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SubjectRe: Change in functionality of futex() system call.
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On Mon, 2011-06-06 at 17:23 +0200, Eric Dumazet wrote:
> Le lundi 06 juin 2011 à 09:28 -0500, David Oliver a écrit :
> > Hello,
> >
> > The functionality of the futex() system call appears to have changed
> > between versions 2.6.18 and 2.6.32.28.
> >
> > Specifically, performing a FUTEX_WAIT on a read-only mapped location
> > results in an EFAULT. Although other operations, such as FUTEX_WAKE,
> > are only meaningful for writable locations, FUTEX_WAIT is useful for
> > processes with read-only access to a memory-mapped file.
> >
> > The code below illustrates the changed behavior (each of the EXPECT
> > operations succeed on the older kernel, the ASSERTs pass in each
> > case), assuming the file /tmp/futex_test exists and contains int(42).
> >
> > With the older kernel, the syscall() suspends until another process
> > changes the file and issues a FUTEX_WAKE, whereas the new behavior is
> > for an EFAULT error, independent of the file contents.
> >
> > Let me know if you need further clarification.
> >
> > Cheers!
> >
> > David Oliver.
> >
> >
> > #include <errno.h>
> > #include <fcntl.h>
> > #include <stdint.h>
> > typedef uint32_t u32; // for futex.h
> > #include <linux/futex.h>
> > #include <sys/mman.h>
> > #include <sys/syscall.h>
> > #include <unistd.h>
> > #include "gtest/gtest.h" // test framework to illustrate issue.
> >
> >
> > TEST(Futex, futex_in_read_only_file_is_ok) {
> > int fd = open("/tmp/futex_test", O_RDONLY);
> > ASSERT_GE(fd, 0);
> > int* futex = static_cast<int *>(mmap(0, sizeof(int), PROT_READ,
> > MAP_SHARED, fd, 0));
> > ASSERT_NE((int *)(0), futex);
> >
> > int rc = syscall(SYS_futex, futex, FUTEX_WAIT, 42, 0, 0, 0);
> >
> > EXPECT_NE(-1, rc); // fails.
> > if (rc == -1) {
> > EXPECT_NE(errno, EFAULT); // fails.
> > }
> > }
> >
>
> Right you are, this came from commit 7485d0d3758e8e6491a5 (futexes:
> Remove rw parameter from get_futex_key()) in 2.6.33
>
> commit 7485d0d3758e8e6491a5c9468114e74dc050785d
> Author: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
> Date: Tue Jan 5 16:32:43 2010 +0900
>
> futexes: Remove rw parameter from get_futex_key()
>
> 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.

Urgh,. maybe something like the below but with more conditionals that
enable the extra logic only for FUTEX_WAIT..

The idea is to try a RO gup() when the RW gup() fails so as not to slow
down the common path of writable anonymous maps and bail when we used
the RO path on anonymous memory.

---
diff --git a/kernel/futex.c b/kernel/futex.c
index fe28dc2..11f2ad1 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -234,7 +234,7 @@ get_futex_key(u32 __user *uaddr, int fshared, union futex_key *key)
unsigned long address = (unsigned long)uaddr;
struct mm_struct *mm = current->mm;
struct page *page, *page_head;
- int err;
+ int err, ro = 0;

/*
* The futex address must be "naturally" aligned.
@@ -262,6 +262,10 @@ get_futex_key(u32 __user *uaddr, int fshared, union futex_key *key)

again:
err = get_user_pages_fast(address, 1, 1, &page);
+ if (err == -EFAULT) {
+ err = get_user_pages_fast(address, 1, 0, &page);
+ ro = 1;
+ }
if (err < 0)
return err;

@@ -316,6 +320,11 @@ again:
* the object not the particular process.
*/
if (PageAnon(page_head)) {
+ if (ro) {
+ err = -EFAULT;
+ goto out;
+ }
+
key->both.offset |= FUT_OFF_MMSHARED; /* ref taken on mm */
key->private.mm = mm;
key->private.address = address;
@@ -327,9 +336,10 @@ again:

get_futex_key_refs(key);

+out:
unlock_page(page_head);
put_page(page_head);
- return 0;
+ return err;
}

static inline void put_futex_key(union futex_key *key)
--
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