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[Qemu-devel] [PATCH] linux-user: Don't use sigfillset() on uc->uc_sigmas
[Qemu-devel] [PATCH] linux-user: Don't use sigfillset() on uc->uc_sigmask
Tue, 14 Jun 2016 12:49:18 +0100
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>
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
@@ -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 37fb60f..26e5e94 100644
@@ -639,8 +639,16 @@ 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.
+ memset(&uc->uc_sigmask, 0xff, SIGSET_T_SIZE);
diff --git a/linux-user/syscall.c b/linux-user/syscall.c
index 121b89f..202e387 100644
@@ -124,11 +124,6 @@ int __clone2(int (*fn)(void *), void *child_stack_base,
#define VFAT_IOCTL_READDIR_BOTH _IOR('r', 1, struct
#define VFAT_IOCTL_READDIR_SHORT _IOR('r', 2, struct
-/* 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)
- [Qemu-devel] [PATCH] linux-user: Don't use sigfillset() on uc->uc_sigmask,
Peter Maydell <=