[PATCH 1/1] include: Add memory clobber to GCC asm statement in NtCurrentTeb().

[Jinoh Kang]

From: Jinoh Kang

Today, NtCurrentTeb() has a single asm statement when compiling for the
i386, x86-64, or ARM architecture with GCC.  This single asm statement
has neither the "volatile" qualifier nor the "memory" clobber.

This can provoke undefined behavior if the current TEB changes between
NtCurrentTeb() calls.  This is because GCC assumes that the asm
statement does not depend on memory or the thread-local register.  In
fact, given the same address of the "teb" variable, GCC assumes that the
asm statement produces exactly the same value on every invocaton.

This primarily causes issues when switching to another fiber from a
thread that is different from the thread on which the fiber was last
executed.  Theoretically, however, this may also cause issues when the
optimizer aliases the "teb" variable to another variable that is shared
between threads, and perform global optimization that can work across
multiple threads in runtime (if any).

Fix this by adding the "memory" clobber to the asm statements that
computes the current TEB address.
---
 include/winnt.h | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/include/winnt.h b/include/winnt.h
index 66f6b7ad809..97a9deac21d 100644
--- a/include/winnt.h
+++ b/include/winnt.h
@@ -2238,7 +2238,7 @@ struct _TEB;
 static FORCEINLINE struct _TEB * WINAPI NtCurrentTeb(void)
 {
     struct _TEB *teb;
-    __asm__(".byte 0x64\n\tmovl (0x18),%0" : "=r" (teb));
+    __asm__(".byte 0x64\n\tmovl (0x18),%0" : "=r" (teb) :: "memory");
     return teb;
 }
 #elif defined(__i386__) && defined(_MSC_VER) && !defined(WINE_UNIX_LIB)
@@ -2253,7 +2253,7 @@ static FORCEINLINE struct _TEB * WINAPI NtCurrentTeb(void)
 static FORCEINLINE struct _TEB * WINAPI NtCurrentTeb(void)
 {
     struct _TEB *teb;
-    __asm__(".byte 0x65\n\tmovq (0x30),%0" : "=r" (teb));
+    __asm__(".byte 0x65\n\tmovq (0x30),%0" : "=r" (teb) :: "memory");
     return teb;
 }
 #elif defined(__x86_64__) && defined(_MSC_VER) && !defined(WINE_UNIX_LIB)
@@ -2267,7 +2267,7 @@ static FORCEINLINE struct _TEB * WINAPI NtCurrentTeb(void)
 static FORCEINLINE struct _TEB * WINAPI NtCurrentTeb(void)
 {
     struct _TEB *teb;
-    __asm__("mrc p15, 0, %0, c13, c0, 2" : "=r" (teb));
+    __asm__("mrc p15, 0, %0, c13, c0, 2" : "=r" (teb) :: "memory");
     return teb;
 }
 #elif defined(__arm__) && defined(_MSC_VER) && !defined(WINE_UNIX_LIB)