David Holmes found a bug in the RT patch with respect to
pthread_cond_timedwait. After trying his test program on the latest git
from mainline, I found the bug was there too. The bug he was seeing
that his test program showed, was that if one were to do a "Ctrl-Z" on a
process that was in the pthread_cond_timedwait, and then did a "bg" on
that process, it would return with a "-ETIMEDOUT" but early. That is,
the timer would go off early.
Looking into this, I found the source of the problem. And it is a rather
nasty bug at that.
Here's the relevant code from kernel/futex.c: (not in order in the file)
[...]
smlinkage long sys_futex(u32 __user *uaddr, int op, u32 val,
struct timespec __user *utime, u32 __user *uaddr2,
u32 val3)
{
struct timespec ts;
ktime_t t, *tp = NULL;
u32 val2 = 0;
int cmd = op & FUTEX_CMD_MASK;
if (utime && (cmd == FUTEX_WAIT || cmd == FUTEX_LOCK_PI)) {
if (copy_from_user(&ts, utime, sizeof(ts)) != 0)
return -EFAULT;
if (!timespec_valid(&ts))
return -EINVAL;
t = timespec_to_ktime(ts);
if (cmd == FUTEX_WAIT)
t = ktime_add(ktime_get(), t);
tp = &t;
}
[...]
return do_futex(uaddr, op, val, tp, uaddr2, val2, val3);
}
[...]
long do_futex(u32 __user *uaddr, int op, u32 val, ktime_t *timeout,
u32 __user *uaddr2, u32 val2, u32 val3)
{
int ret;
int cmd = op & FUTEX_CMD_MASK;
struct rw_semaphore *fshared = NULL;
if (!(op & FUTEX_PRIVATE_FLAG))
fshared = ¤t->mm->mmap_sem;
switch (cmd) {
case FUTEX_WAIT:
ret = futex_wait(uaddr, fshared, val, timeout);
[...]
static int futex_wait(u32 __user *uaddr, struct rw_semaphore *fshared,
u32 val, ktime_t *abs_time)
{
[...]
struct restart_block *restart;
restart = ¤t_thread_info()->restart_block;
restart->fn = futex_wait_restart;
restart->arg0 = (unsigned long)uaddr;
restart->arg1 = (unsigned long)val;
restart->arg2 = (unsigned long)abs_time;
restart->arg3 = 0;
if (fshared)
restart->arg3 |= ARG3_SHARED;
return -ERESTART_RESTARTBLOCK;
[...]
static long futex_wait_restart(struct restart_block *restart)
{
u32 __user *uaddr = (u32 __user *)restart->arg0;
u32 val = (u32)restart->arg1;
ktime_t *abs_time = (ktime_t *)restart->arg2;
struct rw_semaphore *fshared = NULL;
restart->fn = do_no_restart_syscall;
if (restart->arg3 & ARG3_SHARED)
fshared = ¤t->mm->mmap_sem;
return (long)futex_wait(uaddr, fshared, val, abs_time);
}
So when the futex_wait is interrupt by a signal we break out of the
hrtimer code and set up or return from signal. This code does not return
back to userspace, so we set up a RESTARTBLOCK. The bug here is that we
save the "abs_time" which is a pointer to the stack variable "ktime_t t"
from sys_futex.
This returns and unwinds the stack before we get to call our signal. On
return from the signal we go to futex_wait_restart, where we update all
the parameters for futex_wait and call it. But here we have a problem
where abs_time is no longer valid.
I verified this with print statements, and sure enough, what abs_time
was set to ends up being garbage when we get to futex_wait_restart.
The solution I did to solve this is to allocate a temporary buffer when
setting up the block and free it in futex_wait_restart. This patch
allows David's test program to actually pass.
Signed-off-by: Steven Rostedt <[email protected]>
diff --git a/kernel/futex.c b/kernel/futex.c
index 9dc591a..74be1cb 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1288,11 +1288,27 @@ static int futex_wait(u32 __user *uaddr, struct rw_semaphore *fshared,
return -ERESTARTSYS;
else {
struct restart_block *restart;
+ ktime_t *tmp_time = NULL;
+
+ /*
+ * abs_time is on the stack and is not a parameter.
+ * If we save it, it will be overridden on return and
+ * what is sent to futex_wait_restart will be corrupted.
+ */
+ if (abs_time) {
+ tmp_time = kmalloc(sizeof(*tmp_time), GFP_KERNEL);
+ if (unlikely(!tmp_time))
+ /* Ahh, what else can we do?? */
+ printk(KERN_WARNING
+ "Can't allocate temp timer storage\n");
+ else
+ *tmp_time = *abs_time;
+ }
restart = ¤t_thread_info()->restart_block;
restart->fn = futex_wait_restart;
restart->arg0 = (unsigned long)uaddr;
restart->arg1 = (unsigned long)val;
- restart->arg2 = (unsigned long)abs_time;
+ restart->arg2 = (unsigned long)tmp_time;
restart->arg3 = 0;
if (fshared)
restart->arg3 |= ARG3_SHARED;
@@ -1312,13 +1328,19 @@ static long futex_wait_restart(struct restart_block *restart)
{
u32 __user *uaddr = (u32 __user *)restart->arg0;
u32 val = (u32)restart->arg1;
- ktime_t *abs_time = (ktime_t *)restart->arg2;
+ ktime_t *tmp_time = (ktime_t *)restart->arg2;
+ ktime_t t;
struct rw_semaphore *fshared = NULL;
+ if (tmp_time) {
+ t = *tmp_time;
+ kfree(tmp_time);
+ tmp_time = &t;
+ }
restart->fn = do_no_restart_syscall;
if (restart->arg3 & ARG3_SHARED)
fshared = ¤t->mm->mmap_sem;
- return (long)futex_wait(uaddr, fshared, val, abs_time);
+ return (long)futex_wait(uaddr, fshared, val, tmp_time);
}
--
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