[PATCH v2] um: Fix misaligned stack in stub_exe

[David Gow <davidgow@google.com>]

The stub_exe could segfault when built with some compilers (e.g. gcc
13.2.0), as SSE instructions which relied on stack alignment could be
generated, but the stack was misaligned.

This seems to be due to the __start entry point being run with a 16-byte
aligned stack, but the x86_64 SYSV ABI wanting the stack to be so
aligned _before_ a function call (so it is misaligned when the function
is entered due to the return address being pushed). The function
prologue then realigns it. Because the entry point is never _called_,
and hence there is no return address, the prologue is therefore actually
misaligning it, and causing the generated movaps instructions to
SIGSEGV. This results in the following error:
start_userspace : expected SIGSTOP, got status = 139

Force the compiler to emit code to re-align the stack in real_init(), so
that the generated SSE code doesn't crash. This isn't necessarily the
_correct_ way of solving the problem, but it avoids the need to rewrite
__start in assembly for each architecture for now.

Fixes: 32e8eaf263d9 ("um: use execveat to create userspace MMs")
Signed-off-by: David Gow <davidgow@google.com>
---

Changes since v1:
https://lore.kernel.org/linux-um/20241017231007.1500497-2-davidgow@google.com/
- Use force_arg_align_pointer on real_init() instead of naked on
  __start, which works with clang.

 arch/um/kernel/skas/stub_exe.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

---
diff --git a/arch/um/kernel/skas/stub_exe.c b/arch/um/kernel/skas/stub_exe.c
index 04f75c577f1a..50fded2384e6 100644
--- a/arch/um/kernel/skas/stub_exe.c
+++ b/arch/um/kernel/skas/stub_exe.c
@@ -6,7 +6,7 @@
 
 void _start(void);
 
-noinline static void real_init(void)
+noinline __attribute__((force_align_arg_pointer)) static void real_init(void)
 {
 	struct stub_init_data init_data;
 	unsigned long res;
-- 
2.47.0.105.g07ac214952-goog