[PATCH v10 09/11] arm64: Make the unwind loop in unwind() similar to other architectures

madvenka
2021-10-15T02:59:36Z

From: "Madhavan T. Venkataraman" <madvenka@linux.microsoft.com>

Change the loop in unwind()
===========================

Change the unwind loop in unwind() to:

unwind_start(&frame, fp, pc);
while (unwind_continue(tsk, &frame, fn, data))
unwind_next(tsk, &frame);

New function unwind_continue()
==============================

Define a new function unwind_continue() that is used in the unwind loop
to check for conditions that terminate a stack trace.

The conditions checked are:

- If the bottom of the stack has been reached, terminate.

- If the consume_entry() function returns false, the caller of
unwind has asked to terminate the stack trace. So, terminate.

- If unwind_next() failed for some reason (like stack corruption),
terminate.

Do not return an error value from unwind_next()
===============================================

We want to check for terminating conditions only in unwind_continue() from
the unwinder loop. So, do not return an error value from unwind_next().
Simply set a flag in the stackframe and check the flag in unwind_continue().

Signed-off-by: Madhavan T. Venkataraman <madvenka@linux.microsoft.com>
---
arch/arm64/include/asm/stacktrace.h | 3 ++
arch/arm64/kernel/stacktrace.c | 78 ++++++++++++++++++-----------
2 files changed, 53 insertions(+), 28 deletions(-)

diff --git a/arch/arm64/include/asm/stacktrace.h b/arch/arm64/include/asm/stacktrace.h
index c239f357d779..ba2180c7d5cd 100644
--- a/arch/arm64/include/asm/stacktrace.h
+++ b/arch/arm64/include/asm/stacktrace.h
@@ -49,6 +49,8 @@ struct stack_info {
*
* @graph: When FUNCTION_GRAPH_TRACER is selected, holds the index of a
* replacement lr value in the ftrace graph stack.
+ *
+ * @failed: Unwind failed.
*/
struct stackframe {
unsigned long fp;
@@ -59,6 +61,7 @@ struct stackframe {
#ifdef CONFIG_FUNCTION_GRAPH_TRACER
int graph;
#endif
+ bool failed;
};

extern void dump_backtrace(struct pt_regs *regs, struct task_struct *tsk,
diff --git a/arch/arm64/kernel/stacktrace.c b/arch/arm64/kernel/stacktrace.c
index f4f3575f71fd..8e9e6f38c975 100644
--- a/arch/arm64/kernel/stacktrace.c
+++ b/arch/arm64/kernel/stacktrace.c
@@ -54,6 +54,7 @@ static void notrace unwind_start(struct stackframe *frame, unsigned long fp,
bitmap_zero(frame->stacks_done, __NR_STACK_TYPES);
frame->prev_fp = 0;
frame->prev_type = STACK_TYPE_UNKNOWN;
+ frame->failed = false;
}

NOKPROBE_SYMBOL(unwind_start);
@@ -65,24 +66,26 @@ NOKPROBE_SYMBOL(unwind_start);
* records (e.g. a cycle), determined based on the location and fp value of A
* and the location (but not the fp value) of B.
*/
-static int notrace unwind_next(struct task_struct *tsk,
- struct stackframe *frame)
+static void notrace unwind_next(struct task_struct *tsk,
+ struct stackframe *frame)
{
unsigned long fp = frame->fp;
struct stack_info info;

- /* Final frame; nothing to unwind */
- if (fp == (unsigned long)task_pt_regs(tsk)->stackframe)
- return -ENOENT;
-
- if (fp & 0x7)
- return -EINVAL;
+ if (fp & 0x7) {
+ frame->failed = true;
+ return;
+ }

- if (!on_accessible_stack(tsk, fp, 16, &info))
- return -EINVAL;
+ if (!on_accessible_stack(tsk, fp, 16, &info)) {
+ frame->failed = true;
+ return;
+ }

- if (test_bit(info.type, frame->stacks_done))
- return -EINVAL;
+ if (test_bit(info.type, frame->stacks_done)) {
+ frame->failed = true;
+ return;
+ }

/*
* As stacks grow downward, any valid record on the same stack must be
@@ -98,8 +101,10 @@ static int notrace unwind_next(struct task_struct *tsk,
* stack.
*/
if (info.type == frame->prev_type) {
- if (fp <= frame->prev_fp)
- return -EINVAL;
+ if (fp <= frame->prev_fp) {
+ frame->failed = true;
+ return;
+ }
} else {
set_bit(frame->prev_type, frame->stacks_done);
}
@@ -124,19 +129,44 @@ static int notrace unwind_next(struct task_struct *tsk,
* So replace it to an original value.
*/
ret_stack = ftrace_graph_get_ret_stack(tsk, frame->graph++);
- if (WARN_ON_ONCE(!ret_stack))
- return -EINVAL;
+ if (WARN_ON_ONCE(!ret_stack)) {
+ frame->failed = true;
+ return;
+ }
frame->pc = ret_stack->ret;
}
#endif /* CONFIG_FUNCTION_GRAPH_TRACER */

frame->pc = ptrauth_strip_insn_pac(frame->pc);
-
- return 0;
}

NOKPROBE_SYMBOL(unwind_next);

+static bool notrace unwind_continue(struct task_struct *task,
+ struct stackframe *frame,
+ stack_trace_consume_fn consume_entry,
+ void *cookie)
+{
+ if (frame->failed) {
+ /* PC is suspect. Cannot consume it. */
+ return false;
+ }
+
+ if (!consume_entry(cookie, frame->pc)) {
+ /* Caller terminated the unwind. */
+ frame->failed = true;
+ return false;
+ }
+
+ if (frame->fp == (unsigned long)task_pt_regs(task)->stackframe) {
+ /* Final frame; nothing to unwind */
+ return false;
+ }
+ return true;
+}
+
+NOKPROBE_SYMBOL(unwind_continue);
+
static void notrace unwind(struct task_struct *tsk,
unsigned long fp, unsigned long pc,
bool (*fn)(void *, unsigned long),
@@ -145,16 +175,8 @@ static void notrace unwind(struct task_struct *tsk,
struct stackframe frame;

unwind_start(&frame, fp, pc);
-
- while (1) {
- int ret;
-
- if (!fn(data, frame.pc))
- break;
- ret = unwind_next(tsk, &frame);
- if (ret < 0)
- break;
- }
+ while (unwind_continue(tsk, &frame, fn, data))
+ unwind_next(tsk, &frame);
}

NOKPROBE_SYMBOL(unwind);
--
2.25.1