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Commit 5be71b61 authored by Frederic Weisbecker's avatar Frederic Weisbecker Committed by Ingo Molnar
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tracing/syscalls: protect thread flag toggling from races 



Impact: fix syscall tracer enable/disable race

The current thread flag toggling is racy as shown in the following
scenario:

- task A is the last user of syscall tracing, it releases the
  TIF_SYSCALL_FTRACE on each tasks

- at the same time task B start syscall tracing. refcount == 0 so
  it sets up TIF_SYSCALL_FTRACE on each tasks.

The effect of the mixup is unpredictable.
So this fix adds a mutex on {start,stop}_syscall_tracing().

Reported-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Reported-by: default avatarIngo Molnar <mingo@elte.hu>
LKML-Reference: <1237151439-6755-3-git-send-email-fweisbec@gmail.com>
Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
parent 64044345

kernel/trace/trace_syscalls.c

+19 −5
Original line number Diff line number Diff line
@@ -5,7 +5,11 @@ 
#include "trace_output.h"

#include "trace.h"



static atomic_t refcount;

/* Keep a counter of the syscall tracing users */

static int refcount;



/* Prevent from races on thread flags toggling */

static DEFINE_MUTEX(syscall_trace_lock);



/* Option to display the parameters types */

enum {
@@ -96,9 +100,11 @@ void start_ftrace_syscalls(void) 
	unsigned long flags;

	struct task_struct *g, *t;



	mutex_lock(&syscall_trace_lock);



	/* Don't enable the flag on the tasks twice */

	if (atomic_inc_return(&refcount) != 1)

		return;

	if (++refcount != 1)

		goto unlock;



	arch_init_ftrace_syscalls();

	read_lock_irqsave(&tasklist_lock, flags);
@@ -108,6 +114,9 @@ void start_ftrace_syscalls(void) 
	} while_each_thread(g, t);



	read_unlock_irqrestore(&tasklist_lock, flags);



unlock:

	mutex_unlock(&syscall_trace_lock);

}



void stop_ftrace_syscalls(void)
@@ -115,9 +124,11 @@ void stop_ftrace_syscalls(void) 
	unsigned long flags;

	struct task_struct *g, *t;



	mutex_lock(&syscall_trace_lock);



	/* There are perhaps still some users */

	if (atomic_dec_return(&refcount))

		return;

	if (--refcount)

		goto unlock;



	read_lock_irqsave(&tasklist_lock, flags);
@@ -126,6 +137,9 @@ void stop_ftrace_syscalls(void) 
	} while_each_thread(g, t);



	read_unlock_irqrestore(&tasklist_lock, flags);



unlock:

	mutex_unlock(&syscall_trace_lock);

}



void ftrace_syscall_enter(struct pt_regs *regs)