linux-kernel - [PATCH 1/2] lib/string.c: Add wcslen()

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Message-Id: <20250325-string-add-wcslen-for-llvm-opt-v1-1-b8f1e2c17888@kernel.org>
Date: Tue, 25 Mar 2025 08:45:18 -0700
From: Nathan Chancellor <nathan@...nel.org>
To: Kees Cook <kees@...nel.org>
Cc: Andy Shevchenko <andy@...nel.org>, 
 Nick Desaulniers <nick.desaulniers+lkml@...il.com>, 
 Bill Wendling <morbo@...gle.com>, Justin Stitt <justinstitt@...gle.com>, 
 Ard Biesheuvel <ardb@...nel.org>, linux-kernel@...r.kernel.org, 
 linux-hardening@...r.kernel.org, llvm@...ts.linux.dev, 
 linux-efi@...r.kernel.org, stable@...r.kernel.org, 
 Nathan Chancellor <nathan@...nel.org>
Subject: [PATCH 1/2] lib/string.c: Add wcslen()

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. Give
wcslen() an internal prototype to avoid -Wmissing-prototypes (a global
prototype needs a little extra work and it is not currently needed).

Cc: stable@...r.kernel.org
Link: https://github.com/llvm/llvm-project/commit/9694844d7e36fd5e01011ab56b64f27b867aa72d [1]
Signed-off-by: Nathan Chancellor <nathan@...nel.org>
---
 lib/string.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/lib/string.c b/lib/string.c
index eb4486ed40d2..bbee8a9e4d83 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.h>
 #include <linux/stddef.h>
 #include <linux/string.h>
 #include <linux/types.h>
@@ -429,6 +430,17 @@ size_t strnlen(const char *s, size_t count)
 EXPORT_SYMBOL(strnlen);
 #endif

+size_t wcslen(const wchar_t *s);
+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