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[2/2] um: Disable auto variable initialization for stub_exe.c

Message ID 20241016-uml-fix-stub_exe-clang-v1-2-3d6381dc5a78@kernel.org
State Accepted
Headers show
Series [1/2] um: Fix passing '-n' to linker for stub_exe | expand

Commit Message

Nathan Chancellor Oct. 16, 2024, 9:12 p.m. UTC 
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@kernel.org>
---
 arch/um/kernel/skas/Makefile | 5 +++++
 1 file changed, 5 insertions(+)
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Patch

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