linux-kernel - [PATCH v2 4/9] x86/head/32: fix the end of the stack for idle tasks

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Date:   Wed, 21 Sep 2016 16:04:02 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Brian Gerst <brgerst@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Nilay Vaish <nilayvaish@...il.com>
Subject: [PATCH v2 4/9] x86/head/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@...hat.com>
---
 arch/x86/kernel/head_32.S | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/head_32.S b/arch/x86/kernel/head_32.S
index 617fba2..7aac1b9 100644
--- a/arch/x86/kernel/head_32.S
+++ b/arch/x86/kernel/head_32.S
@@ -288,7 +288,8 @@ 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
 
@@ -469,8 +470,9 @@ 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"
 
-- 
2.7.4