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Date: Wed, 30 Oct 2019 11:12:00 +0100 (CET)
From: Miroslav Benes <mbenes@...e.cz>
To: Heiko Carstens <heiko.carstens@...ibm.com>
cc: gor@...ux.ibm.com, borntraeger@...ibm.com, jpoimboe@...hat.com,
joe.lawrence@...hat.com, linux-s390@...r.kernel.org,
linux-kernel@...r.kernel.org, jikos@...nel.org, pmladek@...e.com,
nstange@...e.de, live-patching@...r.kernel.org
Subject: Re: [PATCH v2 0/3] s390/livepatch: Implement reliable stack tracing
for the consistency model
On Tue, 29 Oct 2019, Heiko Carstens wrote:
> On Tue, Oct 29, 2019 at 03:39:01PM +0100, Miroslav Benes wrote:
> > - I tried to use the existing infrastructure as much as possible with
> > one exception. I kept unwind_next_frame_reliable() next to the
> > ordinary unwind_next_frame(). I did not come up with a nice solution
> > how to integrate it. The reliable unwinding is executed on a task
> > stack only, which leads to a nice simplification. My integration
> > attempts only obfuscated the existing unwind_next_frame() which is
> > already not easy to read. Ideas are definitely welcome.
>
> Ah, now I see. So patch 2 seems to be leftover(?). Could you just send
> how the result would look like?
>
> I'd really like to have only one function, since some of the sanity
> checks you added also make sense for what we already have - so code
> would diverge from the beginning.
Ok, that is understandable. I tried a bit harder and the outcome does not
look as bad as my previous attempts (read, I gave up too early).
I deliberately split unwind_reliable/!unwind_reliable case in "No
back-chain, look for a pt_regs structure" branch, because the purpose is
different there. In !unwind_reliable case we can continue on a different
stack (if I understood the code correctly when I analyzed it in the past.
I haven't found a good documentation unfortunately :(). While in
unwind_realiable case we just check if there are pt_regs in the right
place on a task stack and stop. If there are not, error out.
It applies on top of the patch set. Only compile tested though. If it
looks ok-ish to you, I'll work on it.
Thanks
Miroslav
---
diff --git a/arch/s390/include/asm/unwind.h b/arch/s390/include/asm/unwind.h
index 87d1850d195a..282c158a3c2a 100644
--- a/arch/s390/include/asm/unwind.h
+++ b/arch/s390/include/asm/unwind.h
@@ -43,7 +43,7 @@ struct unwind_state {
void __unwind_start(struct unwind_state *state, struct task_struct *task,
struct pt_regs *regs, unsigned long first_frame,
bool unwind_reliable);
-bool unwind_next_frame(struct unwind_state *state);
+bool unwind_next_frame(struct unwind_state *state, bool unwind_reliable);
bool unwind_next_frame_reliable(struct unwind_state *state);
unsigned long unwind_get_return_address(struct unwind_state *state);
@@ -75,7 +75,7 @@ static inline struct pt_regs *unwind_get_entry_regs(struct unwind_state *state)
#define unwind_for_each_frame(state, task, regs, first_frame, unwind_reliable) \
for (unwind_start(state, task, regs, first_frame, unwind_reliable); \
!unwind_done(state); \
- unwind_next_frame(state))
+ unwind_next_frame(state, unwind_reliable))
static inline void unwind_init(void) {}
static inline void unwind_module_init(struct module *mod, void *orc_ip,
diff --git a/arch/s390/kernel/stacktrace.c b/arch/s390/kernel/stacktrace.c
index cff9ba0715e6..c5e3a37763f7 100644
--- a/arch/s390/kernel/stacktrace.c
+++ b/arch/s390/kernel/stacktrace.c
@@ -38,7 +38,7 @@ int arch_stack_walk_reliable(stack_trace_consume_fn consume_entry,
for (unwind_start(&state, task, NULL, 0, true);
!unwind_done(&state) && !unwind_error(&state);
- unwind_next_frame_reliable(&state)) {
+ unwind_next_frame(&state, true)) {
addr = unwind_get_return_address(&state);
if (!addr)
diff --git a/arch/s390/kernel/unwind_bc.c b/arch/s390/kernel/unwind_bc.c
index 8d3a1d137ad0..2a7c88b58089 100644
--- a/arch/s390/kernel/unwind_bc.c
+++ b/arch/s390/kernel/unwind_bc.c
@@ -36,7 +36,7 @@ static bool update_stack_info(struct unwind_state *state, unsigned long sp)
return true;
}
-bool unwind_next_frame(struct unwind_state *state)
+bool unwind_next_frame(struct unwind_state *state, bool unwind_reliable)
{
struct stack_info *info = &state->stack_info;
struct stack_frame *sf;
@@ -58,28 +58,59 @@ bool unwind_next_frame(struct unwind_state *state)
} else {
sf = (struct stack_frame *) state->sp;
sp = READ_ONCE_NOCHECK(sf->back_chain);
- if (likely(sp)) {
- /* Non-zero back-chain points to the previous frame */
- if (unlikely(outside_of_stack(state, sp))) {
- if (!update_stack_info(state, sp))
- goto out_err;
- }
+ /*
+ * unwind_reliable case: Idle tasks are special. The final
+ * back-chain points to nodat_stack. See CALL_ON_STACK() in
+ * smp_start_secondary() callback used in __cpu_up(). We just
+ * accept it, go to else branch and look for pt_regs.
