Re: [PATCH] module: make perm const, change BUG_ON to BUILD_BUG_ON

[Rusty Russell]

Dan Streetman <ddstreet@xxxxxxxx> writes:
> Change the struct kernel_param.perm field to a const, as it should never
> be changed.  Add inline functions that return a const value, which are
> compiled out, for kparam_[un]block_sysfs_r/w() macros to use; and change
> the BUG_ON() in those macros to BUILD_BUG_ON().

Nice win!

I think it's slightly clearer if we just declare:

        static __always_unused const u16 __param_perm_##name = (perm).

And indeed, gcc (at least 4.9) eliminates it correctly.

So, here's your patch with that mod, and the param.c noise removed.
It's in my pending queue for the moment.

I still am leaning towards just removing these wrappers and exposing
the (new) per-module mutex though.  Happy for that patch to replace this
one, if you send it.

Thanks,
Rusty.

From: Dan Streetman <ddstreet@xxxxxxxx>
Subject: module: make perm const, change BUG_ON to BUILD_BUG_ON

Change the struct kernel_param.perm field to a const, as it should never
be changed.  Add a static const name for this (which gcc compiles out)
for kparam_[un]block_sysfs_r/w() macros to use; and change the BUG_ON()
in those macros to BUILD_BUG_ON().

This will enforce that the kernel_param permissions are never changed,
and it will fail at build instead of runtime for any misuse of
sysfs param blocking.

I initially wanted to just use the const param.perm in the block_sysfs
checks, but unfortunately that also requires the param itself to be
const, in order to do const folding into a constant compiletime
condition to BUILD_BUG_ON(), and on some archs the params aren't defined
as const:

/* On alpha, ia64 and ppc64 relocations to global data cannot go into
   read-only sections (which is part of respective UNIX ABI on these
   platforms). So 'const' makes no sense and even causes compile failures
   with some compilers. */

Signed-off-by: Dan Streetman <ddstreet@xxxxxxxx>
Signed-off-by: Rusty Russell <rusty@xxxxxxxxxxxxxxx> (slight tweak)

diff --git a/include/linux/moduleparam.h b/include/linux/moduleparam.h
index 7e0079936396..347e11f3bc32 100644
--- a/include/linux/moduleparam.h
+++ b/include/linux/moduleparam.h
@@ -68,7 +68,7 @@ enum {
 struct kernel_param {
 	const char *name;
 	const struct kernel_param_ops *ops;
-	u16 perm;
+	const u16 perm;
 	s8 level;
 	u8 flags;
 	union {
@@ -215,8 +215,10 @@ struct kparam_array
 /* This is the fundamental function for registering boot/module
    parameters. */
 #define __module_param_call(prefix, name, ops, arg, perm, level, flags)	\
+	/* These allow kparam_block_sysfs_*() to use BUILD_BUG_ON() */	\
+	static __always_unused const u16 __param_perm_##name = (perm);	\
 	/* Default value instead of permissions? */			\
-	static const char __param_str_##name[] = prefix #name; \
+	static const char __param_str_##name[] = prefix #name;		\
 	static struct kernel_param __moduleparam_const __param_##name	\
 	__used								\
     __attribute__ ((unused,__section__ ("__param"),aligned(sizeof(void *)))) \
@@ -244,20 +246,20 @@ __check_old_set_param(int (*oldset)(const char *, struct kernel_param *))
  *
  * There's no point blocking write on a paramter that isn't writable via sysfs!
  */
-#define kparam_block_sysfs_write(name)			\
-	do {						\
-		BUG_ON(!(__param_##name.perm & 0222));	\
-		__kernel_param_lock();			\
+#define kparam_block_sysfs_write(name)					\
+	do {								\
+		BUILD_BUG_ON(!(__param_perm_##name & 0222));		\
+		__kernel_param_lock();					\
 	} while (0)
 
 /**
  * kparam_unblock_sysfs_write - allows sysfs to write to a parameter again.
  * @name: the name of the parameter
  */
-#define kparam_unblock_sysfs_write(name)		\
-	do {						\
-		BUG_ON(!(__param_##name.perm & 0222));	\
-		__kernel_param_unlock();		\
+#define kparam_unblock_sysfs_write(name)				\
+	do {								\
+		BUILD_BUG_ON(!(__param_perm_##name & 0222));		\
+		__kernel_param_unlock();				\
 	} while (0)
 
 /**
@@ -266,20 +268,20 @@ __check_old_set_param(int (*oldset)(const char *, struct kernel_param *))
  *
  * This also blocks sysfs writes.
  */
-#define kparam_block_sysfs_read(name)			\
-	do {						\
-		BUG_ON(!(__param_##name.perm & 0444));	\
-		__kernel_param_lock();			\
+#define kparam_block_sysfs_read(name)					\
+	do {								\
+		BUILD_BUG_ON(!(__param_perm_##name & 0444));		\
+		__kernel_param_lock();					\
 	} while (0)
 
 /**
  * kparam_unblock_sysfs_read - allows sysfs to read a parameter again.
  * @name: the name of the parameter
  */
-#define kparam_unblock_sysfs_read(name)			\
-	do {						\
-		BUG_ON(!(__param_##name.perm & 0444));	\
-		__kernel_param_unlock();		\
+#define kparam_unblock_sysfs_read(name)					\
+	do {								\
+		BUILD_BUG_ON(!(__param_perm_##name & 0444));		\
+		__kernel_param_unlock();				\
 	} while (0)
 
 #ifdef CONFIG_SYSFS
diff --git a/kernel/params.c b/kernel/params.c
index 0b9bbdf830cb..15715a3efb50 100644
--- a/kernel/params.c
+++ b/kernel/params.c
@@ -395,12 +395,11 @@ EXPORT_SYMBOL(param_ops_invbool);
 
 int param_set_bint(const char *val, const struct kernel_param *kp)
 {
-	struct kernel_param boolkp;
+	/* Match bool exactly, by re-using it. */
+	struct kernel_param boolkp = *kp;
 	bool v;
 	int ret;
 
-	/* Match bool exactly, by re-using it. */
-	boolkp = *kp;
 	boolkp.arg = &v;
 
 	ret = param_set_bool(val, &boolkp);
@@ -480,9 +479,8 @@ static int param_array_get(char *buffer, const struct kernel_param *kp)
 {
 	int i, off, ret;
 	const struct kparam_array *arr = kp->arr;
-	struct kernel_param p;
+	struct kernel_param p = *kp;
 
-	p = *kp;
 	for (i = off = 0; i < (arr->num ? *arr->num : arr->max); i++) {
 		if (i)
 			buffer[off++] = ',';
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/