task stacks, which have a stack frame header and a real return address
before the pt_regs area.  This inconsistency can be confusing for stack
unwinders.  It also hides useful information about what asm code was
involved in calling into C.

Fix that by changing the initial code jumps to calls.  Also add infinite
loops after the calls to make it clear that the calls don't return, and
to hang if they do.

Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Nilay Vaish <nilayvaish@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/2588f34b6fbac4ae6f6f9ead2a78d7f8d58a6341.1474480779.git.jpoimboe@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/arch/x86/kernel/head_32.S b/arch/x86/kernel/head_32.S
index 65e62256df432ccb873f1919ad988e10d32b561f..9a6f8e820ae13d1ef3b40640ffc56cdbf62d28b0 100644 (file)
--- a/arch/x86/kernel/head_32.S
+++ b/arch/x86/kernel/head_32.S
@@ -289,7 +289,8 @@ num_subarch_entries = (. - subarch_entries) / 4
 ENTRY(start_cpu0)
        movl initial_stack, %ecx
        movl %ecx, %esp
-       jmp  *(initial_code)
+       call *(initial_code)
+1:     jmp 1b
 ENDPROC(start_cpu0)
 #endif
 
@@ -470,8 +471,9 @@ ENTRY(startup_32_smp)
        xorl %eax,%eax                  # Clear LDT
        lldt %ax
 
-       pushl $0                # fake return address for unwinder
-       jmp *(initial_code)
+       call *(initial_code)
+1:     jmp 1b
+ENDPROC(startup_32_smp)
 
 #include "verify_cpu.S"