x86/show_trace_log_lvl: ensure stack pointer is aligned, again

From: Vernon Lovejoy <[email protected]>

The commit e335bb51cc15 ("x86/unwind: Ensure stack pointer is aligned") tried to align the stack pointer in show_trace_log_lvl(), otherwise the "stack < stack_info.end" check can't guarantee that the last read does not go past the end of the stack.

However, we have the same problem with the initial value of the stack pointer, it can also be unaligned. So without this patch this trivial kernel module

	#include <linux/module.h>

	static int init(void)
	{
		asm volatile("sub    $0x4,%rsp");
		dump_stack();
		asm volatile("add    $0x4,%rsp");

		return -EAGAIN;
	}

	module_init(init);
	MODULE_LICENSE("GPL");

crashes the kernel.

Fixes e335bb51cc15 ("x86/unwind: Ensure stack pointer is aligned")

Signed-off-by Vernon Lovejoy <[email protected]>

Signed-off-by Oleg Nesterov <[email protected]>

---
 arch/x86/kernel/dumpstack.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

arch/x86/kernel/dumpstack.c +5 -2

index 0bf6779187dd..f18ca44c904b 100644
@@ -195,7 +195,6 @@ static void show_trace_log_lvl(struct task_struct *task, struct pt_regs *regs,
 	printk("%sCall Trace:\n", log_lvl);
 
 	unwind_start(&state, task, regs, stack);
-	stack = stack ? : get_stack_pointer(task, regs);
 	regs = unwind_get_entry_regs(&state, &partial);
 
 	/*

@@ -214,9 +213,13 @@ static void show_trace_log_lvl(struct task_struct *task, struct pt_regs *regs,
 	 * - hardirq stack
 	 * - entry stack
 	 */
-	for ( ; stack; stack = PTR_ALIGN(stack_info.next_sp, sizeof(long))) {
+	for (stack = stack ?: get_stack_pointer(task, regs);
+	     stack;
+	     stack = stack_info.next_sp) {
 		const char *stack_name;
 
+		stack = PTR_ALIGN(stack, sizeof(long));
+
 		if (get_stack_info(stack, task, &stack_info, &visit_mask)) {
 			/*
 			 * We weren't on a valid stack.  It's possible that

Signature · 1 line

2.25.1.362.g51ebf55

Josh, this is boring.

We are changing this simple fix for the second time, precisely following your recommendations. And for the second time, you mysteriously disappear afterward.

Please tell us if you finally agree with this version or if you think we should make changes again.