Re: [Qemu-devel] [PATCH v2 17/19] linux-user: Use both si_code and si_signo when converting siginfo_t

[Riku Voipio]

On Wed, Jun 08, 2016 at 09:30:35AM +0300, Riku Voipio wrote:
> On Fri, May 27, 2016 at 03:51:59PM +0100, Peter Maydell wrote:
> > The siginfo_t struct includes a union. The correct way to identify
> > which fields of the union are relevant is complicated, because we
> > have to use a combination of the si_code and si_signo to figure out
> > which of the union's members are valid.  (Within the host kernel it
> > is always possible to tell, but the kernel carefully avoids giving
> > userspace the high 16 bits of si_code, so we don't have the
> > information to do this the easy way...) We therefore make our best
> > guess, bearing in mind that a guest can spoof most of the si_codes
> > via rt_sigqueueinfo() if it likes.  Once we have made our guess, we
> > record it in the top 16 bits of the si_code, so that tswap_siginfo()
> > later can use it.  tswap_siginfo() then strips these top bits out
> > before writing si_code to the guest (sign-extending the lower bits).
> > 
> > This fixes a bug where fields were sometimes wrong; in particular
> > the LTP kill10 test went into an infinite loop because its signal
> > handler got a si_pid value of 0 rather than the pid of the sending
> > process.
> > 
> > As part of this change, we switch to using __put_user() in the
> > tswap_siginfo code which writes out the byteswapped values to
> > the target memory, in case the target memory pointer is not
> > sufficiently aligned for the host CPU's requirements.
> 
> At least on Debian jessie, this blows up a selection of architectures:
> 
>   CC    microblazeel-linux-user/linux-user/signal.o
>   CC    cris-linux-user/linux-user/signal.o
>   CC    microblaze-linux-user/linux-user/signal.o
>   CC    sparc-linux-user/linux-user/signal.o
>   CC    sparc64-linux-user/linux-user/signal.o
>   CC    x86_64-linux-user/linux-user/signal.o
>   CC    unicore32-linux-user/linux-user/signal.o
>   CC    sparc32plus-linux-user/linux-user/signal.o
> /home/voipio/linaro/qemu/linux-user/signal.c: In function 
> ‘host_to_target_siginfo’:
> /home/voipio/linaro/qemu/linux-user/signal.c:387:10: error: 
> ‘tgt_tmp._sifields._sigchld._stime’ may be used uninitialized in this 
> function [-Werror=maybe-uninitialized]
>          __put_user(info->_sifields._sigchld._stime,

On the second look, this compile fail only happens if the next patch
that introduces tgt_tmp variable is also applied. Dropping that patch
instead.

> /home/voipio/linaro/qemu/linux-user/signal.c:403:22: note: 
> ‘tgt_tmp._sifields._sigchld._stime’ was declared here
>      target_siginfo_t tgt_tmp;
>                       ^
> /home/voipio/linaro/qemu/linux-user/signal.c:385:10: error: 
> ‘tgt_tmp._sifields._sigchld._utime’ may be used uninitialized in this 
> function [-Werror=maybe-uninitialized]
>          __put_user(info->_sifields._sigchld._utime,
>           ^
> /home/voipio/linaro/qemu/linux-user/signal.c:403:22: note: 
> ‘tgt_tmp._sifields._sigchld._utime’ was declared here
>      target_siginfo_t tgt_tmp;
>                       ^
> /home/voipio/linaro/qemu/linux-user/signal.c:383:10: error: 
> ‘tgt_tmp._sifields._sigchld._status’ may be used uninitialized in this 
> function [-Werror=maybe-uninitialized]
>          __put_user(info->_sifields._sigchld._status,
>           ^
> /home/voipio/linaro/qemu/linux-user/signal.c:403:22: note: 
> ‘tgt_tmp._sifields._sigchld._status’ was declared here
>      target_siginfo_t tgt_tmp;
>                       ^
> cc1: all warnings being treated as errors
> 
> These appear to be the architectures where setup_rt_frame isn't implemented.
