[Patch] hvm/vidirian: Avoid printing page_to_mfn(NULL) on error paths.

[Patch] hvm/vidirian: Avoid printing page_to_mfn(NULL) on error paths.

[Andrew Cooper]

While working in the viridian code, I noticed that 4cb6c4f4941

"x86/hvm: Use get_page_from_gfn() instead of get_gfn()/put_gfn."

introduced two error paths where page_to_mfn(NULL) would be formatted and
presented as a bad MFN.  This provides junk in the warning rather than
something useful.

These two codepaths are fixed up to match their counterpart in
wrmsr_hypervisor_regs()

While auditing the other changes from 4cb6c4f4941, I noticed a small
optimisation which could be made by changing the order of the validity checks
to remove 6 NULL pointer checks.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
CC: Keir Fraser <keir@xen.org>
CC: Jan Beulich <JBeulich@suse.com>
CC: Tim Deegan <tim@xen.org>

--

To help identify similar errors in the future, I wonder whether __page_to_mfn
(and perhaps some of the other transformation functions) should turn into a
static inline which asserts that its struct page_info pointer is within the
bounds of the frame table.
---
 xen/arch/x86/hvm/hvm.c      |   28 ++++++++++++----------------
 xen/arch/x86/hvm/viridian.c |    4 ++--
 2 files changed, 14 insertions(+), 18 deletions(-)

diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c
index de81e45..6fcd95a 100644
--- a/xen/arch/x86/hvm/hvm.c
+++ b/xen/arch/x86/hvm/hvm.c
@@ -2499,27 +2499,25 @@ static enum hvm_copy_result __hvm_copy(
 
         page = get_page_from_gfn(curr->domain, gfn, &p2mt, P2M_UNSHARE);
 
+        if ( !page )
+            return HVMCOPY_bad_gfn_to_mfn;
+
         if ( p2m_is_paging(p2mt) )
         {
-            if ( page )
-                put_page(page);
+            put_page(page);
             p2m_mem_paging_populate(curr->domain, gfn);
             return HVMCOPY_gfn_paged_out;
         }
         if ( p2m_is_shared(p2mt) )
         {
-            if ( page )
-                put_page(page);
+            put_page(page);
             return HVMCOPY_gfn_shared;
         }
         if ( p2m_is_grant(p2mt) )
         {
-            if ( page )
-                put_page(page);
+            put_page(page);
             return HVMCOPY_unhandleable;
         }
-        if ( !page )
-            return HVMCOPY_bad_gfn_to_mfn;
 
         p = (char *)__map_domain_page(page) + (addr & ~PAGE_MASK);
 
@@ -2597,27 +2595,25 @@ static enum hvm_copy_result __hvm_clear(paddr_t addr, int size)
 
         page = get_page_from_gfn(curr->domain, gfn, &p2mt, P2M_UNSHARE);
 
+        if ( !page )
+            return HVMCOPY_bad_gfn_to_mfn;
+
         if ( p2m_is_paging(p2mt) )
         {
-            if ( page )
-                put_page(page);
+            put_page(page);
             p2m_mem_paging_populate(curr->domain, gfn);
             return HVMCOPY_gfn_paged_out;
         }
         if ( p2m_is_shared(p2mt) )
         {
-            if ( page )
-                put_page(page);
+            put_page(page);
             return HVMCOPY_gfn_shared;
         }
         if ( p2m_is_grant(p2mt) )
         {
-            if ( page )
-                put_page(page);
+            put_page(page);
             return HVMCOPY_unhandleable;
         }
-        if ( !page )
-            return HVMCOPY_bad_gfn_to_mfn;
 
         p = (char *)__map_domain_page(page) + (addr & ~PAGE_MASK);
 
diff --git a/xen/arch/x86/hvm/viridian.c b/xen/arch/x86/hvm/viridian.c
index d5462f2..0a0a4da 100644
--- a/xen/arch/x86/hvm/viridian.c
+++ b/xen/arch/x86/hvm/viridian.c
@@ -157,7 +157,7 @@ static void enable_hypercall_page(struct domain *d)
         if ( page )
             put_page(page);
         gdprintk(XENLOG_WARNING, "Bad GMFN %lx (MFN %lx)\n", gmfn,
-                 page_to_mfn(page));
+                 page ? page_to_mfn(page) : ~0UL);
         return;
     }
 
@@ -202,7 +202,7 @@ static void initialize_apic_assist(struct vcpu *v)
         if ( page )
             put_page(page);
         gdprintk(XENLOG_WARNING, "Bad GMFN %lx (MFN %lx)\n", gmfn,
-                 page_to_mfn(page));
+                 page ? page_to_mfn(page) : ~0UL);
         return;
     }
 
-- 
1.7.10.4

Re: [Patch] hvm/vidirian: Avoid printing page_to_mfn(NULL) on error paths.

[Keir Fraser]

On 08/10/2013 21:17, "Andrew Cooper" <andrew.cooper3@citrix.com> wrote:

> While working in the viridian code, I noticed that 4cb6c4f4941
> 
> "x86/hvm: Use get_page_from_gfn() instead of get_gfn()/put_gfn."
> 
> introduced two error paths where page_to_mfn(NULL) would be formatted and
> presented as a bad MFN.  This provides junk in the warning rather than
> something useful.
> 
> These two codepaths are fixed up to match their counterpart in
> wrmsr_hypervisor_regs()
> 
> While auditing the other changes from 4cb6c4f4941, I noticed a small
> optimisation which could be made by changing the order of the validity checks
> to remove 6 NULL pointer checks.
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
> CC: Keir Fraser <keir@xen.org>
> CC: Jan Beulich <JBeulich@suse.com>
> CC: Tim Deegan <tim@xen.org>

Acked-by: Keir Fraser <keir@xen.org>

Re: [Patch] hvm/vidirian: Avoid printing page_to_mfn(NULL) on error paths.

[Jan Beulich]

>>> On 08.10.13 at 22:17, Andrew Cooper <andrew.cooper3@citrix.com> wrote:
> --- a/xen/arch/x86/hvm/viridian.c
> +++ b/xen/arch/x86/hvm/viridian.c
> @@ -157,7 +157,7 @@ static void enable_hypercall_page(struct domain *d)
>          if ( page )
>              put_page(page);
>          gdprintk(XENLOG_WARNING, "Bad GMFN %lx (MFN %lx)\n", gmfn,
> -                 page_to_mfn(page));
> +                 page ? page_to_mfn(page) : ~0UL);
>          return;
>      }
>  
> @@ -202,7 +202,7 @@ static void initialize_apic_assist(struct vcpu *v)
>          if ( page )
>              put_page(page);
>          gdprintk(XENLOG_WARNING, "Bad GMFN %lx (MFN %lx)\n", gmfn,
> -                 page_to_mfn(page));
> +                 page ? page_to_mfn(page) : ~0UL);
>          return;
>      }
>  

So why did you not use INVALID_MFN here, other than you had
suggested earlier?

Jan