Re: [RFC] semantics of singlestepping vs. tracer exiting

From: Oleg Nesterov
Date: Mon Sep 03 2012 - 12:59:53 EST

On 09/03, Oleg Nesterov wrote:
>
> This is another reason to move enable/disable step into ptrace_stop().
> And in fact I had the patches a loong ago, but we need to cleanup
> the usage of PT_SINGLESTEP/PT_BLOCKSTEP first. The tracer should
> simply set/clear these PT_ flags and resume the tracee which should
> check them and do user_*_single_step() in response.

Found these patches, see the attachments.... And this also fixes the
problems with DEBUGCTLMSR_BTF.

The patches should be re-diffed, but I need to recall why I didn't
send them, perhaps I noticed some problem...

Oleg.
[PATCH 1/3] ptrace_resume: set/clear PT_SINGLESTEP/PT_BLOCKSTEP

Contrary to the comment in ptrace.h, PT_SINGLESTEP is only used on
arch/xtensa, and PT_BLOCKSTEP is not used at all.

Change the arch independent ptrace_resume() to set/clear these bits
before user_enable_*_step/user_disable_single_step() and remove this
this code from arch/xtensa/kernel/ptrace.c.

Also change ptrace_init_task() to prevent the copying of these bits.

This doesn't make any difference on xtensa, and other arches do not
use these flags so far.

But, thereafter we can check task->ptrace & PT_*STEP to figure out if
this tracer wants the stepping and unlike TIF_SINGLESTEP it is always
correct in this sense and it is not arch dependent.

Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
---

include/linux/ptrace.h | 4 ++--
kernel/ptrace.c | 3 +++
arch/xtensa/kernel/ptrace.c | 2 --
3 files changed, 5 insertions(+), 4 deletions(-)

--- ptrace/include/linux/ptrace.h~1_use_PT_STEP 2011-06-28 17:50:27.000000000 +0200
+++ ptrace/include/linux/ptrace.h 2011-07-03 21:55:17.000000000 +0200
@@ -104,7 +104,6 @@

#define PT_TRACE_MASK 0x000003f4

-/* single stepping state bits (used on ARM and PA-RISC) */
#define PT_SINGLESTEP_BIT 31
#define PT_SINGLESTEP (1<<PT_SINGLESTEP_BIT)
#define PT_BLOCKSTEP_BIT 30
@@ -220,7 +219,8 @@ static inline void ptrace_init_task(stru
child->parent = child->real_parent;
child->ptrace = 0;
if (unlikely(ptrace) && (current->ptrace & PT_PTRACED)) {
- child->ptrace = current->ptrace;
+ child->ptrace = current->ptrace &
+ ~(PT_SINGLESTEP | PT_BLOCKSTEP);
__ptrace_link(child, current->parent);
}

--- ptrace/kernel/ptrace.c~1_use_PT_STEP 2011-06-28 17:50:27.000000000 +0200
+++ ptrace/kernel/ptrace.c 2011-07-03 21:55:17.000000000 +0200
@@ -599,13 +599,16 @@ static int ptrace_resume(struct task_str
clear_tsk_thread_flag(child, TIF_SYSCALL_EMU);
#endif

+ child->ptrace &= ~(PT_SINGLESTEP | PT_BLOCKSTEP);
if (is_singleblock(request)) {
if (unlikely(!arch_has_block_step()))
return -EIO;
+ child->ptrace |= PT_BLOCKSTEP;
user_enable_block_step(child);
} else if (is_singlestep(request) || is_sysemu_singlestep(request)) {
if (unlikely(!arch_has_single_step()))
return -EIO;
+ child->ptrace |= PT_SINGLESTEP;
user_enable_single_step(child);
} else {
user_disable_single_step(child);
--- ptrace/arch/xtensa/kernel/ptrace.c~1_use_PT_STEP 2011-04-06 21:33:43.000000000 +0200
+++ ptrace/arch/xtensa/kernel/ptrace.c 2011-07-03 20:30:57.000000000 +0200
@@ -33,12 +33,10 @@

void user_enable_single_step(struct task_struct *child)
{
- child->ptrace |= PT_SINGLESTEP;
}

void user_disable_single_step(struct task_struct *child)
{
- child->ptrace &= ~PT_SINGLESTEP;
}

/*
[PATCH 2/3] ptrace: shift user_*_step() from ptrace_resume() to ptrace_stop() path

Ignoring the buggy PTRACE_KILL, ptrace_resume() calls user_*_step()
when the tracee sleeps in ptrace_stop(). Now that ptrace_resume()
sets PT_SINGLE* flags, we can reassign user_*_step() from the tracer
to the tracee.