> 
> 
> > Signed-off-by: Peter Maydell <address@hidden>
> > ---
> >  linux-user/signal.c       | 165 
> > ++++++++++++++++++++++++++++++++--------------
> >  linux-user/syscall_defs.h |  15 +++++
> >  2 files changed, 131 insertions(+), 49 deletions(-)
> > 
> > diff --git a/linux-user/signal.c b/linux-user/signal.c
> > index b21d6bf..8ea0cbf 100644
> > --- a/linux-user/signal.c
> > +++ b/linux-user/signal.c
> > @@ -17,6 +17,7 @@
> >   *  along with this program; if not, see <http://www.gnu.org/licenses/>.
> >   */
> >  #include "qemu/osdep.h"
> > +#include "qemu/bitops.h"
> >  #include <sys/ucontext.h>
> >  #include <sys/resource.h>
> >  
> > @@ -274,70 +275,129 @@ static inline void 
> > host_to_target_siginfo_noswap(target_siginfo_t *tinfo,
> >                                                   const siginfo_t *info)
> >  {
> >      int sig = host_to_target_signal(info->si_signo);
> > +    int si_code = info->si_code;
> > +    int si_type;
> >      tinfo->si_signo = sig;
> >      tinfo->si_errno = 0;
> >      tinfo->si_code = info->si_code;
> >  
> > -    if (sig == TARGET_SIGILL || sig == TARGET_SIGFPE || sig == 
> > TARGET_SIGSEGV
> > -            || sig == TARGET_SIGBUS || sig == TARGET_SIGTRAP) {
> > -        /* Should never come here, but who knows. The information for
> > -           the target is irrelevant.  */
> > -        tinfo->_sifields._sigfault._addr = 0;
> > -    } else if (sig == TARGET_SIGIO) {
> > -        tinfo->_sifields._sigpoll._band = info->si_band;
> > -        tinfo->_sifields._sigpoll._fd = info->si_fd;
> > -    } else if (sig == TARGET_SIGCHLD) {
> > -        tinfo->_sifields._sigchld._pid = info->si_pid;
> > -        tinfo->_sifields._sigchld._uid = info->si_uid;
> > -        tinfo->_sifields._sigchld._status
> > +    /* This is awkward, because we have to use a combination of
> > +     * the si_code and si_signo to figure out which of the union's
> > +     * members are valid. (Within the host kernel it is always possible
> > +     * to tell, but the kernel carefully avoids giving userspace the
> > +     * high 16 bits of si_code, so we don't have the information to
> > +     * do this the easy way...) We therefore make our best guess,
> > +     * bearing in mind that a guest can spoof most of the si_codes
> > +     * via rt_sigqueueinfo() if it likes.
> > +     *
> > +     * Once we have made our guess, we record it in the top 16 bits of
> > +     * the si_code, so that tswap_siginfo() later can use it.
> > +     * tswap_siginfo() will strip these top bits out before writing
> > +     * si_code to the guest (sign-extending the lower bits).
> > +     */
> > +
> > +    switch (si_code) {
> > +    case SI_USER:
> > +    case SI_TKILL:
> > +    case SI_KERNEL:
> > +        /* Sent via kill(), tkill() or tgkill(), or direct from the kernel.
> > +         * These are the only unspoofable si_code values.
