Commit fc3d1bad authored by Brad Smith's avatar Brad Smith Committed by Peter Maydell
Browse files

oslib-posix: Use MAP_STACK in qemu_alloc_stack() on OpenBSD 



Use MAP_STACK in qemu_alloc_stack() on OpenBSD.

Added to our 6.4 release.

MAP_STACK      Indicate that the mapping is used as a stack.  This
               flag must be used in combination with MAP_ANON and
               MAP_PRIVATE.

Implement MAP_STACK option for mmap().  Synchronous faults (pagefault and
syscall) confirm the stack register points at MAP_STACK memory, otherwise
SIGSEGV is delivered. sigaltstack() and pthread_attr_setstack() are modified
to create a MAP_STACK sub-region which satisfies alignment requirements.
Observe that MAP_STACK can only be set/cleared by mmap(), which zeroes the
contents of the region -- there is no mprotect() equivalent operation, so
there is no MAP_STACK-adding gadget.

Signed-off-by: default avatarBrad Smith <brad@comstyle.com>
Reviewed-by: default avatarKamil Rytarowski <n54@gmx.com>
Reviewed-by: default avatarPeter Maydell <peter.maydell@linaro.org>
Message-id: 20181019125239.GA13884@humpty.home.comstyle.com
Signed-off-by: default avatarPeter Maydell <peter.maydell@linaro.org>
parent 03c1ca1c

util/oslib-posix.c

+13 −2
Original line number Diff line number Diff line
@@ -596,6 +596,7 @@ pid_t qemu_fork(Error **errp) 
void *qemu_alloc_stack(size_t *sz)

{

    void *ptr, *guardpage;

    int flags;

#ifdef CONFIG_DEBUG_STACK_USAGE

    void *ptr2;

#endif
@@ -610,8 +611,18 @@ void *qemu_alloc_stack(size_t *sz) 
    /* allocate one extra page for the guard page */

    *sz += pagesz;



    ptr = mmap(NULL, *sz, PROT_READ | PROT_WRITE,

               MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);

    flags = MAP_PRIVATE | MAP_ANONYMOUS;

#if defined(MAP_STACK) && defined(__OpenBSD__)

    /* Only enable MAP_STACK on OpenBSD. Other OS's such as

     * Linux/FreeBSD/NetBSD have a flag with the same name

     * but have differing functionality. OpenBSD will SEGV

     * if it spots execution with a stack pointer pointing

     * at memory that was not allocated with MAP_STACK.

     */

    flags |= MAP_STACK;

#endif



    ptr = mmap(NULL, *sz, PROT_READ | PROT_WRITE, flags, -1, 0);

    if (ptr == MAP_FAILED) {

        perror("failed to allocate memory for stack");

        abort();