From 20a485d66e41411f81adb9e75cb3bfaa528ddc74 Mon Sep 17 00:00:00 2001
From: Andrew Cooper <andrew.cooper3@citrix.com>
Date: Fri, 27 May 2016 08:53:53 +0100
Subject: [PATCH] Helpers to retrieve %ss and %esp from cpu_regs

In 32bit, if not stack switch occurs, this information isn't present in an
exception frame.  As a result, regs->sp and regs->ss may actually alias the
interrupted stack frame.  To avoid accidental incorrect use, prefix the names
in cpu_regs with an underscore.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
---
 arch/x86/traps.c         | 28 ++++++++++++++++++++++++++++
 include/arch/x86/regs.h  | 12 ++++++++----
 include/arch/x86/traps.h |  7 +++++++
 3 files changed, 43 insertions(+), 4 deletions(-)

diff --git a/arch/x86/traps.c b/arch/x86/traps.c
index cad7ea6..74d5c54 100644
--- a/arch/x86/traps.c
+++ b/arch/x86/traps.c
@@ -55,6 +55,34 @@ static bool is_trap_or_interrupt(const struct cpu_regs *regs)
     return true;
 }
 
+unsigned long cpu_regs_sp(const struct cpu_regs *regs)
+{
+#ifdef __x86_64__
+    return regs->_sp;
+#else
+    unsigned int cs = read_cs();
+
+    if ( (regs->cs & 3) > (cs & 3) )
+        return regs->_sp;
+
+    return (unsigned long)regs + offsetof(struct cpu_regs, _sp);
+#endif
+}
+
+unsigned int cpu_regs_ss(const struct cpu_regs *regs)
+{
+#ifdef __x86_64__
+    return regs->_ss;
+#else
+    unsigned int cs = read_cs();
+
+    if ( (regs->cs & 3) > (cs & 3) )
+        return regs->_ss;
+
+    return read_ss();
+#endif
+}
+
 /*
  * C entry-point for exceptions, after the per-environment stubs have suitably
  * adjusted the stack.
diff --git a/include/arch/x86/regs.h b/include/arch/x86/regs.h
index 1fb5340..618b562 100644
--- a/include/arch/x86/regs.h
+++ b/include/arch/x86/regs.h
@@ -7,6 +7,8 @@
 
 #define DECL_REG(n)                             \
     union { uint32_t e ## n; unsigned long n; }
+#define _DECL_REG(n)                            \
+    union { uint32_t _e ## n; unsigned long _ ## n; }
 
 struct cpu_regs {
     DECL_REG(bp);
@@ -24,8 +26,8 @@ struct cpu_regs {
     DECL_REG(ip);
     uint16_t cs, _pad1[1];
     DECL_REG(flags);
-    DECL_REG(sp);          /* Won't be valid if stack */
-    uint16_t ss, _pad0[1]; /* switch didn't occur.    */
+    _DECL_REG(sp);          /* Won't be valid if stack */
+    uint16_t _ss, _pad0[1]; /* switch didn't occur.    */
 /* Top of stack. */
 };
 
@@ -33,6 +35,8 @@ struct cpu_regs {
 
 #define DECL_REG(n)                             \
     union { uint64_t r ## n; uint32_t e ## n; unsigned long n; }
+#define _DECL_REG(n)                                                \
+    union { uint64_t _r ## n; uint32_t _e ## n; unsigned long _ ## n; }
 
 struct cpu_regs {
     uint64_t r15;
@@ -58,8 +62,8 @@ struct cpu_regs {
     DECL_REG(ip);
     uint16_t cs, _pad1[3];
     DECL_REG(flags);
-    DECL_REG(sp);
-    uint16_t ss, _pad0[3];
+    _DECL_REG(sp);
+    uint16_t _ss, _pad0[3];
 /* Top of stack. */
 };
 
diff --git a/include/arch/x86/traps.h b/include/arch/x86/traps.h
index 600aa76..48687f6 100644
--- a/include/arch/x86/traps.h
+++ b/include/arch/x86/traps.h
@@ -16,6 +16,13 @@ void arch_init_traps(void);
  */
 void __noreturn arch_crash_hard(void);
 
+/*
+ * Return the correct %ss/%esp from an exception.  In 32bit if no stack switch
+ * occurs, an exception frame doesn't contain this information.
+ */
+unsigned long cpu_regs_sp(const struct cpu_regs *regs);
+unsigned int  cpu_regs_ss(const struct cpu_regs *regs);
+
 extern uint8_t boot_stack[2 * PAGE_SIZE];
 
 #if defined(CONFIG_PV)
-- 
2.39.5