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[Qemu-devel] [PULL v2 04/24] linux-user: Don't use sigfillset() on uc->u

From: riku . voipio
Subject: [Qemu-devel] [PULL v2 04/24] linux-user: Don't use sigfillset() on uc->uc_sigmask
Date: Tue, 28 Jun 2016 22:12:38 +0300

From: Peter Maydell <address@hidden>

The kernel and libc have different ideas about what a sigset_t
is -- for the kernel it is only _NSIG / 8 bytes in size (usually
8 bytes), but for libc it is much larger, 128 bytes. In most
situations the difference doesn't matter, because if you pass a
pointer to a libc sigset_t to the kernel it just acts on the first
8 bytes of it, but for the ucontext_t* argument to a signal handler
it trips us up. The kernel allocates this ucontext_t on the stack
according to its idea of the sigset_t type, but the type of the
ucontext_t defined by the libc headers uses the libc type, and
so do the manipulator functions like sigfillset(). This means that
 (1) sizeof(uc->uc_sigmask) is much larger than the actual
     space used on the stack
 (2) sigfillset(&uc->uc_sigmask) will write garbage 0xff bytes
     off the end of the structure, which can trash data that
     was on the stack before the signal handler was invoked,
     and may result in a crash after the handler returns

To avoid this, we use a memset() of the correct size to fill
the signal mask rather than using the libc function.

This fixes a problem where we would crash at least some of the
time on an i386 host when a signal was taken.

Signed-off-by: Peter Maydell <address@hidden>
Reviewed-by: Laurent Vivier <address@hidden>
Signed-off-by: Riku Voipio <address@hidden>
 linux-user/qemu.h    |  5 +++++
 linux-user/signal.c  | 10 +++++++++-
 linux-user/syscall.c |  5 -----
 3 files changed, 14 insertions(+), 6 deletions(-)

diff --git a/linux-user/qemu.h b/linux-user/qemu.h
index 56f29c3..e8a5aed 100644
--- a/linux-user/qemu.h
+++ b/linux-user/qemu.h
@@ -20,6 +20,11 @@
 #define THREAD __thread
+/* This is the size of the host kernel's sigset_t, needed where we make
+ * direct system calls that take a sigset_t pointer and a size.
+ */
+#define SIGSET_T_SIZE (_NSIG / 8)
 /* This struct is used to hold certain information about the image.
  * Basically, it replicates in user space what would be certain
  * task_struct fields in the kernel
diff --git a/linux-user/signal.c b/linux-user/signal.c
index e2d55ff..9d98045 100644
--- a/linux-user/signal.c
+++ b/linux-user/signal.c
@@ -636,8 +636,16 @@ static void host_signal_handler(int host_signum, siginfo_t 
      * code in case the guest code provokes one in the window between
      * now and it getting out to the main loop. Signals will be
      * unblocked again in process_pending_signals().
+     *
+     * WARNING: we cannot use sigfillset() here because the uc_sigmask
+     * field is a kernel sigset_t, which is much smaller than the
+     * libc sigset_t which sigfillset() operates on. Using sigfillset()
+     * would write 0xff bytes off the end of the structure and trash
+     * data on the struct.
+     * We can't use sizeof(uc->uc_sigmask) either, because the libc
+     * headers define the struct field with the wrong (too large) type.
-    sigfillset(&uc->uc_sigmask);
+    memset(&uc->uc_sigmask, 0xff, SIGSET_T_SIZE);
     sigdelset(&uc->uc_sigmask, SIGSEGV);
     sigdelset(&uc->uc_sigmask, SIGBUS);
diff --git a/linux-user/syscall.c b/linux-user/syscall.c
index 3dfaea9..5166ff9 100644
--- a/linux-user/syscall.c
+++ b/linux-user/syscall.c
@@ -123,11 +123,6 @@ int __clone2(int (*fn)(void *), void *child_stack_base,
 #define        VFAT_IOCTL_READDIR_BOTH         _IOR('r', 1, struct 
linux_dirent [2])
 #define        VFAT_IOCTL_READDIR_SHORT        _IOR('r', 2, struct 
linux_dirent [2])
-/* This is the size of the host kernel's sigset_t, needed where we make
- * direct system calls that take a sigset_t pointer and a size.
- */
-#define SIGSET_T_SIZE (_NSIG / 8)
 #undef _syscall0
 #undef _syscall1
 #undef _syscall2

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