> > +         */
> > +        tinfo->_sifields._kill._pid = info->si_pid;
> > +        tinfo->_sifields._kill._uid = info->si_uid;
> > +        si_type = QEMU_SI_KILL;
> > +        break;
> > +    default:
> > +        /* Everything else is spoofable. Make best guess based on signal */
> > +        switch (sig) {
> > +        case TARGET_SIGCHLD:
> > +            tinfo->_sifields._sigchld._pid = info->si_pid;
> > +            tinfo->_sifields._sigchld._uid = info->si_uid;
> > +            tinfo->_sifields._sigchld._status
> >                  = host_to_target_waitstatus(info->si_status);
> > -        tinfo->_sifields._sigchld._utime = info->si_utime;
> > -        tinfo->_sifields._sigchld._stime = info->si_stime;
> > -    } else if (sig >= TARGET_SIGRTMIN) {
> > -        tinfo->_sifields._rt._pid = info->si_pid;
> > -        tinfo->_sifields._rt._uid = info->si_uid;
> > -        /* XXX: potential problem if 64 bit */
> > -        tinfo->_sifields._rt._sigval.sival_ptr
> > +            tinfo->_sifields._sigchld._utime = info->si_utime;
> > +            tinfo->_sifields._sigchld._stime = info->si_stime;
> > +            si_type = QEMU_SI_CHLD;
> > +            break;
> > +        case TARGET_SIGIO:
> > +            tinfo->_sifields._sigpoll._band = info->si_band;
> > +            tinfo->_sifields._sigpoll._fd = info->si_fd;
> > +            si_type = QEMU_SI_POLL;
> > +            break;
> > +        default:
> > +            /* Assume a sigqueue()/mq_notify()/rt_sigqueueinfo() source. */
> > +            tinfo->_sifields._rt._pid = info->si_pid;
> > +            tinfo->_sifields._rt._uid = info->si_uid;
> > +            /* XXX: potential problem if 64 bit */
> > +            tinfo->_sifields._rt._sigval.sival_ptr
> >                  = (abi_ulong)(unsigned long)info->si_value.sival_ptr;
> > +            si_type = QEMU_SI_RT;
> > +            break;
> > +        }
> > +        break;
> >      }
> > +
> > +    tinfo->si_code = deposit32(si_code, 16, 16, si_type);
> >  }
> >  
> >  static void tswap_siginfo(target_siginfo_t *tinfo,
> >                            const target_siginfo_t *info)
> >  {
> > -    int sig = info->si_signo;
> > -    tinfo->si_signo = tswap32(sig);
> > -    tinfo->si_errno = tswap32(info->si_errno);
> > -    tinfo->si_code = tswap32(info->si_code);
> > -
> > -    if (sig == TARGET_SIGILL || sig == TARGET_SIGFPE || sig == 
> > TARGET_SIGSEGV
> > -        || sig == TARGET_SIGBUS || sig == TARGET_SIGTRAP) {
> > -        tinfo->_sifields._sigfault._addr
> > -            = tswapal(info->_sifields._sigfault._addr);
> > -    } else if (sig == TARGET_SIGIO) {
> > -        tinfo->_sifields._sigpoll._band
> > -            = tswap32(info->_sifields._sigpoll._band);
> > -        tinfo->_sifields._sigpoll._fd = 
> > tswap32(info->_sifields._sigpoll._fd);
> > -    } else if (sig == TARGET_SIGCHLD) {
> > -        tinfo->_sifields._sigchld._pid
> > -            = tswap32(info->_sifields._sigchld._pid);
> > -        tinfo->_sifields._sigchld._uid
> > -            = tswap32(info->_sifields._sigchld._uid);
> > -        tinfo->_sifields._sigchld._status
> > -            = tswap32(info->_sifields._sigchld._status);
> > -        tinfo->_sifields._sigchld._utime
> > -            = tswapal(info->_sifields._sigchld._utime);
> > -        tinfo->_sifields._sigchld._stime
> > -            = tswapal(info->_sifields._sigchld._stime);
> > -    } else if (sig >= TARGET_SIGRTMIN) {
> > -        tinfo->_sifields._rt._pid = tswap32(info->_sifields._rt._pid);
> > -        tinfo->_sifields._rt._uid = tswap32(info->_sifields._rt._uid);
> > -        tinfo->_sifields._rt._sigval.sival_ptr
> > -            = tswapal(info->_sifields._rt._sigval.sival_ptr);
> > +    int si_type = extract32(info->si_code, 16, 16);
> > +    int si_code = sextract32(info->si_code, 0, 16);
> > +
> > +    __put_user(info->si_signo, &tinfo->si_signo);
> > +    __put_user(info->si_errno, &tinfo->si_errno);
> > +    __put_user(si_code, &tinfo->si_code);
> > +
> > +    /* We can use our internal marker of which fields in the structure
> > +     * are valid, rather than duplicating the guesswork of
> > +     * host_to_target_siginfo_noswap() here.
