linux-kernel - [PATCH 2/2] um: Disable auto variable initialization for stub

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Message-Id: <20241016-uml-fix-stub_exe-clang-v1-2-3d6381dc5a78@kernel.org>
Date: Wed, 16 Oct 2024 14:12:38 -0700
From: Nathan Chancellor <nathan@...nel.org>
To: Richard Weinberger <richard@....at>, 
 Anton Ivanov <anton.ivanov@...bridgegreys.com>, 
 Johannes Berg <johannes@...solutions.net>
Cc: Nick Desaulniers <ndesaulniers@...gle.com>, 
 Bill Wendling <morbo@...gle.com>, Justin Stitt <justinstitt@...gle.com>, 
 Benjamin Berg <benjamin.berg@...el.com>, linux-um@...ts.infradead.org, 
 linux-kernel@...r.kernel.org, llvm@...ts.linux.dev, patches@...ts.linux.dev, 
 Nathan Chancellor <nathan@...nel.org>
Subject: [PATCH 2/2] um: Disable auto variable initialization for
 stub_exe.c

When automatic variable initialization is enabled via
CONFIG_INIT_STACK_ALL_{PATTERN,ZERO}, clang will insert a call to
memset() to initialize an object created with __builtin_alloca(). This
ultimately breaks the build when linking stub_exe because it is a
standalone executable that does not include or link against memset().

  ld: arch/um/kernel/skas/stub_exe.o: in function `_start':
  arch/um/kernel/skas/stub_exe.c:83:(.ltext+0x15): undefined reference to `memset'

Disable automatic variable initialization for stub_exe.c by passing the
default value of 'uninitialized' to '-ftrivial-auto-var-init', which
avoids generating the call to memset(). This code is small and runs
quickly as it is just designed to set up an environment, so stack
variable initialization is unnecessary overhead for little gain.

Fixes: 32e8eaf263d9 ("um: use execveat to create userspace MMs")
Signed-off-by: Nathan Chancellor <nathan@...nel.org>
---
 arch/um/kernel/skas/Makefile | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/arch/um/kernel/skas/Makefile b/arch/um/kernel/skas/Makefile
index f93db893b8236cf805edd01b41501d51dd8f0a35..f6a219074772283f3c6c6d8d6ccaa8a1cfc24e33 100644
--- a/arch/um/kernel/skas/Makefile
+++ b/arch/um/kernel/skas/Makefile
@@ -39,6 +39,11 @@ targets += stub_exe.dbg stub_exe $(stub_exe_objs-y)
 
 CFLAGS_stub.o := $(CFLAGS_NO_HARDENING)
 CFLAGS_stub_exe.o := $(CFLAGS_NO_HARDENING)
+
+# Clang will call memset() from __builtin_alloca() when stack variable
+# initialization is enabled, which is used in stub_exe.c.
+CFLAGS_stub_exe.o += $(call cc-option, -ftrivial-auto-var-init=uninitialized)
+
 UNPROFILE_OBJS := stub.o stub_exe.o
 KCOV_INSTRUMENT := n
 

-- 
2.47.0