linux-kernel - [PATCH] kbuild: Add '-fno-builtin-wcslen'

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Message-Id: <20250407-fno-builtin-wcslen-v1-1-6775ce759b15@kernel.org>
Date: Mon, 07 Apr 2025 16:22:12 -0700
From: Nathan Chancellor <nathan@...nel.org>
To: Linus Torvalds <torvalds@...ux-foundation.org>, 
 Masahiro Yamada <masahiroy@...nel.org>
Cc: Kees Cook <kees@...nel.org>, Nicolas Schier <nicolas.schier@...ux.dev>, 
 Nick Desaulniers <nick.desaulniers+lkml@...il.com>, 
 Bill Wendling <morbo@...gle.com>, Justin Stitt <justinstitt@...gle.com>, 
 linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org, 
 llvm@...ts.linux.dev, stable@...r.kernel.org, 
 Nathan Chancellor <nathan@...nel.org>
Subject: [PATCH] kbuild: Add '-fno-builtin-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)

Disable this optimization with '-fno-builtin-wcslen', which prevents the
compiler from assuming that wcslen() is available in the kernel's C
library

Cc: stable@...r.kernel.org
Link: https://github.com/llvm/llvm-project/commit/9694844d7e36fd5e01011ab56b64f27b867aa72d [1]
Signed-off-by: Nathan Chancellor <nathan@...nel.org>
---
 Makefile | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/Makefile b/Makefile
index 38689a0c3605..f42418556507 100644
--- a/Makefile
+++ b/Makefile
@@ -1068,6 +1068,9 @@ ifdef CONFIG_CC_IS_GCC
 KBUILD_CFLAGS   += -fconserve-stack
 endif
 
+# Ensure compilers do not transform certain loops into calls to wcslen()
+KBUILD_CFLAGS += -fno-builtin-wcslen
+
 # change __FILE__ to the relative path to the source directory
 ifdef building_out_of_srctree
 KBUILD_CPPFLAGS += $(call cc-option,-ffile-prefix-map=$(srcroot)/=)

---
base-commit: 0af2f6be1b4281385b618cb86ad946eded089ac8
change-id: 20250407-fno-builtin-wcslen-90a858ae7d54

Best regards,
-- 
Nathan Chancellor <nathan@...nel.org>