[PATCH] futex: remove rw parameter from get_futex_key()

[KOSAKI Motohiro]

Currently, futex have two problem.

A) current futex doesn't handle private file mappings properly.

get_futex_key() use PageAnon() to distinguish file and anon. it can
makes following bad scenario.

  1) thread-A call futex(private-mapping, FUTEX_WAIT). it makes to
     sleep on file mapping object.
  2) thread-B write a variable and it makes cow.
  3) thread-B call futex(private-mapping, FUTEX_WAKE). it wake up
     sleeped thread on the anonymous page. (but it's nothing)

following testcase reproduce this issue.

actual result)
       FUTEX_WAIT thread never wake up.

expect result)
       FUTEX_WAIT thread wake up by FUTEX_WAKE.

--------------------------------------------------------------------
 #include <stdio.h>
 #include <stdlib.h>
 #include <syscall.h>
 #include <unistd.h>
 #include <errno.h>
 #include <linux/futex.h>
 #include <pthread.h>

char pad[4096] = {1};
int val = 1;
char pad2[4096] = {1};

void * futex_wait(void *arg)
{
	int ret;

	fprintf(stderr, "futex wait\n");
	ret = syscall( SYS_futex, &val, FUTEX_WAIT, 1, NULL, NULL, NULL);
	if (ret != 0 && errno != EWOULDBLOCK) {
		perror("futex error.\n");
		exit(1);
	}
	fprintf(stderr, "futex_wait: ret = %d, errno = %d\n", ret, errno);

	return NULL;
}

int main(int argc, char **argv)
{
	pthread_t thr;
	int ret;

	ret = pthread_create(&thr, NULL, futex_wait, NULL);
	if (ret < 0) {
		fprintf(stderr, "pthread_create error\n");
		exit(1);
	}

	sleep(10);

	fprintf(stderr, "futex wake\n");
	val = 2;
	ret = syscall( SYS_futex, &val, FUTEX_WAKE, 1, NULL, NULL, NULL);
	fprintf(stderr, "futex_wake: ret = %d, errno = %d\n", ret, errno);

	fprintf(stderr, "join\n");
	pthread_join(thr, NULL);

	return 0;
}
--------------------------------------------------------------------

B) current futex doesn't handle zero page properly.

read mode get_user_pages() can return zero page. but current futex code doesn't
handle it at all. Then, zero page makes infinite loop internally.

following testcase can reproduce this issue.

actual result)
	FUTEX_WAIT never return and waste cpu time 100%.

expected result)
	FUTEX_WAIT return immediately.

------------------------------------------------------------------
 #include <stdio.h>
 #include <stdlib.h>
 #include <sys/mman.h>
 #include <syscall.h>
 #include <sys/time.h>
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <fcntl.h>
 #include <unistd.h>
 #include <errno.h>
 #include <linux/futex.h>
 #include <pthread.h>

int main(int argc, char **argv)
{
	long page_size;
	int ret;
	void *buf;

	page_size = sysconf(_SC_PAGESIZE);

	buf = mmap(NULL, page_size, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, 0, 0);
	if (buf == (void *)-1) {
		perror("mmap error.\n");
		exit(1);
	}

	fprintf(stderr, "futex wait\n");
	ret = syscall( SYS_futex, buf, FUTEX_WAIT, 1, NULL, NULL, NULL);
	if (ret != 0 && errno != EWOULDBLOCK) {
		perror("futex error.\n");
		exit(1);
	}
	fprintf(stderr, "futex_wait: ret = %d, errno = %d\n", ret, errno);

	return 0;
}
------------------------------------------------------------------

The solution is to use write mode get_user_page() always for page lookup.
It prevent to lookup both file page of private mappings and zero page.

performance concern:

Probaly very little. because glibc always initialize variables for futex
before to call futex(). It mean glibc user never seen the overhead of this
patch.

compatibility concern:

This patch have few compatibility break. After this patch, FUTEX_WAIT require
writable access to futex variables (read-only mappings makes EFAULT).
But practically it's no problem. again glibc always initalize variables for futex
explicitly. nobody use read-only mappings.

Reported-by: Hugh Dickins <hugh.dickins@xxxxxxxxxxxxx>
Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
---
 kernel/futex.c |   27 ++++++++++++---------------
 1 files changed, 12 insertions(+), 15 deletions(-)

