proc: calculate end pointer for /proc/*/* lookup at compile time

Compilers like to transform loops like

	for (i = 0; i < n; i++) {
		[use p[i]]
	}

into
	for (p = p0; p < end; p++) {
		...
	}

Do it by hand, so that it results in overall simpler loop
and smaller code.

Space savings:

	$ ./scripts/bloat-o-meter ../vmlinux-001 ../obj/vmlinux
	add/remove: 0/0 grow/shrink: 2/1 up/down: 4/-9 (-5)
	Function                                     old     new   delta
	proc_tid_base_lookup                          17      19      +2
	proc_tgid_base_lookup                         17      19      +2
	proc_pident_lookup                           179     170      -9

The same could be done to proc_pident_readdir(), but the code becomes
bigger for some reason.

[sfr@canb.auug.org.au: merge fix for proc_pident_lookup() API change]
  Link: http://lkml.kernel.org/r/20190131160135.4a8ae70b@canb.auug.org.au
Link: http://lkml.kernel.org/r/20190114200422.GB9680@avx2Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: James Morris <jmorris@namei.org>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Casey Schaufler <casey@schaufler-ca.com>
Cc: Kees Cook <keescook@chromium.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/proc/base.c

@@ -2459,11 +2459,10 @@ static struct dentry *proc_pident_instantiate(struct dentry *dentry,
 
 static struct dentry *proc_pident_lookup(struct inode *dir, 
 					 struct dentry *dentry,
-					 const struct pid_entry *ents,
-					 unsigned int nents)
+					 const struct pid_entry *p,
+					 const struct pid_entry *end)
 {
 	struct task_struct *task = get_proc_task(dir);
-	const struct pid_entry *p, *last;
 	struct dentry *res = ERR_PTR(-ENOENT);
 
 	if (!task)
@@ -2473,8 +2472,7 @@ static struct dentry *proc_pident_lookup(struct inode *dir,
 	 * Yes, it does not scale. And it should not. Don't add
 	 * new entries into /proc/<tgid>/ without very good reasons.
 	 */
-	last = &ents[nents];
-	for (p = ents; p < last; p++) {
+	for (; p < end; p++) {
 		if (p->len != dentry->d_name.len)
 			continue;
 		if (!memcmp(dentry->d_name.name, p->name, p->len)) {
@@ -2610,7 +2608,7 @@ static struct dentry *proc_##LSM##_attr_dir_lookup(struct inode *dir, \
 { \
 	return proc_pident_lookup(dir, dentry, \
 				  LSM##_attr_dir_stuff, \
-				  ARRAY_SIZE(LSM##_attr_dir_stuff)); \
+				  LSM##_attr_dir_stuff + ARRAY_SIZE(LSM##_attr_dir_stuff)); \
 } \
 \
 static const struct inode_operations proc_##LSM##_attr_dir_inode_ops = { \
@@ -2655,7 +2653,8 @@ static struct dentry *proc_attr_dir_lookup(struct inode *dir,
 				struct dentry *dentry, unsigned int flags)
 {
 	return proc_pident_lookup(dir, dentry,
-				  attr_dir_stuff, ARRAY_SIZE(attr_dir_stuff));
+				  attr_dir_stuff,
+				  attr_dir_stuff + ARRAY_SIZE(attr_dir_stuff));
 }
 
 static const struct inode_operations proc_attr_dir_inode_operations = {
@@ -3091,7 +3090,8 @@ static const struct file_operations proc_tgid_base_operations = {
 static struct dentry *proc_tgid_base_lookup(struct inode *dir, struct dentry *dentry, unsigned int flags)
 {
 	return proc_pident_lookup(dir, dentry,
-				  tgid_base_stuff, ARRAY_SIZE(tgid_base_stuff));
+				  tgid_base_stuff,
+				  tgid_base_stuff + ARRAY_SIZE(tgid_base_stuff));
 }
 
 static const struct inode_operations proc_tgid_base_inode_operations = {
@@ -3463,7 +3463,8 @@ static int proc_tid_base_readdir(struct file *file, struct dir_context *ctx)
 static struct dentry *proc_tid_base_lookup(struct inode *dir, struct dentry *dentry, unsigned int flags)
 {
 	return proc_pident_lookup(dir, dentry,
-				  tid_base_stuff, ARRAY_SIZE(tid_base_stuff));
+				  tid_base_stuff,
+				  tid_base_stuff + ARRAY_SIZE(tid_base_stuff));
 }
 
 static const struct file_operations proc_tid_base_operations = {