[RFC PATCH] arm: SIGBUS on unsupported ARMv6 unaligned accesses

[Robin Murphy]

This patch changes the default behaviour for userspace alignment faults
on v6 from silent fixup to SIGBUS. This only affects code that violates
the v6 unaligned access model - bad assembly/JIT code or high-level
language code in violation of the relevant language spec - which should
be corrected rather than unwittingly relying on performance-degrading
fixups. Fixup behaviour can still be controlled from boot parameters or
at runtime for compatibility with existing incorrect software.

There will be no impact for v5 and earlier.

Signed-off-by: Robin Murphy <robin.murphy at arm.com>
---

This is very much the hard-line approach to the issue, but userspace
really shouldn't be depending on the ability to execute architecturally
invalid code; ultimately this is an ABI that deserves to be broken.
The only trouble with going in softly and just turning on the warning
alone is that it should already be rare enough to be easily missed.
"Break everything AND scream about it" as the default isn't going to
make any friends, but does make bad code as visible as possible.

Anyone is of course free to turn warnings off, and/or fixups back on -
defaulting to UM_SIGNAL on UM_IGNORE is to ensure fixup has to be
explicitly requested, by which point they should be well aware of what
they're doing and why.

It would be useful to know if people are actually running v6+ userspaces
that would be non-trivially impacted by this before considering
progressing beyond RFC.

 arch/arm/mm/alignment.c |    6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/arm/mm/alignment.c b/arch/arm/mm/alignment.c
index 9240364..9a93315 100644
--- a/arch/arm/mm/alignment.c
+++ b/arch/arm/mm/alignment.c
@@ -106,10 +106,10 @@ static int safe_usermode(int new_usermode, bool warn)
 	 * making any progress.
 	 */
 	if (cpu_is_v6_unaligned() && !(new_usermode & (UM_FIXUP | UM_SIGNAL))) {
-		new_usermode |= UM_FIXUP;
+		new_usermode |= UM_SIGNAL;
 
 		if (warn)
-			printk(KERN_WARNING "alignment: ignoring faults is unsafe on this CPU.  Defaulting to fixup mode.\n");
+			printk(KERN_WARNING "alignment: ignoring faults is unsafe on this CPU.  Defaulting to signal mode.\n");
 	}
 
 	return new_usermode;
@@ -971,7 +971,7 @@ static int __init alignment_init(void)
 		cr_alignment &= ~CR_A;
 		cr_no_alignment &= ~CR_A;
 		set_cr(cr_alignment);
-		ai_usermode = safe_usermode(ai_usermode, false);
+		ai_usermode = safe_usermode(ai_usermode | UM_WARN, false);
 	}
 #endif
 
-- 
1.7.9.5