diff --git a/kernel/futex.c b/kernel/futex.c
index 8e3c3ff..d9b3a22 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -203,8 +203,6 @@ static void drop_futex_key_refs(union futex_key *key)
  * @uaddr:	virtual address of the futex
  * @fshared:	0 for a PROCESS_PRIVATE futex, 1 for PROCESS_SHARED
  * @key:	address where result is stored.
- * @rw:		mapping needs to be read/write (values: VERIFY_READ,
- * 		VERIFY_WRITE)
  *
  * Returns a negative error code or 0
  * The key words are stored in *key on success.
@@ -216,7 +214,7 @@ static void drop_futex_key_refs(union futex_key *key)
  * lock_page() might sleep, the caller should not hold a spinlock.
  */
 static int
-get_futex_key(u32 __user *uaddr, int fshared, union futex_key *key, int rw)
+get_futex_key(u32 __user *uaddr, int fshared, union futex_key *key)
 {
 	unsigned long address = (unsigned long)uaddr;
 	struct mm_struct *mm = current->mm;
@@ -239,7 +237,7 @@ get_futex_key(u32 __user *uaddr, int fshared, union futex_key *key, int rw)
 	 *        but access_ok() should be faster than find_vma()
 	 */
 	if (!fshared) {
-		if (unlikely(!access_ok(rw, uaddr, sizeof(u32))))
+		if (unlikely(!access_ok(VERIFY_WRITE, uaddr, sizeof(u32))))
 			return -EFAULT;
 		key->private.mm = mm;
 		key->private.address = address;
@@ -248,7 +246,7 @@ get_futex_key(u32 __user *uaddr, int fshared, union futex_key *key, int rw)
 	}
 
 again:
-	err = get_user_pages_fast(address, 1, rw == VERIFY_WRITE, &page);
+	err = get_user_pages_fast(address, 1, 1, &page);
 	if (err < 0)
 		return err;
 
@@ -867,7 +865,7 @@ static int futex_wake(u32 __user *uaddr, int fshared, int nr_wake, u32 bitset)
 	if (!bitset)
 		return -EINVAL;
 
-	ret = get_futex_key(uaddr, fshared, &key, VERIFY_READ);
+	ret = get_futex_key(uaddr, fshared, &key);
 	if (unlikely(ret != 0))
 		goto out;
 
@@ -913,10 +911,10 @@ futex_wake_op(u32 __user *uaddr1, int fshared, u32 __user *uaddr2,
 	int ret, op_ret;
 
 retry:
-	ret = get_futex_key(uaddr1, fshared, &key1, VERIFY_READ);
+	ret = get_futex_key(uaddr1, fshared, &key1);
 	if (unlikely(ret != 0))
 		goto out;
-	ret = get_futex_key(uaddr2, fshared, &key2, VERIFY_WRITE);
+	ret = get_futex_key(uaddr2, fshared, &key2);
 	if (unlikely(ret != 0))
 		goto out_put_key1;
 
@@ -1175,11 +1173,10 @@ retry:
 		pi_state = NULL;
 	}
 
-	ret = get_futex_key(uaddr1, fshared, &key1, VERIFY_READ);
+	ret = get_futex_key(uaddr1, fshared, &key1);
 	if (unlikely(ret != 0))
 		goto out;
-	ret = get_futex_key(uaddr2, fshared, &key2,
-			    requeue_pi ? VERIFY_WRITE : VERIFY_READ);
+	ret = get_futex_key(uaddr2, fshared, &key2);
 	if (unlikely(ret != 0))
 		goto out_put_key1;
 
@@ -1738,7 +1735,7 @@ static int futex_wait_setup(u32 __user *uaddr, u32 val, int fshared,
 	 */
 retry:
 	q->key = FUTEX_KEY_INIT;
-	ret = get_futex_key(uaddr, fshared, &q->key, VERIFY_READ);
+	ret = get_futex_key(uaddr, fshared, &q->key);
 	if (unlikely(ret != 0))
 		return ret;
 
@@ -1904,7 +1901,7 @@ static int futex_lock_pi(u32 __user *uaddr, int fshared,
 	q.requeue_pi_key = NULL;
 retry:
 	q.key = FUTEX_KEY_INIT;
-	ret = get_futex_key(uaddr, fshared, &q.key, VERIFY_WRITE);
+	ret = get_futex_key(uaddr, fshared, &q.key);
 	if (unlikely(ret != 0))
 		goto out;
 
@@ -2023,7 +2020,7 @@ retry:
 	if ((uval & FUTEX_TID_MASK) != task_pid_vnr(current))
 		return -EPERM;
 
-	ret = get_futex_key(uaddr, fshared, &key, VERIFY_WRITE);
+	ret = get_futex_key(uaddr, fshared, &key);
 	if (unlikely(ret != 0))
 		goto out;
 
@@ -2215,7 +2212,7 @@ static int futex_wait_requeue_pi(u32 __user *uaddr, int fshared,
 	rt_waiter.task = NULL;
 
 	key2 = FUTEX_KEY_INIT;
-	ret = get_futex_key(uaddr2, fshared, &key2, VERIFY_WRITE);
+	ret = get_futex_key(uaddr2, fshared, &key2);
 	if (unlikely(ret != 0))
 		goto out;
 
-- 
1.6.5.2




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