Introduce ptrace_finish_stop(), it is called by ptrace_stop() after
schedule(). Move user_*_step() call sites from ptrace_resume() to
ptrace_finish_stop().

This way:

- we can remove user_disable_single_step() from detach paths.

This is the main motivation, we can implement asynchronous
detach.

- this makes the detach-on-exit more correct, we do not leak
TIF_SINGLESTEP if the tracer dies.

- user_enable_*_step(tsk) can be implemented more efficiently
if tsk == current, we can avoid access_process_vm().

Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
---

include/linux/ptrace.h | 1 +
kernel/signal.c | 1 +
kernel/ptrace.c | 16 ++++++++++++----
3 files changed, 14 insertions(+), 4 deletions(-)

--- ptrace/include/linux/ptrace.h~2_ptrace_finish_resume 2011-07-03 21:55:17.000000000 +0200
+++ ptrace/include/linux/ptrace.h 2011-07-03 21:55:37.000000000 +0200
@@ -112,6 +112,7 @@
#include <linux/compiler.h> /* For unlikely. */
#include <linux/sched.h> /* For struct task_struct. */

+extern void ptrace_finish_stop(void);

extern long arch_ptrace(struct task_struct *child, long request,
unsigned long addr, unsigned long data);
--- ptrace/kernel/signal.c~2_ptrace_finish_resume 2011-07-03 21:55:17.000000000 +0200
+++ ptrace/kernel/signal.c 2011-07-03 21:55:37.000000000 +0200
@@ -1879,6 +1879,7 @@ static void ptrace_stop(int exit_code, i
*/
try_to_freeze();

+ ptrace_finish_stop();
/*
* We are back. Now reacquire the siglock before touching
* last_siginfo, so that we are sure to have synchronized with
--- ptrace/kernel/ptrace.c~2_ptrace_finish_resume 2011-07-03 21:55:17.000000000 +0200
+++ ptrace/kernel/ptrace.c 2011-07-03 21:55:37.000000000 +0200
@@ -581,6 +581,18 @@ static int ptrace_setsiginfo(struct task
#define is_sysemu_singlestep(request) 0
#endif

+void ptrace_finish_stop(void)
+{
+ struct task_struct *task = current;
+
+ if (task->ptrace & PT_BLOCKSTEP)
+ user_enable_block_step(task);
+ else if (task->ptrace & PT_SINGLESTEP)
+ user_enable_single_step(task);
+ else
+ user_disable_single_step(task);
+}
+
static int ptrace_resume(struct task_struct *child, long request,
unsigned long data)
{
@@ -604,14 +616,10 @@ static int ptrace_resume(struct task_str
if (unlikely(!arch_has_block_step()))
return -EIO;
child->ptrace |= PT_BLOCKSTEP;
- user_enable_block_step(child);
} else if (is_singlestep(request) || is_sysemu_singlestep(request)) {
if (unlikely(!arch_has_single_step()))
return -EIO;
child->ptrace |= PT_SINGLESTEP;
- user_enable_single_step(child);
- } else {
- user_disable_single_step(child);
}

child->exit_code = data;
[PATCH 3/3] x86: remove ptrace_disable()->user_disable_single_step()

Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
---

arch/x86/kernel/ptrace.c | 1 -
1 file changed, 1 deletion(-)

--- ptrace/arch/x86/kernel/ptrace.c~3_detach_dont_disable_step 2011-06-07 19:20:02.000000000 +0200
+++ ptrace/arch/x86/kernel/ptrace.c 2011-07-03 21:56:42.000000000 +0200
@@ -807,7 +807,6 @@ static int ioperm_get(struct task_struct
*/
void ptrace_disable(struct task_struct *child)
{
- user_disable_single_step(child);
#ifdef TIF_SYSCALL_EMU
clear_tsk_thread_flag(child, TIF_SYSCALL_EMU);
#endif