x86: xsave: set FP, SSE bits in the xsave header in the user sigcontext

If a processor implementation discern that a processor state component is in
its initialized state, it may modify the corresponding bit in the
xsave header.xstate_bv as '0'. State in the memory layout setup by 'xsave'
will be consistent with the bit values in the header.

During signal handling, legacy applications may change the FP/SSE bits
in the sigcontext memory layout without touching the FP/SSE header bits
in the xsave header. So always set FP/SSE bits in the xsave header
while saving the sigcontext state to the user space. During signal return,
this will enable the kernel to capture any changes to the FP/SSE bits by the
legacy applications which don't touch xsave headers.

xsave aware apps can change the xstate_bv in the xsave header aswell
as change any contents in the memory layout. xrestor as part of sigreturn
will capture all the changes.

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

1 files changed, 14 insertions, 0 deletions

diff --git a/arch/x86/kernel/i387.c b/arch/x86/kernel/i387.c
index 45723f1..1f20608 100644
--- a/arch/x86/kernel/i387.c
+++ b/arch/x86/kernel/i387.c
@@ -468,9 +468,23 @@ static int save_i387_fxsave(struct _fpstate_ia32 __user *buf)
 
 static int save_i387_xsave(void __user *buf)
 {
+	struct task_struct *tsk = current;
 	struct _fpstate_ia32 __user *fx = buf;
 	int err = 0;
 
+	/*
+	 * For legacy compatible, we always set FP/SSE bits in the bit
+	 * vector while saving the state to the user context.
+	 * This will enable us capturing any changes(during sigreturn) to
+	 * the FP/SSE bits by the legacy applications which don't touch
+	 * xstate_bv in the xsave header.
+	 *
+	 * xsave aware applications can change the xstate_bv in the xsave
+	 * header as well as change any contents in the memory layout.
+	 * xrestore as part of sigreturn will capture all the changes.
+	 */
+	tsk->thread.xstate->xsave.xsave_hdr.xstate_bv |= XSTATE_FPSSE;
+
 	if (save_i387_fxsave(fx) < 0)
 		return -1;