Skip to content

Commit

Permalink
tracing/syscalls: protect thread flag toggling from races
Browse files Browse the repository at this point in the history
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: Andrew Morton <akpm@linux-foundation.org>
Reported-by: Ingo Molnar <mingo@elte.hu>
LKML-Reference: <1237151439-6755-3-git-send-email-fweisbec@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
  • Loading branch information
Frederic Weisbecker authored and Ingo Molnar committed Mar 16, 2009
1 parent 6404434 commit 5be71b6
Showing 1 changed file with 19 additions and 5 deletions.
24 changes: 19 additions & 5 deletions kernel/trace/trace_syscalls.c
Original file line number Diff line number Diff line change
Expand Up @@ -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 {
Expand Down Expand Up @@ -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);
Expand All @@ -108,16 +114,21 @@ 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)
{
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);

Expand All @@ -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)
Expand Down

0 comments on commit 5be71b6

Please sign in to comment.