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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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