Re: [PATCH v2] um: Fix misaligned stack in stub_exe

[Johannes Berg <johannes@sipsolutions.net>]

Thanks :)
> 
> 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.

I already applied it, so need to fix on top of it now, not replace it.

However I was just playing with the below - was just looking at the size
though, but what do you think?

johannes


From 57c5a80a4db2de33a11a5a20fcbea8f3643844f5 Mon Sep 17 00:00:00 2001
From: Johannes Berg <johannes.berg@intel.com>
Date: Tue, 22 Oct 2024 11:48:21 +0200
Subject: [PATCH] um: make stub_exe _start() pure inline asm

Since __attribute__((naked)) cannot be used with functions
containing C statements, just generate the few instructions
it needs in assembly directly.

Fixes: 8508a5e0e9db ("um: Fix misaligned stack in stub_exe")
Link: https://lore.kernel.org/linux-um/CABVgOSntH-uoOFMP5HwMXjx_f1osMnVdhgKRKm4uz6DFm2Lb8Q@mail.gmail.com/
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
---
 arch/um/kernel/skas/stub_exe.c      | 8 +-------
 arch/x86/um/shared/sysdep/stub_32.h | 8 ++++++++
 arch/x86/um/shared/sysdep/stub_64.h | 8 ++++++++
 3 files changed, 17 insertions(+), 7 deletions(-)

diff --git a/arch/um/kernel/skas/stub_exe.c b/arch/um/kernel/skas/stub_exe.c
index 722ce6267476..a61f9c008233 100644
--- a/arch/um/kernel/skas/stub_exe.c
+++ b/arch/um/kernel/skas/stub_exe.c
@@ -81,11 +81,5 @@ noinline static void real_init(void)
 
 __attribute__((naked)) void _start(void)
 {
-	char *alloc;
-
-	/* Make enough space for the stub (including space for alignment) */
-	alloc = __builtin_alloca((1 + 2 * STUB_DATA_PAGES - 1) * UM_KERN_PAGE_SIZE);
-	asm volatile("" : "+r,m"(alloc) : : "memory");
-
-	real_init();
+	stub_start(real_init);
 }
diff --git a/arch/x86/um/shared/sysdep/stub_32.h b/arch/x86/um/shared/sysdep/stub_32.h
index 631a18d0ff44..760e8ce8093f 100644
--- a/arch/x86/um/shared/sysdep/stub_32.h
+++ b/arch/x86/um/shared/sysdep/stub_32.h
@@ -123,4 +123,12 @@ static __always_inline void *get_stub_data(void)
 
 	return (void *)ret;
 }
+
+#define stub_start(fn)							\
+	asm volatile (							\
+		"subl %0,%%esp ;"					\
+		"movl %1, %%eax ; "					\
+		"call *%%eax ;"						\
+		:: "i" ((STUB_DATA_PAGES + 1) * UM_KERN_PAGE_SIZE),	\
+		   "i" (&fn))
 #endif
diff --git a/arch/x86/um/shared/sysdep/stub_64.h b/arch/x86/um/shared/sysdep/stub_64.h
index 17153dfd780a..148bf423289e 100644
--- a/arch/x86/um/shared/sysdep/stub_64.h
+++ b/arch/x86/um/shared/sysdep/stub_64.h
@@ -126,4 +126,12 @@ static __always_inline void *get_stub_data(void)
 
 	return (void *)ret;
 }
+
+#define stub_start(fn)							\
+	asm volatile (							\
+		"subq %0,%%rsp ;"					\
+		"movq %1,%%rax ;"					\
+		"call *%%rax ;"						\
+		:: "i" ((STUB_DATA_PAGES + 1) * UM_KERN_PAGE_SIZE),	\
+		   "i" (&fn))
 #endif
-- 
2.47.0