capability: just use a 'u64' instead of a 'u32[2]' array

Back in 2008 we extended the capability bits from 32 to 64, and we did
it by extending the single 32-bit capability word from one word to an
array of two words.  It was then obfuscated by hiding the "2" behind two
macro expansions, with the reasoning being that maybe it gets extended
further some day.

That reasoning may have been valid at the time, but the last thing we
want to do is to extend the capability set any more.  And the array of
values not only causes source code oddities (with loops to deal with
it), but also results in worse code generation.  It's a lose-lose
situation.

So just change the 'u32[2]' into a 'u64' and be done with it.

We still have to deal with the fact that the user space interface is
designed around an array of these 32-bit values, but that was the case
before too, since the array layouts were different (ie user space
doesn't use an array of 32-bit values for individual capability masks,
but an array of 32-bit slices of multiple masks).

So that marshalling of data is actually simplified too, even if it does
remain somewhat obscure and odd.

This was all triggered by my reaction to the new "cap_isidentical()"
introduced recently.  By just using a saner data structure, it went from

	unsigned __capi;
	CAP_FOR_EACH_U32(__capi) {
		if (a.cap[__capi] != b.cap[__capi])
			return false;
	}
	return true;

to just being

	return a.val == b.val;

instead.  Which is rather more obvious both to humans and to compilers.

Cc: Mateusz Guzik <mjguzik@gmail.com>
Cc: Casey Schaufler <casey@schaufler-ca.com>
Cc: Serge Hallyn <serge@hallyn.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Paul Moore <paul@paul-moore.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 18 insertions, 23 deletions

diff --git a/kernel/umh.c b/kernel/umh.c
index fbf872c624cb..2a4708277335 100644
--- a/kernel/umh.c
+++ b/kernel/umh.c
@@ -501,9 +501,9 @@ static int proc_cap_handler(struct ctl_table *table, int write,
 			 void *buffer, size_t *lenp, loff_t *ppos)
 {
 	struct ctl_table t;
-	unsigned long cap_array[_KERNEL_CAPABILITY_U32S];
-	kernel_cap_t new_cap;
-	int err, i;
+	unsigned long cap_array[2];
+	kernel_cap_t new_cap, *cap;
+	int err;
 
 	if (write && (!capable(CAP_SETPCAP) ||
 		      !capable(CAP_SYS_MODULE)))
@@ -514,14 +514,16 @@ static int proc_cap_handler(struct ctl_table *table, int write,
 	 * userspace if this is a read.
 	 */
 	spin_lock(&umh_sysctl_lock);
-	for (i = 0; i < _KERNEL_CAPABILITY_U32S; i++)  {
-		if (table->data == CAP_BSET)
-			cap_array[i] = usermodehelper_bset.cap[i];
-		else if (table->data == CAP_PI)
-			cap_array[i] = usermodehelper_inheritable.cap[i];
-		else
-			BUG();
-	}
+	if (table->data == CAP_BSET)
+		cap = &usermodehelper_bset;
+	else if (table->data == CAP_PI)
+		cap = &usermodehelper_inheritable;
+	else
+		BUG();
+
+	/* Legacy format: capabilities are exposed as two 32-bit values */
+	cap_array[0] = (u32) cap->val;
+	cap_array[1] = cap->val >> 32;
 	spin_unlock(&umh_sysctl_lock);
 
 	t = *table;
@@ -535,22 +537,15 @@ static int proc_cap_handler(struct ctl_table *table, int write,
 	if (err < 0)
 		return err;
 
-	/*
-	 * convert from the sysctl array of ulongs to the kernel_cap_t
-	 * internal representation
-	 */
-	for (i = 0; i < _KERNEL_CAPABILITY_U32S; i++)
-		new_cap.cap[i] = cap_array[i];
+	new_cap.val = (u32)cap_array[0];
+	new_cap.val += (u64)cap_array[1] << 32;
 
 	/*
 	 * Drop everything not in the new_cap (but don't add things)
 	 */
 	if (write) {
 		spin_lock(&umh_sysctl_lock);
-		if (table->data == CAP_BSET)
-			usermodehelper_bset = cap_intersect(usermodehelper_bset, new_cap);
-		if (table->data == CAP_PI)
-			usermodehelper_inheritable = cap_intersect(usermodehelper_inheritable, new_cap);
+		*cap = cap_intersect(*cap, new_cap);
 		spin_unlock(&umh_sysctl_lock);
 	}
 
@@ -561,14 +556,14 @@ struct ctl_table usermodehelper_table[] = {
 	{
 		.procname	= "bset",
 		.data		= CAP_BSET,
-		.maxlen		= _KERNEL_CAPABILITY_U32S * sizeof(unsigned long),
+		.maxlen		= 2 * sizeof(unsigned long),
 		.mode		= 0600,
 		.proc_handler	= proc_cap_handler,
 	},
 	{
 		.procname	= "inheritable",
 		.data		= CAP_PI,
-		.maxlen		= _KERNEL_CAPABILITY_U32S * sizeof(unsigned long),
+		.maxlen		= 2 * sizeof(unsigned long),
 		.mode		= 0600,
 		.proc_handler	= proc_cap_handler,
 	},