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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: Dave Martin <dave.martin@linaro.org>
Acked-by: Nicolas Pitre <nicolas.pitre@linaro.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
This commit is contained in:
Dave Martin 2011-07-28 14:28:52 +01:00 committed by Russell King
parent bf912d99e9
commit 088c01f1e3
1 changed files with 30 additions and 12 deletions
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42
arch/arm/mm/alignment.c
View File

@ -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,