asmlinkage_protect replaces prevent_tail_call

The prevent_tail_call() macro works around the problem of the compiler
clobbering argument words on the stack, which for asmlinkage functions
is the caller's (user's) struct pt_regs.  The tail/sibling-call
optimization is not the only way that the compiler can decide to use
stack argument words as scratch space, which we have to prevent.
Other optimizations can do it too.

Until we have new compiler support to make "asmlinkage" binding on the
compiler's own use of the stack argument frame, we have work around all
the manifestations of this issue that crop up.

More cases seem to be prevented by also keeping the incoming argument
variables live at the end of the function.  This makes their original
stack slots attractive places to leave those variables, so the compiler
tends not clobber them for something else.  It's still no guarantee, but
it handles some observed cases that prevent_tail_call() did not.

Signed-off-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 2 insertions, 2 deletions

diff --git a/arch/x86/kernel/tls.c b/arch/x86/kernel/tls.c
index 022bcaa3b42e..ab6bf375a307 100644
--- a/arch/x86/kernel/tls.c
+++ b/arch/x86/kernel/tls.c
@@ -92,7 +92,7 @@ int do_set_thread_area(struct task_struct *p, int idx,
 asmlinkage int sys_set_thread_area(struct user_desc __user *u_info)
 {
         int ret = do_set_thread_area(current, -1, u_info, 1);
-        prevent_tail_call(ret);
+        asmlinkage_protect(1, ret, u_info);
         return ret;
 }
 
@@ -142,7 +142,7 @@ int do_get_thread_area(struct task_struct *p, int idx,
 asmlinkage int sys_get_thread_area(struct user_desc __user *u_info)
 {
         int ret = do_get_thread_area(current, -1, u_info);
-        prevent_tail_call(ret);
+        asmlinkage_protect(1, ret, u_info);
         return ret;
 }