Re: [PATCH] compat: Fix endian issue in union sigval

From: Catalin Marinas
Date: Sat Feb 14 2015 - 06:32:32 EST

On Fri, Feb 13, 2015 at 04:56:29PM -0500, Chris Metcalf wrote:
> On 2/13/2015 5:44 AM, Catalin Marinas wrote:
> >On Fri, Feb 13, 2015 at 04:00:43PM +0800, Bamvor Jian Zhang wrote:
> >>diff --git a/arch/arm64/kernel/signal32.c b/arch/arm64/kernel/signal32.c
> >>index e299de396e9b..32601939a3c8 100644
> >>--- a/arch/arm64/kernel/signal32.c
> >>+++ b/arch/arm64/kernel/signal32.c
> >>@@ -154,8 +154,7 @@ int copy_siginfo_to_user32(compat_siginfo_t __user *to, const siginfo_t *from)
> >> case __SI_TIMER:
> >> err |= __put_user(from->si_tid, &to->si_tid);
> >> err |= __put_user(from->si_overrun, &to->si_overrun);
> >>- err |= __put_user((compat_uptr_t)(unsigned long)from->si_ptr,
> >>- &to->si_ptr);
> >>+ err |= __put_user(from->si_int, &to->si_int);
> >> break;
> >> case __SI_POLL:
> >> err |= __put_user(from->si_band, &to->si_band);
> >>@@ -184,7 +183,7 @@ int copy_siginfo_to_user32(compat_siginfo_t __user *to, const siginfo_t *from)
> >> case __SI_MESGQ: /* But this is */
> >> err |= __put_user(from->si_pid, &to->si_pid);
> >> err |= __put_user(from->si_uid, &to->si_uid);
> >>- err |= __put_user((compat_uptr_t)(unsigned long)from->si_ptr, &to->si_ptr);
> >>+ err |= __put_user(from->si_int, &to->si_int);
> >> break;
> >> case __SI_SYS:
> >> err |= __put_user((compat_uptr_t)(unsigned long)
> I must be confused here, but I don't see that these do anything different.
> If we are writing 32 bits to to->si_ptr or to->si_int, either way the high 32 bits
> are irrelevant. So whether we read it from from->si_ptr and massage the high bits,
> or just read it from from->si_int as a straight-up 32-bit quantity, either way it
> seems we should end up writing the same bits to userspace.
> I would understand the argument if we were overlaying the si_ptr/si_int union
> from a kernel-side siginfo_t where si_ptr and si_int are different sizes
> onto userspace, but it doesn't seem we ever do that.

That's the problem, "from" above is a kernel siginfo_t while "to" is a
compat_siginfo_t. The call paths go something like:

1. user populates sival_int compat_sigevent and invokes
2. kernel get_compat_sigevent() copies compat_sigevent into the native
sigevent. compat and native sival_int are the same, no problem so
far. The other half of 64-bit sival_ptr is zeroed by a memset in this
function (this other half can be top or bottom, depending on
3. signal is about to be delivered to user via arch code. The
compat_ptr(from->si_ptr) conversion always takes the least
significant part of the native si_ptr. On big endian 64-bit, this is
zero because get_compat_sigevent() populated the top part of si_ptr
with si_int.

So delivering such signals to compat user always sets si_int to 0.
Little endian is fine.

A similar example is sys_timer_create() which takes a sigevent argument.
Maybe Bamvor has a test case.

(btw, I'm off for a week, I'll follow up when I get back)

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