[PATCH 1/2] lib: introduce kvasprintf_const

[Rasmus Villemoes <linux@rasmusvillemoes.dk>]

This adds kvasprintf_const which tries to use kstrdup_const if
possible: If the format string contains no % characters, or if the
format string is exactly "%s", we delegate to
kstrdup_const. Otherwise, we fall back to kvasprintf.

Just as for kstrdup_const, the main motivation is to save memory by
reusing .rodata when possible.

The return value should be freed by kfree_const, just like for
kstrdup_const.

There is deliberately no kasprintf_const: In the vast majority of
cases, the format string argument is a literal, so one can determine
statically whether one could instead use kstrdup_const directly (which
would also require one to change all corresponding kfree calls to
kfree_const).

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
---
 include/linux/kernel.h |  2 ++
 lib/kasprintf.c        | 16 ++++++++++++++++
 2 files changed, 18 insertions(+)

diff --git a/include/linux/kernel.h b/include/linux/kernel.h
index 5582410727cb..2c13f747ac2e 100644
--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -413,6 +413,8 @@ extern __printf(2, 3)
 char *kasprintf(gfp_t gfp, const char *fmt, ...);
 extern __printf(2, 0)
 char *kvasprintf(gfp_t gfp, const char *fmt, va_list args);
+extern __printf(2, 0)
+const char *kvasprintf_const(gfp_t gfp, const char *fmt, va_list args);
 
 extern __scanf(2, 3)
 int sscanf(const char *, const char *, ...);
diff --git a/lib/kasprintf.c b/lib/kasprintf.c
index 32f12150fc4f..f194e6e593e1 100644
--- a/lib/kasprintf.c
+++ b/lib/kasprintf.c
@@ -31,6 +31,22 @@ char *kvasprintf(gfp_t gfp, const char *fmt, va_list ap)
 }
 EXPORT_SYMBOL(kvasprintf);
 
+/*
+ * If fmt contains no % (or is exactly %s), use kstrdup_const. If fmt
+ * (or the sole vararg) points to rodata, we will then save a memory
+ * allocation and string copy. In any case, the return value should be
+ * freed using kfree_const().
+ */
+const char *kvasprintf_const(gfp_t gfp, const char *fmt, va_list ap)
+{
+	if (!strchr(fmt, '%'))
+		return kstrdup_const(fmt, gfp);
+	if (!strcmp(fmt, "%s"))
+		return kstrdup_const(va_arg(ap, const char*), gfp);
+	return kvasprintf(gfp, fmt, ap);
+}
+EXPORT_SYMBOL(kvasprintf_const);
+
 char *kasprintf(gfp_t gfp, const char *fmt, ...)
 {
 	va_list ap;
-- 
2.1.3