[RFC Patch 1/2] x86/traps: Refactor show_trace()

[Andrew Cooper <andrew.cooper3@citrix.com>]

Before, show_trace() had two implementations depending on
CONFIG_FRAME_POINTER.  Some parts were common, while the loops to wander up
the stack were different.

The version aided by frame pointers had a special case for function calls on
wild function pointers, but this doesn't need to be a special case.

After the refactoring, there are now two implementations of __show_trace()
which differ depending on CONFIG_FRAME_POINTER, and a single show_trace()
with the common bits, including the logic for wild function pointers.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>

---

The new wild pointer logic is rather larger than its pre-refactor version, but
is rather more legible.  I cant think of a way to compact it without making it
substantially less legible.
---
 xen/arch/x86/traps.c |   66 +++++++++++++++++++++++++++++---------------------
 1 file changed, 39 insertions(+), 27 deletions(-)

diff --git a/xen/arch/x86/traps.c b/xen/arch/x86/traps.c
index 57dbd0c..324b0f1 100644
--- a/xen/arch/x86/traps.c
+++ b/xen/arch/x86/traps.c
@@ -192,14 +192,13 @@ static void show_guest_stack(struct vcpu *v, struct cpu_user_regs *regs)
 
 #if !defined(CONFIG_FRAME_POINTER)
 
-static void show_trace(struct cpu_user_regs *regs)
+/* Stack trace from pointers found in stack, unaided by frame pointers.  For
+ * caller convenience, this has the same prototype as its alternative, and
+ * simply ignores the rbp parameter.
+ */
+static void __show_trace(unsigned long sp, unsigned long bp)
 {
-    unsigned long *stack = ESP_BEFORE_EXCEPTION(regs), addr;
-
-    printk("Xen call trace:\n   ");
-
-    printk("[<%p>]", _p(regs->eip));
-    print_symbol(" %s\n   ", regs->eip);
+    unsigned long *stack = (unsigned long *)sp, addr;
 
     while ( ((long)stack & (STACK_SIZE-BYTES_PER_LONG)) != 0 )
     {
@@ -210,36 +209,22 @@ static void show_trace(struct cpu_user_regs *regs)
             print_symbol(" %s\n   ", addr);
         }
     }
-
-    printk("\n");
 }
 
 #else
 
-static void show_trace(struct cpu_user_regs *regs)
+/* Stack trace from frames in the stack, using frame pointers */
+static void __show_trace(unsigned long sp, unsigned long bp)
 {
     unsigned long *frame, next, addr, low, high;
 
-    printk("Xen call trace:\n   ");
-
-    /*
-     * If RIP is not pointing into hypervisor code then someone may have
-     * called into oblivion. Peek to see if they left a return address at
-     * top of stack.
-     */
-    addr = is_active_kernel_text(regs->eip) ||
-           !is_active_kernel_text(*ESP_BEFORE_EXCEPTION(regs)) ?
-           regs->eip : *ESP_BEFORE_EXCEPTION(regs);
-    printk("[<%p>]", _p(addr));
-    print_symbol(" %s\n   ", addr);
-
     /* Bounds for range of valid frame pointer. */
-    low  = (unsigned long)(ESP_BEFORE_EXCEPTION(regs) - 2);
+    low  = sp - 2*sizeof(unsigned long);
     high = (low & ~(STACK_SIZE - 1)) + 
         (STACK_SIZE - sizeof(struct cpu_info) - 2*sizeof(unsigned long));
 
     /* The initial frame pointer. */
-    next = regs->ebp;
+    next = bp;
 
     for ( ; ; )
     {
@@ -272,12 +257,39 @@ static void show_trace(struct cpu_user_regs *regs)
 
         low = (unsigned long)&frame[2];
     }
-
-    printk("\n");
 }
 
 #endif
 
+static void show_trace(const struct cpu_user_regs *regs)
+{
+    unsigned long sp = regs->rsp;
+    printk("Xen call trace:\n   ");
+
+    /* If RIP looks sensible, or the top of the stack doesn't look sensible,
+     * print RIP at the top of the stack trace. */
+    if ( is_active_kernel_text(regs->rip) ||
+         !is_active_kernel_text(regs->rsp) )
+    {
+        printk("[<%p>]", _p(regs->rip));
+        print_symbol(" %s\n   ", regs->rip);
+    }
+    /* else RIP looks bad but the top of the stack looks ok.  Perhaps we
+     * followed a wild function pointer, so lets assume the top of the stack is
+     * a return address.  Skip past it so__show_trace() doesn't print it
+     * again. */
+    else
+    {
+        printk("[<%p>]", _p(sp));
+        print_symbol(" %s\n   ", sp);
+        sp += sizeof (unsigned long);
+    }
+
+    __show_trace(sp, regs->rbp);
+
+    printk("\n");
+}
+
 void show_stack(struct cpu_user_regs *regs)
 {
     unsigned long *stack = ESP_BEFORE_EXCEPTION(regs), addr;
-- 
1.7.10.4