[PATCH 2/2] x86/dumpstack: Fix broken unwinding from exception stacks

[Jann Horn]

Commit 2e4be0d011f2 ("x86/show_trace_log_lvl: Ensure stack pointer is
aligned, again") was intended to ensure alignment of the stack pointer; but
it also moved the initialization of the "stack" variable down into the loop
header. This was likely intended as a no-op cleanup, since the commit
message does not mention it; however, this caused a behavioral change
because the value of "regs" is different between the two places.

Originally, get_stack_pointer() used the regs provided by the caller; after
that commit, get_stack_pointer() instead uses the regs at the top of the
stack frame the unwinder is looking at. Often, there are no such regs at
all, and "regs" is NULL, causing get_stack_pointer() to fall back to the
task's current stack pointer, which is not what we want here, but probably
happens to mostly work. Other times, the original regs will point to
another regs frame - in that case, the linear guess unwind logic in
show_trace_log_lvl() will start unwinding too far up the stack, causing the
first frame found by the proper unwinder to never be visited, resulting in
a stack trace consisting purely of guess lines.

Fix it by moving the "stack = " assignment back where it belongs.

Fixes: 2e4be0d011f2 ("x86/show_trace_log_lvl: Ensure stack pointer is aligned, again")
Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
---
 arch/x86/kernel/dumpstack.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/dumpstack.c b/arch/x86/kernel/dumpstack.c
index a7d562697e50..b2b118a8c09b 100644
--- a/arch/x86/kernel/dumpstack.c
+++ b/arch/x86/kernel/dumpstack.c
@@ -195,6 +195,7 @@ 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);
 
 	/*
@@ -213,9 +214,7 @@ static void show_trace_log_lvl(struct task_struct *task, struct pt_regs *regs,
 	 * - hardirq stack
 	 * - entry stack
 	 */
-	for (stack = stack ?: get_stack_pointer(task, regs);
-	     stack;
-	     stack = stack_info.next_sp) {
+	for (; stack; stack = stack_info.next_sp) {
 		const char *stack_name;
 
 		stack = PTR_ALIGN(stack, sizeof(long));

-- 
2.49.0.395.g12beb8f557-goog