x86, xsave: dynamically allocate sigframes fpstate instead of static allocation

dynamically allocate fpstate on the stack, instead of static allocation
in the current sigframe layout on the user stack. This will allow the
fpstate structure to grow in the future, which includes extended state
information supporting xsave/xrstor.

signal handlers will be able to access the fpstate pointer from the
sigcontext structure asusual, with no change. For the non RT sigframe's
(which are supported only for 32bit apps), current static fpstate layout
in the sigframe will be unused(so that we don't change the extramask[]
offset in the sigframe and thus prevent breaking app's which modify
extramask[]).

Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 12 insertions, 2 deletions

diff --git a/arch/x86/kernel/sigframe.h b/arch/x86/kernel/sigframe.h
index 72bbb519d2d..6dd7e2b70a4 100644
--- a/arch/x86/kernel/sigframe.h
+++ b/arch/x86/kernel/sigframe.h
@@ -3,9 +3,18 @@ struct sigframe {
         char __user *pretcode;
         int sig;
         struct sigcontext sc;
-        struct _fpstate fpstate;
+        /*
+         * fpstate is unused. fpstate is moved/allocated after
+         * retcode[] below. This movement allows to have the FP state and the
+         * future state extensions (xsave) stay together.
+         * And at the same time retaining the unused fpstate, prevents changing
+         * the offset of extramask[] in the sigframe and thus prevent any
+         * legacy application accessing/modifying it.
+         */
+        struct _fpstate fpstate_unused;
         unsigned long extramask[_NSIG_WORDS-1];
         char retcode[8];
+        /* fp state follows here */
 };
 
 struct rt_sigframe {
@@ -15,13 +24,14 @@ struct rt_sigframe {
         void __user *puc;
         struct siginfo info;
         struct ucontext uc;
-        struct _fpstate fpstate;
         char retcode[8];
+        /* fp state follows here */
 };
 #else
 struct rt_sigframe {
         char __user *pretcode;
         struct ucontext uc;
         struct siginfo info;
+        /* fp state follows here */
 };
 #endif