xenoprof: Handle 32-bit guest stacks properly in a 64-bit hypervisor

Quoted 1 line

Quoted 1 line · Marcus Granado

xenoprof: Handle 32-bit guest stacks properly in a 64-bit hypervisor

The dump_guest_backtrace() function attempted to walk the stack based on the assumption that the guest and hypervisor pointer sizes were the same; thus any 32-bit guest running under 64-bit hypervisor would have unreliable results.

In 64-bit mode, read the 32-bit stack frame properly.

Signed-off-by Marcus Granado <[email protected]>

Signed-off-by George Dunlap <[email protected]>

diff -r f6953e89913f -r 21e7744a52b1 xen/arch/x86/oprofile/backtrace.c

xen/arch/x86/oprofile/backtrace.c Wed Jan 18 17:35:06 2012 +0000 renamed from xen/arch/x86/oprofile/backtrace.c Wed Jan 18 17:23:02 2012 +0000 +13 -6

+} __attribute__((packed));
+#endif
+
  static struct frame_head *
  dump_hypervisor_backtrace(struct domain *d, struct vcpu *vcpu,
                struct frame_head * head, int mode)

@@ -35,19 +42,46 @@ dump_hypervisor_backtrace(struct domain
      return head->ebp;
  }
 
+#ifdef CONFIG_X86_64
+static inline int is_32bit_vcpu(struct vcpu *vcpu)
+{
+    if (is_hvm_vcpu(vcpu))
+        return !hvm_long_mode_enabled(vcpu);
+    else
+        return is_pv_32bit_vcpu(vcpu);
+}
+#endif
+
  static struct frame_head *
  dump_guest_backtrace(struct domain *d, struct vcpu *vcpu,
               struct frame_head * head, int mode)
  {
      struct frame_head bufhead[2];
      XEN_GUEST_HANDLE(char) guest_head = guest_handle_from_ptr(head, char);
-
-    /* Also check accessibility of one struct frame_head beyond */
-    if (!guest_handle_okay(guest_head, sizeof(bufhead)))
-        return 0;
-    if (__copy_from_guest_offset((char *)bufhead, guest_head, 0,
-                                 sizeof(bufhead)))

xen/arch/x86/oprofile/backtrace.c Wed Jan 18 17:35:06 2012 +0000 renamed from xen/arch/x86/oprofile/backtrace.c Wed Jan 18 17:23:02 2012 +0000 +15 -0

+                                     sizeof(bufhead32)))
+            return 0;
+        bufhead[0].ebp=(struct frame_head *)(unsigned long)bufhead32[0].ebp;
+        bufhead[0].ret=bufhead32[0].ret;
+    }
+    else
+#endif
+    {
+        /* Also check accessibility of one struct frame_head beyond */
+        if (!guest_handle_okay(guest_head, sizeof(bufhead)))
+            return 0;
+        if (__copy_from_guest_offset((char *)bufhead, guest_head, 0,
+                                     sizeof(bufhead)))
+            return 0;
+    }
 
      if (!xenoprof_add_trace(d, vcpu, bufhead[0].ret, mode))
          return 0;

If you're adding a compat mode guest case here, then you should also use compat mode accessors (compat_handle_okay(), __copy_from_compat_offset()), implying that you also have a local handle variable of the appropriate type (and perhaps moving the native one down into the 'else' body).

Quoted 1 line

Quoted 1 line · Marcus Granado

I'm trying to understand the compat handle. It is not clear to me how to map one from head (a 64-bit pointer), since COMPAT_HANDLE seems to store

a 32-bit compat_ptr_t value in its structure. Ideally, what I would like to do is

COMPAT_HANDLE(char) guest_head = map_guest_handle_to_compat_handle (guest_handle_from_ptr(head, char));
or
COMPAT_HANDLE(char) guest_head = compat_handle_from_ptr(head, char)); but I can't find any equivalent functions in any header.

The following line compiles,
COMPAT_HANDLE(char) guest_head = { (full_ptr_t)head };
but it looks like, in this case, the compat handle structure in compat.h will truncate the most significant bits from the head pointer, so compat_handle_okay(guest_head,...) and __copy_from_compat(...,guest_head,...) below will be using a truncated pointer:

      56 static struct frame_head *
      57 dump_guest_backtrace(struct domain *d, struct vcpu *vcpu,
      58                      struct frame_head * head, int mode)
      59 {
      60     struct frame_head bufhead[2];
      61
      62 #ifdef CONFIG_X86_64
      63     if ( is_32bit_vcpu(vcpu) )
      64     {
      65         COMPAT_HANDLE(char) guest_head = { (full_ptr_t)head };
      66         struct frame_head_32bit bufhead32[2];
      67         /* Also check accessibility of one struct frame_head 

beyond */
68 if (!compat_handle_okay(guest_head, sizeof(bufhead32)))
69 return 0;
70 if (__copy_from_compat((char *)bufhead32, guest_head,
71 sizeof(bufhead32)))
72 return 0;
73 bufhead[0].ebp=(struct frame_head

*)(full_ptr_t)bufhead32[0].ebp;
      74         bufhead[0].ret=bufhead32[0].ret;
      75     }
      76     else
      77 #endif

Any advice? Maybe the best option in this case is to avoid the compat* functions and to use the original guest* functions instead.

Thanks,
Marcus