From 2c118f50d7fd4d9aefc4533a26f83338b2906b7a Mon Sep 17 00:00:00 2001 From: Jann Horn Date: Tue, 25 Mar 2025 03:01:23 +0100 Subject: [PATCH] x86/dumpstack: Fix inaccurate unwinding from exception stacks due to misplaced assignment 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 Signed-off-by: Ingo Molnar Link: https://lore.kernel.org/r/20250325-2025-03-unwind-fixes-v1-2-acd774364768@google.com --- 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 91639d1e4ec2..c6fefd4585f8 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)); -- 2.25.1