Commit 088c01f1 authored by Dave Martin's avatar Dave Martin Committed by Russell King
Browse files

ARM: 7007/1: alignment: Prevent ignoring of faults with ARMv6 unaligned access model 



Currently, it's possible to set the kernel to ignore alignment
faults when changing the alignment fault handling mode at runtime
via /proc/sys/alignment, even though this is undesirable on ARMv6
and above, where it can result in infinite spins where an un-fixed-
up instruction repeatedly faults.

In addition, the kernel clobbers any alignment mode specified on
the command-line if running on ARMv6 or above.

This patch factors out the necessary safety check into a couple of
new helper functions, and checks and modifies the fault handling
mode as appropriate on boot and on writes to /proc/cpu/alignment.

Prior to ARMv6, the behaviour is unchanged.

For ARMv6 and above, the behaviour changes as follows:

  * Attempting to ignore faults on ARMv6 results in the mode being
    forced to UM_FIXUP instead.  A warning is printed if this
    happened as a result of a write to /proc/cpu/alignment.  The
    user's UM_WARN bit (if present) is still honoured.

  * An alignment= argument from the kernel command-line is now
    honoured, except that the kernel will modify the specified mode
    as described above.  This is allows modes such as UM_SIGNAL and
    UM_WARN to be active immediately from boot, which is useful for
    debugging purposes.

Signed-off-by: default avatarDave Martin <dave.martin@linaro.org>
Acked-by: default avatarNicolas Pitre <nicolas.pitre@linaro.org>
Signed-off-by: default avatarRussell King <rmk+kernel@arm.linux.org.uk>
parent bf912d99

arch/arm/mm/alignment.c

+30 −12
Original line number Diff line number Diff line
@@ -95,6 +95,33 @@ static const char *usermode_action[] = { 
	"signal+warn"

};



/* Return true if and only if the ARMv6 unaligned access model is in use. */

static bool cpu_is_v6_unaligned(void)

{

	return cpu_architecture() >= CPU_ARCH_ARMv6 && (cr_alignment & CR_U);

}



static int safe_usermode(int new_usermode, bool warn)

{

	/*

	 * ARMv6 and later CPUs can perform unaligned accesses for

	 * most single load and store instructions up to word size.

	 * LDM, STM, LDRD and STRD still need to be handled.

	 *

	 * Ignoring the alignment fault is not an option on these

	 * CPUs since we spin re-faulting the instruction without

	 * making any progress.

	 */

	if (cpu_is_v6_unaligned() && !(new_usermode & (UM_FIXUP | UM_SIGNAL))) {

		new_usermode |= UM_FIXUP;



		if (warn)

			printk(KERN_WARNING "alignment: ignoring faults is unsafe on this CPU.  Defaulting to fixup mode.\n");

	}



	return new_usermode;

}



static int alignment_proc_show(struct seq_file *m, void *v)

{

	seq_printf(m, "User:\t\t%lu\n", ai_user);
@@ -125,7 +152,7 @@ static ssize_t alignment_proc_write(struct file *file, const char __user *buffer 
		if (get_user(mode, buffer))

			return -EFAULT;

		if (mode >= '0' && mode <= '5')

			ai_usermode = mode - '0';

			ai_usermode = safe_usermode(mode - '0', true);

	}

	return count;

}
@@ -926,20 +953,11 @@ static int __init alignment_init(void) 
		return -ENOMEM;

#endif



	/*

	 * ARMv6 and later CPUs can perform unaligned accesses for

	 * most single load and store instructions up to word size.

	 * LDM, STM, LDRD and STRD still need to be handled.

	 *

	 * Ignoring the alignment fault is not an option on these

	 * CPUs since we spin re-faulting the instruction without

	 * making any progress.

	 */

	if (cpu_architecture() >= CPU_ARCH_ARMv6 && (cr_alignment & CR_U)) {

	if (cpu_is_v6_unaligned()) {

		cr_alignment &= ~CR_A;

		cr_no_alignment &= ~CR_A;

		set_cr(cr_alignment);

		ai_usermode = UM_FIXUP;

		ai_usermode = safe_usermode(ai_usermode, false);

	}



	hook_fault_code(1, do_alignment, SIGBUS, BUS_ADRALN,

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