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Subject[PATCH v15 3/6] arm64: Make the unwind loop in unwind() similar to other architectures
Date
From: "Madhavan T. Venkataraman" <madvenka@linux.microsoft.com>

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

Change the unwind loop in unwind() to:

while (unwind_continue(state, consume_entry, cookie))
unwind_next(state);

This is easy to understand and maintain.

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 (final frame) 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 unwind_state and check the flag in unwind_continue().

Final FP
========

Introduce a new field "final_fp" in "struct unwind_state". Initialize this
to the final frame of the stack trace:

task_pt_regs(task)->stackframe

This is where the stacktrace must terminate if it is successful. Add an
explicit comment to that effect.

Signed-off-by: Madhavan T. Venkataraman <madvenka@linux.microsoft.com>
Reviewed-by: Mark Brown <broonie@kernel.org>
---
arch/arm64/kernel/stacktrace.c | 78 ++++++++++++++++++++++------------
1 file changed, 52 insertions(+), 26 deletions(-)

diff --git a/arch/arm64/kernel/stacktrace.c b/arch/arm64/kernel/stacktrace.c
index 8e43444d50e2..c749129aba5a 100644
--- a/arch/arm64/kernel/stacktrace.c
+++ b/arch/arm64/kernel/stacktrace.c
@@ -40,6 +40,10 @@
* value.
*
* @task: The task being unwound.
+ *
+ * @final_fp: Pointer to the final frame.
+ *
+ * @failed: Unwind failed.
*/
struct unwind_state {
unsigned long fp;
@@ -51,6 +55,8 @@ struct unwind_state {
struct llist_node *kr_cur;
#endif
struct task_struct *task;
+ unsigned long final_fp;
+ bool failed;
};

static void unwind_init_common(struct unwind_state *state,
@@ -73,6 +79,10 @@ static void unwind_init_common(struct unwind_state *state,
bitmap_zero(state->stacks_done, __NR_STACK_TYPES);
state->prev_fp = 0;
state->prev_type = STACK_TYPE_UNKNOWN;
+ state->failed = false;
+
+ /* Stack trace terminates here. */
+ state->final_fp = (unsigned long)task_pt_regs(task)->stackframe;
}

/*
@@ -126,6 +136,25 @@ static inline void unwind_init_from_task(struct unwind_state *state,
state->pc = thread_saved_pc(task);
}

+static bool notrace unwind_continue(struct unwind_state *state,
+ stack_trace_consume_fn consume_entry,
+ void *cookie)
+{
+ if (state->failed) {
+ /* PC is suspect. Cannot consume it. */
+ return false;
+ }
+
+ if (!consume_entry(cookie, state->pc)) {
+ /* Caller terminated the unwind. */
+ state->failed = true;
+ return false;
+ }
+
+ return state->fp != state->final_fp;
+}
+NOKPROBE_SYMBOL(unwind_continue);
+
/*
* Unwind from one frame record (A) to the next frame record (B).
*
@@ -133,24 +162,26 @@ static inline void unwind_init_from_task(struct unwind_state *state,
* 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 unwind_state *state)
+static void notrace unwind_next(struct unwind_state *state)
{
struct task_struct *tsk = state->task;
unsigned long fp = state->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) {
+ state->failed = true;
+ return;
+ }

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

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

/*
* As stacks grow downward, any valid record on the same stack must be
@@ -166,8 +197,10 @@ static int notrace unwind_next(struct unwind_state *state)
* stack.
*/
if (info.type == state->prev_type) {
- if (fp <= state->prev_fp)
- return -EINVAL;
+ if (fp <= state->prev_fp) {
+ state->failed = true;
+ return;
+ }
} else {
set_bit(state->prev_type, state->stacks_done);
}
@@ -195,8 +228,10 @@ static int notrace unwind_next(struct unwind_state *state)
*/
orig_pc = ftrace_graph_ret_addr(tsk, NULL, state->pc,
(void *)state->fp);
- if (WARN_ON_ONCE(state->pc == orig_pc))
- return -EINVAL;
+ if (WARN_ON_ONCE(state->pc == orig_pc)) {
+ state->failed = true;
+ return;
+ }
state->pc = orig_pc;
}
#endif /* CONFIG_FUNCTION_GRAPH_TRACER */
@@ -204,23 +239,14 @@ static int notrace unwind_next(struct unwind_state *state)
if (is_kretprobe_trampoline(state->pc))
state->pc = kretprobe_find_ret_addr(tsk, (void *)state->fp, &state->kr_cur);
#endif
-
- return 0;
}
NOKPROBE_SYMBOL(unwind_next);

static void notrace unwind(struct unwind_state *state,
stack_trace_consume_fn consume_entry, void *cookie)
{
- while (1) {
- int ret;
-
- if (!consume_entry(cookie, state->pc))
- break;
- ret = unwind_next(state);
- if (ret < 0)
- break;
- }
+ while (unwind_continue(state, consume_entry, cookie))
+ unwind_next(state);
}
NOKPROBE_SYMBOL(unwind);

--
2.25.1
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