kernel: add do_compat_sigaltstack() helper; remove in-kernel call to compat syscall

Using this helper allows us to avoid the in-kernel call to the
compat_sys_sigaltstack() syscall.

This patch is part of a series which removes in-kernel calls to syscalls.
On this basis, the syscall entry path can be streamlined. For details, see
http://lkml.kernel.org/r/20180325162527.GA17492@light.dominikbrodowski.net

Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>

diff --git a/kernel/signal.c b/kernel/signal.c
index 985c617..f044666 100644 (file)
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -3573,9 +3573,8 @@ int __save_altstack(stack_t __user *uss, unsigned long sp)
 }
 
 #ifdef CONFIG_COMPAT
-COMPAT_SYSCALL_DEFINE2(sigaltstack,
-                       const compat_stack_t __user *, uss_ptr,
-                       compat_stack_t __user *, uoss_ptr)
+static int do_compat_sigaltstack(const compat_stack_t __user *uss_ptr,
+                                compat_stack_t __user *uoss_ptr)
 {
        stack_t uss, uoss;
        int ret;
@@ -3602,9 +3601,16 @@ COMPAT_SYSCALL_DEFINE2(sigaltstack,
        return ret;
 }
 
+COMPAT_SYSCALL_DEFINE2(sigaltstack,
+                       const compat_stack_t __user *, uss_ptr,
+                       compat_stack_t __user *, uoss_ptr)
+{
+       return do_compat_sigaltstack(uss_ptr, uoss_ptr);
+}
+
 int compat_restore_altstack(const compat_stack_t __user *uss)
 {
-       int err = compat_sys_sigaltstack(uss, NULL);
+       int err = do_compat_sigaltstack(uss, NULL);
        /* squash all but -EFAULT for now */
        return err == -EFAULT ? err : 0;
 }