[PATCH RFC 1/5] workqueue: fix parse_affn_scope() prefix matching bug

[Breno Leitao <leitao@debian.org>]

parse_affn_scope() uses strncasecmp() with the length of the candidate
name, which means it only checks if the input *starts with* a known
scope name.

Given that the upcoming diff will create "cache_shard" affinity scope,
writing "cache_shard" to a workqueue's affinity_scope sysfs attribute
always matches "cache" first, making it impossible to select
"cache_shard" via sysfs, so, this fix enable it to distinguish "cache"
and "cache_shard"

Fix by replacing the hand-rolled prefix matching loop with
sysfs_match_string(), which uses sysfs_streq() for exact matching
(modulo trailing newlines). Also add the missing const qualifier to
the wq_affn_names[] array declaration.

Note that sysfs_streq() is case-sensitive, unlike the previous
strncasecmp() approach. This is intentional and consistent with
how other sysfs attributes handle string matching in the kernel.

Signed-off-by: Breno Leitao <leitao@debian.org>
---
 kernel/workqueue.c | 10 ++--------
 1 file changed, 2 insertions(+), 8 deletions(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index aeaec79bc09c4..028afc3d14e59 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -404,7 +404,7 @@ struct work_offq_data {
 	u32			flags;
 };
 
-static const char *wq_affn_names[WQ_AFFN_NR_TYPES] = {
+static const char * const wq_affn_names[WQ_AFFN_NR_TYPES] = {
 	[WQ_AFFN_DFL]		= "default",
 	[WQ_AFFN_CPU]		= "cpu",
 	[WQ_AFFN_SMT]		= "smt",
@@ -7063,13 +7063,7 @@ int workqueue_unbound_housekeeping_update(const struct cpumask *hk)
 
 static int parse_affn_scope(const char *val)
 {
-	int i;
-
-	for (i = 0; i < ARRAY_SIZE(wq_affn_names); i++) {
-		if (!strncasecmp(val, wq_affn_names[i], strlen(wq_affn_names[i])))
-			return i;
-	}
-	return -EINVAL;
+	return sysfs_match_string(wq_affn_names, val);
 }
 
 static int wq_affn_dfl_set(const char *val, const struct kernel_param *kp)

-- 
2.52.0