> > +     */
> > +    switch (si_type) {
> > +    case QEMU_SI_KILL:
> > +        __put_user(info->_sifields._kill._pid, 
> > &tinfo->_sifields._kill._pid);
> > +        __put_user(info->_sifields._kill._uid, 
> > &tinfo->_sifields._kill._uid);
> > +        break;
> > +    case QEMU_SI_TIMER:
> > +        __put_user(info->_sifields._timer._timer1,
> > +                   &tinfo->_sifields._timer._timer1);
> > +        __put_user(info->_sifields._timer._timer2,
> > +                   &tinfo->_sifields._timer._timer2);
> > +        break;
> > +    case QEMU_SI_POLL:
> > +        __put_user(info->_sifields._sigpoll._band,
> > +                   &tinfo->_sifields._sigpoll._band);
> > +        __put_user(info->_sifields._sigpoll._fd,
> > +                   &tinfo->_sifields._sigpoll._fd);
> > +        break;
> > +    case QEMU_SI_FAULT:
> > +        __put_user(info->_sifields._sigfault._addr,
> > +                   &tinfo->_sifields._sigfault._addr);
> > +        break;
> > +    case QEMU_SI_CHLD:
> > +        __put_user(info->_sifields._sigchld._pid,
> > +                   &tinfo->_sifields._sigchld._pid);
> > +        __put_user(info->_sifields._sigchld._uid,
> > +                   &tinfo->_sifields._sigchld._uid);
> > +        __put_user(info->_sifields._sigchld._status,
> > +                   &tinfo->_sifields._sigchld._status);
> > +        __put_user(info->_sifields._sigchld._utime,
> > +                   &tinfo->_sifields._sigchld._utime);
> > +        __put_user(info->_sifields._sigchld._stime,
> > +                   &tinfo->_sifields._sigchld._stime);
> > +        break;
> > +    case QEMU_SI_RT:
> > +        __put_user(info->_sifields._rt._pid, &tinfo->_sifields._rt._pid);
> > +        __put_user(info->_sifields._rt._uid, &tinfo->_sifields._rt._uid);
> > +        __put_user(info->_sifields._rt._sigval.sival_ptr,
> > +                   &tinfo->_sifields._rt._sigval.sival_ptr);
> > +        break;
> > +    default:
> > +        g_assert_not_reached();
> >      }
> >  }
> >  
> > -
> >  void host_to_target_siginfo(target_siginfo_t *tinfo, const siginfo_t *info)
> >  {
> >      host_to_target_siginfo_noswap(tinfo, info);
> > @@ -505,6 +565,13 @@ int queue_signal(CPUArchState *env, int sig, 
> > target_siginfo_t *info)
> >  
> >      trace_user_queue_signal(env, sig);
> >  
> > +    /* Currently all callers define siginfo structures which
> > +     * use the _sifields._sigfault union member, so we can
> > +     * set the type here. If that changes we should push this
> > +     * out so the si_type is passed in by callers.
> > +     */
> > +    info->si_code = deposit32(info->si_code, 16, 16, QEMU_SI_FAULT);
> > +
> >      ts->sync_signal.info = *info;
> >      ts->sync_signal.pending = sig;
> >      /* signal that a new signal is pending */
> > diff --git a/linux-user/syscall_defs.h b/linux-user/syscall_defs.h
> > index 34af15a..124754f 100644
> > --- a/linux-user/syscall_defs.h
> > +++ b/linux-user/syscall_defs.h
> > @@ -673,6 +673,21 @@ typedef struct {
> >  
> >  #define TARGET_SI_PAD_SIZE ((TARGET_SI_MAX_SIZE - TARGET_SI_PREAMBLE_SIZE) 
> > / sizeof(int))
> >  
> > +/* Within QEMU the top 16 bits of si_code indicate which of the parts of
> > + * the union in target_siginfo is valid. This only applies between
> > + * host_to_target_siginfo_noswap() and tswap_siginfo(); it does not
> > + * appear either within host siginfo_t or in target_siginfo structures
> > + * which we get from the guest userspace program. (The Linux kernel
> > + * does a similar thing with using the top bits for its own internal
> > + * purposes but not letting them be visible to userspace.)
> > + */
> > +#define QEMU_SI_KILL 0
> > +#define QEMU_SI_TIMER 1
> > +#define QEMU_SI_POLL 2
> > +#define QEMU_SI_FAULT 3
> > +#define QEMU_SI_CHLD 4
> > +#define QEMU_SI_RT 5
> > +
> >  typedef struct target_siginfo {
> >  #ifdef TARGET_MIPS
> >     int si_signo;
> > -- 
> > 1.9.1
> >