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authorPeter Maydell <peter.maydell@linaro.org>2016-06-14 12:49:18 +0100
committerRiku Voipio <riku.voipio@linaro.org>2016-06-26 13:17:20 +0300
commit1d48fdd9d84aab1bd32c1f70947932f5d90f92aa (patch)
treedee8a5037d9909fdb0201fd60230d399ab26e899
parent435da5e7092aa54e12044b9401b42c4a9333c74d (diff)
linux-user: Don't use sigfillset() on uc->uc_sigmask
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 <peter.maydell@linaro.org> Reviewed-by: Laurent Vivier <laurent@vivier.eu> Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
-rw-r--r--linux-user/qemu.h5
-rw-r--r--linux-user/signal.c10
-rw-r--r--linux-user/syscall.c5
3 files changed, 14 insertions, 6 deletions
diff --git a/linux-user/qemu.h b/linux-user/qemu.h
index 56f29c35b5..e8a5aede95 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 e2d55fff97..9d980456ec 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 *info,
* 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 3dfaea9c8e..5166ff9b62 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