+ */
+ if (likely(sp) &&
+ (!unwind_reliable || !(is_idle_task(state->task) &&
+ outside_of_stack(state, sp)))) {
+
+ /*
+ * Non-zero back-chain points to the previous frame. No
+ * need to update stack info when unwind_reliable is
+ * true. We should be on a task stack and everything
+ * else is an error.
+ */
+ if (unlikely(outside_of_stack(state, sp)) &&
+ ((!unwind_reliable && !update_stack_info(state, sp)) ||
+ unwind_reliable))
+ goto out_err;
+
sf = (struct stack_frame *) sp;
ip = READ_ONCE_NOCHECK(sf->gprs[8]);
reliable = true;
} else {
/* No back-chain, look for a pt_regs structure */
sp = state->sp + STACK_FRAME_OVERHEAD;
- if (!on_stack(info, sp, sizeof(struct pt_regs)))
- goto out_stop;
regs = (struct pt_regs *) sp;
- if (READ_ONCE_NOCHECK(regs->psw.mask) & PSW_MASK_PSTATE)
+
+ if (!unwind_reliable) {
+ if (!on_stack(info, sp, sizeof(struct pt_regs)))
+ goto out_stop;
+ if (READ_ONCE_NOCHECK(regs->psw.mask) & PSW_MASK_PSTATE)
+ goto out_stop;
+ ip = READ_ONCE_NOCHECK(regs->psw.addr);
+ reliable = true;
+ } else {
+ if ((unsigned long)regs != info->end - sizeof(struct pt_regs))
+ goto out_err;
+ if (!(state->task->flags & (PF_KTHREAD | PF_IDLE)) &&
+ !user_mode(regs))
+ goto out_err;
+
+ state->regs = regs;
goto out_stop;
- ip = READ_ONCE_NOCHECK(regs->psw.addr);
- reliable = true;
+ }
}
}
+ /* Sanity check: ABI requires SP to be aligned 8 bytes. */
+ if (sp & 0x7)
+ goto out_err;
+
ip = ftrace_graph_ret_addr(state->task, &state->graph_idx,
ip, (void *) sp);
@@ -98,62 +129,6 @@ bool unwind_next_frame(struct unwind_state *state)
}
EXPORT_SYMBOL_GPL(unwind_next_frame);
-bool unwind_next_frame_reliable(struct unwind_state *state)
-{
- struct stack_info *info = &state->stack_info;
- struct stack_frame *sf;
- struct pt_regs *regs;
- unsigned long sp, ip;
-
- sf = (struct stack_frame *) state->sp;
- sp = READ_ONCE_NOCHECK(sf->back_chain);
- /*
- * Idle tasks are special. The final back-chain points to nodat_stack.
- * See CALL_ON_STACK() in smp_start_secondary() callback used in
- * __cpu_up(). We just accept it, go to else branch and look for
- * pt_regs.
- */
- if (likely(sp && !(is_idle_task(state->task) &&
- outside_of_stack(state, sp)))) {
- /* Non-zero back-chain points to the previous frame */
- if (unlikely(outside_of_stack(state, sp)))
- goto out_err;
-
- sf = (struct stack_frame *) sp;
- ip = READ_ONCE_NOCHECK(sf->gprs[8]);
- } else {
- /* No back-chain, look for a pt_regs structure */
- sp = state->sp + STACK_FRAME_OVERHEAD;
- regs = (struct pt_regs *) sp;
- if ((unsigned long)regs != info->end - sizeof(struct pt_regs))
- goto out_err;
- if (!(state->task->flags & (PF_KTHREAD | PF_IDLE)) &&
- !user_mode(regs))
- goto out_err;
-
- state->regs = regs;
- goto out_stop;
- }
-
- /* Sanity check: ABI requires SP to be aligned 8 bytes. */
- if (sp & 0x7)
- goto out_err;
-
- ip = ftrace_graph_ret_addr(state->task, &state->graph_idx,
- ip, (void *) sp);
-
- /* Update unwind state */
- state->sp = sp;
- state->ip = ip;
- return true;
-
-out_err:
- state->error = true;
-out_stop:
- state->stack_info.type = STACK_TYPE_UNKNOWN;
- return false;
-}
-
void __unwind_start(struct unwind_state *state, struct task_struct *task,
struct pt_regs *regs, unsigned long sp,
bool unwind_reliable)
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