From 5be71b61f17b0e3bc8ad0b1a1b7b53ab7d574ebb Mon Sep 17 00:00:00 2001 From: Frederic Weisbecker Date: Sun, 15 Mar 2009 22:10:37 +0100 Subject: [PATCH] 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: Andrew Morton Reported-by: Ingo Molnar LKML-Reference: <1237151439-6755-3-git-send-email-fweisbec@gmail.com> Signed-off-by: Ingo Molnar --- kernel/trace/trace_syscalls.c | 24 +++++++++++++++++++----- 1 file changed, 19 insertions(+), 5 deletions(-) diff --git a/kernel/trace/trace_syscalls.c b/kernel/trace/trace_syscalls.c index 26f9a8679d3d..a2a3af29c943 100644 --- a/kernel/trace/trace_syscalls.c +++ b/kernel/trace/trace_syscalls.c @@ -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) -- 2.20.1