Re: [PATCH 16/20] arm64: signal32: move ilp32 and aarch32 common code to separated file

[Yury Norov]

On Mon, Jun 19, 2017 at 05:16:42PM +0100, James Morse wrote:
> Hi Yury,
> 
> On 04/06/17 13:00, Yury Norov wrote:
> > Signed-off-by: Yury Norov <ynorov@xxxxxxxxxxxxxxxxxx>
> 
> Can I offer a body for the commit message:
> ILP32 needs to mix 32bit struct siginfo and 64bit sigframe for its signal
> handlers. Move the existing compat code for copying siginfo to user space and
> manipulating signal masks into signal32_common.c so it can be used to deliver
> aarch32 and ilp32 signals.

Ok

> > diff --git a/arch/arm64/include/asm/signal32.h b/arch/arm64/include/asm/signal32.h
> > index e68fcce538e1..1c4ede717bd2 100644
> > --- a/arch/arm64/include/asm/signal32.h
> > +++ b/arch/arm64/include/asm/signal32.h
> > @@ -13,6 +13,9 @@
> >   * You should have received a copy of the GNU General Public License
> >   * along with this program.  If not, see <http://www.gnu.org/licenses/>.
> >   */
> > +
> > +#include <asm/signal32_common.h>
> > +
> >  #ifndef __ASM_SIGNAL32_H
> >  #define __ASM_SIGNAL32_H
> 
> Nit: This should go inside the guard.
 
Ok, thanks. Will fix this and all below
 
> > diff --git a/arch/arm64/kernel/signal32_common.c b/arch/arm64/kernel/signal32_common.c
> > new file mode 100644
> > index 000000000000..5bddc25dca12
> > --- /dev/null
> > +++ b/arch/arm64/kernel/signal32_common.c
> > @@ -0,0 +1,135 @@
> [...]
> > +#include <linux/compat.h>
> > +#include <linux/signal.h>
> > +#include <linux/ratelimit.h>
> 
> What do you need ratelimit.h for?
> 
> 
> > +#include <linux/uaccess.h>
> > +
> > +#include <asm/esr.h>
> 
> I can't see anything using these ESR_ macros in here...
> 
> 
> > +#include <asm/fpsimd.h>
> 
> This was for the VFP save/restore code, which you didn't move...
> 
> 
> > +#include <asm/signal32_common.h>
> > +#include <asm/unistd.h>
> 
> [...]
> 
> 
> > +int copy_siginfo_to_user32(compat_siginfo_t __user *to, const siginfo_t *from)
> [...]
> > +	case __SI_FAULT:
> > +		err |= __put_user((compat_uptr_t)(unsigned long)from->si_addr,
> > +				  &to->si_addr);
> 
> This looks tricky. si_addr comes from FAR_EL1 when user-space touches something
> it shouldn't. This could be a 64bit value as ilp32 processes can still branch to
> 64bit addresses in registers and generate loads that cross the invisible 4GB
> boundary. Here you truncate the 64bit address.
> Obviously this can't happen at all with aarch32, and for C programs its into
> undefined-behaviour territory, but it doesn't feel right to pass an address to
> user-space that we know is wrong... but we don't have an alternative.
> 
> This looks like a class of problem particular to ilp32/x32: 'accessed an address
> you can't encode with a signal'. After a quick dig in x86's x32 code, it looks
> like they only pass the first 32bits of si_addr too.
> 
> One option is to mint a new si_code to go with SIGBUS meaning something like
> 'address overflowed si_addr'. Alternatively we could just kill tasks that do this.

New SIGBUS sounds reasonable at the first glance, but I think it should be
discussed widely at first, and the patch that implements it should touch
all arches that may be affected.

Yury