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linux-user: Don't use sigfillset() on uc->uc_sigmask

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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>
This commit is contained in:
Peter Maydell 2016-06-14 12:49:18 +01:00 committed by Riku Voipio
parent 435da5e709
commit 1d48fdd9d8
3 changed files with 14 additions and 6 deletions
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5
linux-user/qemu.h
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@ -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

10
linux-user/signal.c
View File

@ -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);

5
linux-user/syscall.c
View File

@ -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