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Subject[PATCH 11/15] x86/split_lock: Add sysfs interface to control user mode behavior
Date
Add the interface /sys/kernel/split_lock/user_mode to allow user to
choose to either generate SIGBUS (default) when hitting split lock in
user or re-execute the user faulting instruction without generating
SIGBUS signal.

Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>
---
arch/x86/kernel/cpu/split_lock.c | 89 ++++++++++++++++++++++++++++++++++++++++
1 file changed, 89 insertions(+)

diff --git a/arch/x86/kernel/cpu/split_lock.c b/arch/x86/kernel/cpu/split_lock.c
index 5d399b09c1c8..02b461c48b3c 100644
--- a/arch/x86/kernel/cpu/split_lock.c
+++ b/arch/x86/kernel/cpu/split_lock.c
@@ -32,6 +32,19 @@ static DECLARE_DELAYED_WORK(delayed_work, delayed_reenable_split_lock);

static DEFINE_MUTEX(split_lock_mutex);

+enum {
+ USER_MODE_SIGBUS,
+ USER_MODE_RE_EXECUTE,
+ USER_MODE_LAST
+};
+
+static int user_mode_reaction = USER_MODE_SIGBUS;
+
+static const char * const user_modes[USER_MODE_LAST] = {
+ [USER_MODE_SIGBUS] = "sigbus",
+ [USER_MODE_RE_EXECUTE] = "re-execute",
+};
+
/*
* On processors not supporting #AC exception for split lock feature,
* MSR_TEST_CTL may not exist or MSR_TEST_CTL exists but the bit 29 is
@@ -214,6 +227,16 @@ static void delayed_reenable_split_lock(struct work_struct *w)
_setup_split_lock(ENABLE_SPLIT_LOCK_AC);
}

+static unsigned long eflags_ac(struct pt_regs *regs)
+{
+ return regs->flags & X86_EFLAGS_AC;
+}
+
+static unsigned long cr0_am(struct pt_regs *regs)
+{
+ return read_cr0() & X86_CR0_AM;
+}
+
/* Will the faulting instruction be re-executed? */
static bool re_execute(struct pt_regs *regs)
{
@@ -224,6 +247,24 @@ static bool re_execute(struct pt_regs *regs)
if (!user_mode(regs))
return true;

+ /*
+ * Now check if the user faulting instruction can be re-executed.
+ *
+ * If both CR0.AM (Alignment Mask) and EFLAGS.AC (Alignment Check)
+ * are set in user space, any misalignment including split lock
+ * can trigger #AC. In this case, we just issue SIGBUS as standard
+ * #AC handler to the user process because split lock is not the
+ * definite reason for triggering this #AC.
+ *
+ * If either CR0.AM or EFLAGS.AC is zero, the only reason for
+ * triggering this #AC is split lock. So the faulting instruction
+ * can be re-executed if required by user.
+ */
+ if (cr0_am(regs) == 0 || eflags_ac(regs) == 0)
+ /* User faulting instruction will be re-executed if required. */
+ if (user_mode_reaction == USER_MODE_RE_EXECUTE)
+ return true;
+
return false;
}

@@ -323,8 +364,56 @@ static ssize_t enable_store(struct kobject *kobj, struct kobj_attribute *attr,

static struct kobj_attribute split_lock_ac_enable = __ATTR_RW(enable);

+static ssize_t
+user_mode_show(struct kobject *kobj, struct kobj_attribute *attr, char *buf)
+{
+ char *s = buf;
+ int reaction;
+
+ for (reaction = 0; reaction < USER_MODE_LAST; reaction++) {
+ if (reaction == user_mode_reaction)
+ s += sprintf(s, "[%s] ", user_modes[reaction]);
+ else
+ s += sprintf(s, "%s ", user_modes[reaction]);
+ }
+
+ if (s != buf)
+ /* convert the last space to a newline */
+ *(s - 1) = '\n';
+
+ return s - buf;
+}
+
+static ssize_t
+user_mode_store(struct kobject *kobj, struct kobj_attribute *attr,
+ const char *buf, size_t count)
+{
+ int reaction, len, error = -EINVAL;
+ const char * const *s, *p;
+
+ p = memchr(buf, '\n', count);
+ len = p ? p - buf : count;
+
+ mutex_lock(&split_lock_mutex);
+ reaction = USER_MODE_SIGBUS;
+ for (s = &user_modes[reaction]; reaction <= USER_MODE_LAST;
+ s++, reaction++) {
+ if (*s && len == strlen(*s) && !strncmp(buf, *s, len)) {
+ user_mode_reaction = reaction;
+ error = 0;
+ break;
+ }
+ }
+ mutex_unlock(&split_lock_mutex);
+
+ return error ? error : count;
+}
+
+static struct kobj_attribute split_lock_ac_user = __ATTR_RW(user_mode);
+
static struct attribute *split_lock_attrs[] = {
&split_lock_ac_enable.attr,
+ &split_lock_ac_user.attr,
NULL,
};

--
2.5.0
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