[PATCH] uml: workaround host bug in "TT mode vs. NPTL link fix"

A big bug has been diagnosed on hosts running the SKAS patch and built with
CONFIG_REGPARM, due to some missing prevent_tail_call().

On these hosts, this workaround is needed to avoid triggering that bug,
because "to" is kept by GCC only in EBX, which is corrupted at the return of
mmap2().

Since to trigger this bug int 0x80 must be used when doing the call, it rarely
manifests itself, so I'd prefer to get this merged to workaround that host
bug, since it should cause no functional change.  Still, you might prefer to
drop it, I'll leave this to you.

Signed-off-by: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Cc: Jeff Dike <jdike@addtoit.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

arch/um/sys-i386/unmap.c

@@ -15,7 +15,7 @@ int switcheroo(int fd, int prot, void *from, void *to, int size)
 	if(munmap(to, size) < 0){
 		return(-1);
 	}
-	if(mmap2(to, size, prot, MAP_SHARED | MAP_FIXED, fd, 0) != to){
+	if(mmap2(to, size, prot, MAP_SHARED | MAP_FIXED, fd, 0) == (void*) -1 ){
 		return(-1);
 	}
 	if(munmap(from, size) < 0){

arch/um/sys-x86_64/unmap.c

@@ -15,7 +15,7 @@ int switcheroo(int fd, int prot, void *from, void *to, int size)
 	if(munmap(to, size) < 0){
 		return(-1);
 	}
-	if(mmap(to, size, prot, MAP_SHARED | MAP_FIXED, fd, 0) != to){
+	if(mmap(to, size, prot, MAP_SHARED | MAP_FIXED, fd, 0) == (void*) -1){
 		return(-1);
 	}
 	if(munmap(from, size) < 0){