[PATCH v2 2/2] lib/string.c: Add wcslen()

[Nathan Chancellor]

A recent optimization change in LLVM [1] aims to transform certain loop
idioms into calls to strlen() or wcslen(). This change transforms the
first while loop in UniStrcat() into a call to wcslen(), breaking the
build when UniStrcat() gets inlined into alloc_path_with_tree_prefix():

  ld.lld: error: undefined symbol: wcslen
  >>> referenced by nls_ucs2_utils.h:54 (fs/smb/client/../../nls/nls_ucs2_utils.h:54)
  >>>               vmlinux.o:(alloc_path_with_tree_prefix)
  >>> referenced by nls_ucs2_utils.h:54 (fs/smb/client/../../nls/nls_ucs2_utils.h:54)
  >>>               vmlinux.o:(alloc_path_with_tree_prefix)

The kernel does not build with '-ffreestanding' (which would avoid this
transformation) because it does want libcall optimizations in general
and turning on '-ffreestanding' disables the majority of them. While
'-fno-builtin-wcslen' would be more targeted at the problem, it does not
work with LTO.

Add a basic wcslen() to avoid this linkage failure. While no
architecture or FORTIFY_SOURCE overrides this, add it to string.c
instead of string_helpers.c so that it is built with '-ffreestanding',
otherwise the compiler might transform it into a call to itself.

Cc: stable@xxxxxxxxxxxxxxx
Link: https://github.com/llvm/llvm-project/commit/9694844d7e36fd5e01011ab56b64f27b867aa72d [1]
Signed-off-by: Nathan Chancellor <nathan@xxxxxxxxxx>
---
 include/linux/string.h |  2 ++
 lib/string.c           | 11 +++++++++++
 2 files changed, 13 insertions(+)

diff --git a/include/linux/string.h b/include/linux/string.h
index 0403a4ca4c11..4a48f8eac301 100644
--- a/include/linux/string.h
+++ b/include/linux/string.h
@@ -7,6 +7,7 @@
 #include <linux/cleanup.h>	/* for DEFINE_FREE() */
 #include <linux/compiler.h>	/* for inline */
 #include <linux/types.h>	/* for size_t */
+#include <linux/nls_types.h>	/* for wchar_t */
 #include <linux/stddef.h>	/* for NULL */
 #include <linux/err.h>		/* for ERR_PTR() */
 #include <linux/errno.h>	/* for E2BIG */
@@ -203,6 +204,7 @@ extern __kernel_size_t strlen(const char *);
 #ifndef __HAVE_ARCH_STRNLEN
 extern __kernel_size_t strnlen(const char *,__kernel_size_t);
 #endif
+extern __kernel_size_t wcslen(const wchar_t *s);
 #ifndef __HAVE_ARCH_STRPBRK
 extern char * strpbrk(const char *,const char *);
 #endif
diff --git a/lib/string.c b/lib/string.c
index eb4486ed40d2..2c6f8c8f4159 100644
--- a/lib/string.c
+++ b/lib/string.c
@@ -21,6 +21,7 @@
 #include <linux/errno.h>
 #include <linux/limits.h>
 #include <linux/linkage.h>
+#include <linux/nls_types.h>
 #include <linux/stddef.h>
 #include <linux/string.h>
 #include <linux/types.h>
@@ -429,6 +430,16 @@ size_t strnlen(const char *s, size_t count)
 EXPORT_SYMBOL(strnlen);
 #endif
 
+size_t wcslen(const wchar_t *s)
+{
+	const wchar_t *sc;
+
+	for (sc = s; *sc != '\0'; ++sc)
+		/* nothing */;
+	return sc - s;
+}
+EXPORT_SYMBOL(wcslen);
+
 #ifndef __HAVE_ARCH_STRSPN
 /**
  * strspn - Calculate the length of the initial substring of @s which only contain letters in @accept

-- 
2.49.0