Messages in this thread Patch in this message | | | Date | Wed, 20 Jan 2010 20:15:29 -0800 | From | Greg KH <> | Subject | [08/30] futexes: Remove rw parameter from get_futex_key() |
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2.6.32-stable review patch. If anyone has any objections, please let us know.
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From: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
commit 7485d0d3758e8e6491a5c9468114e74dc050785d upstream.
Currently, futexes have two problem:
A) The current futex code doesn't handle private file mappings properly.
get_futex_key() uses PageAnon() to distinguish file and anon, which can cause the following bad scenario:
1) thread-A call futex(private-mapping, FUTEX_WAIT), it sleeps on file mapping object. 2) thread-B writes a variable and it makes it cow. 3) thread-B calls futex(private-mapping, FUTEX_WAKE), it wakes up blocked thread on the anonymous page. (but it's nothing)
B) Current futex code 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.
The solution is to use write mode get_user_page() always for page lookup. It prevents the lookup of both file page of private mappings and zero page.
Performance concerns:
Probaly very little, because glibc always initialize variables for futex before to call futex(). It means glibc users never see the overhead of this patch.
Compatibility concerns:
This patch has few compatibility issues. After this patch, FUTEX_WAIT require writable access to futex variables (read-only mappings makes EFAULT). But practically it's not a problem, glibc always initalizes variables for futexes explicitly - nobody uses read-only mappings.
Reported-by: Hugh Dickins <hugh.dickins@tiscali.co.uk> Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> Acked-by: Peter Zijlstra <peterz@infradead.org> Acked-by: Darren Hart <dvhltc@us.ibm.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> Cc: Nick Piggin <npiggin@suse.de> Cc: Ulrich Drepper <drepper@gmail.com> LKML-Reference: <20100105162633.45A2.A69D9226@jp.fujitsu.com> Signed-off-by: Ingo Molnar <mingo@elte.hu> Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
--- kernel/futex.c | 27 ++++++++++++--------------- 1 file changed, 12 insertions(+), 15 deletions(-)
--- a/kernel/futex.c +++ b/kernel/futex.c @@ -203,8 +203,6 @@ static void drop_futex_key_refs(union fu * @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 fu * 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 fsh * 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 fsh } 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, 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 fs 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 * */ 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 *uad 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 __u 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;
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