[PATCH 3/4] x86: signal: unify the sigaltstack check with other arches

[Stas Sergeev <stsp@list.ru>]

Currently x86's get_sigframe() checks for "current->sas_ss_size"
to determine whether there is a need to switch to sigaltstack.
The common practice used by all other arches is to check for
sas_ss_flags(sp) == 0

This patch makes the code consistent with other arches and also
allows for the further sigaltstack improvements within this patch serie.
The slight complexity of the patch is added by the optimization on
!sigstack check that was requested by Andy Lutomirski: sas_ss_flags(sp)==0
already implies that we are not on a sigstack, so the code is shuffled
to avoid the duplicate checking.

CC: Andy Lutomirski <luto@amacapital.net>
CC: linux-kernel@vger.kernel.org
CC: Thomas Gleixner <tglx@linutronix.de>
CC: Ingo Molnar <mingo@redhat.com>
CC: "H. Peter Anvin" <hpa@zytor.com>
CC: x86@kernel.org
CC: Borislav Petkov <bp@suse.de>
CC: Brian Gerst <brgerst@gmail.com>
CC: Oleg Nesterov <oleg@redhat.com>
CC: Richard Weinberger <richard@nod.at>

Signed-off-by: Stas Sergeev <stsp@users.sourceforge.net>
---
  arch/x86/kernel/signal.c | 23 +++++++++++------------
  1 file changed, 11 insertions(+), 12 deletions(-)

diff --git a/arch/x86/kernel/signal.c b/arch/x86/kernel/signal.c
index cb6282c..285183b 100644
--- a/arch/x86/kernel/signal.c
+++ b/arch/x86/kernel/signal.c
@@ -213,18 +213,17 @@ get_sigframe(struct k_sigaction *ka, struct 
pt_regs *regs, size_t frame_size,
      if (config_enabled(CONFIG_X86_64))
          sp -= 128;

-    if (!onsigstack) {
-        /* This is the X/Open sanctioned signal stack switching. */
-        if (ka->sa.sa_flags & SA_ONSTACK) {
-            if (current->sas_ss_size)
-                sp = current->sas_ss_sp + current->sas_ss_size;
-        } else if (config_enabled(CONFIG_X86_32) &&
-               (regs->ss & 0xffff) != __USER_DS &&
-               !(ka->sa.sa_flags & SA_RESTORER) &&
-               ka->sa.sa_restorer) {
-                /* This is the legacy signal stack switching. */
-                sp = (unsigned long) ka->sa.sa_restorer;
-        }
+    /* This is the X/Open sanctioned signal stack switching.  */
+    if (ka->sa.sa_flags & SA_ONSTACK) {
+        if (sas_ss_flags(sp) == 0)
+            sp = current->sas_ss_sp + current->sas_ss_size;
+    } else if (config_enabled(CONFIG_X86_32) &&
+           !onsigstack &&
+           (regs->ss & 0xffff) != __USER_DS &&
+           !(ka->sa.sa_flags & SA_RESTORER) &&
+           ka->sa.sa_restorer) {
+        /* This is the legacy signal stack switching. */
+        sp = (unsigned long) ka->sa.sa_restorer;
      }

      if (fpu->fpstate_active) {
-- 
2.5.0