x86/fpu: Improve the __sanitize_i387_state() documentation

Improve the comments and add new ones, as this code isn't very obvious.

Reviewed-by: Borislav Petkov <bp@alien8.de>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/arch/x86/kernel/fpu/xsave.c b/arch/x86/kernel/fpu/xsave.c
index 467e4635bd29ee2052902ae222f0fe9d56d3e8d3..f3d30f0c50f91b064d11a7041eedd72c0b9077bb 100644 (file)
--- a/arch/x86/kernel/fpu/xsave.c
+++ b/arch/x86/kernel/fpu/xsave.c
@@ -30,19 +30,23 @@ static unsigned int xstate_comp_offsets[sizeof(xfeatures_mask)*8];
 static unsigned int xfeatures_nr;
 
 /*
- * If a processor implementation discern that a processor state component is
- * in its initialized state it may modify the corresponding bit in the
- * header.xfeatures as '0', with out modifying the corresponding memory
- * layout in the case of xsaveopt. While presenting the xstate information to
- * the user, we always ensure that the memory layout of a feature will be in
- * the init state if the corresponding header bit is zero. This is to ensure
- * that the user doesn't see some stale state in the memory layout during
- * signal handling, debugging etc.
+ * When executing XSAVEOPT (optimized XSAVE), if a processor implementation
+ * detects that an FPU state component is still (or is again) in its
+ * initialized state, it may clear the corresponding bit in the header.xfeatures
+ * field, and can skip the writeout of registers to the corresponding memory layout.
+ *
+ * This means that when the bit is zero, the state component might still contain
+ * some previous - non-initialized register state.
+ *
+ * Before writing xstate information to user-space we sanitize those components,
+ * to always ensure that the memory layout of a feature will be in the init state
+ * if the corresponding header bit is zero. This is to ensure that user-space doesn't
+ * see some stale state in the memory layout during signal handling, debugging etc.
  */
 void __sanitize_i387_state(struct task_struct *tsk)
 {
        struct i387_fxsave_struct *fx = &tsk->thread.fpu.state->fxsave;
-       int feature_bit = 0x2;
+       int feature_bit;
        u64 xfeatures;
 
        if (!fx)
@@ -76,19 +80,25 @@ void __sanitize_i387_state(struct task_struct *tsk)
        if (!(xfeatures & XSTATE_SSE))
                memset(&fx->xmm_space[0], 0, 256);
 
+       /*
+        * First two features are FPU and SSE, which above we handled
+        * in a special way already:
+        */
+       feature_bit = 0x2;
        xfeatures = (xfeatures_mask & ~xfeatures) >> 2;
 
        /*
-        * Update all the other memory layouts for which the corresponding
-        * header bit is in the init state.
+        * Update all the remaining memory layouts according to their
+        * standard xstate layout, if their header bit is in the init
+        * state:
         */
        while (xfeatures) {
                if (xfeatures & 0x1) {
                        int offset = xstate_offsets[feature_bit];
                        int size = xstate_sizes[feature_bit];
 
-                       memcpy(((void *) fx) + offset,
-                              ((void *) init_xstate_buf) + offset,
+                       memcpy((void *)fx + offset,
+                              (void *)init_xstate_buf + offset,
                               size);
                }