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Re: [Qemu-devel] [PATCH] linux-user: Don't use sigfillset() on uc->uc_si
From: |
Laurent Vivier |
Subject: |
Re: [Qemu-devel] [PATCH] linux-user: Don't use sigfillset() on uc->uc_sigmask |
Date: |
Tue, 14 Jun 2016 14:46:08 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.1.0 |
Le 14/06/2016 à 13:49, Peter Maydell a écrit :
> 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>
The "WARNING" in the comments is really welcome...
Reviewed-by: Laurent Vivier <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 37fb60f..26e5e94 100644
> --- a/linux-user/signal.c
> +++ b/linux-user/signal.c
> @@ -639,8 +639,16 @@ 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 121b89f..202e387 100644
> --- a/linux-user/syscall.c
> +++ b/linux-user/syscall.c
> @@ -124,11 +124,6 @@ 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
>