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Re: [libvirt] alignment of data fields when compiling with mingwin



´╗┐You are perfectly right, I have to find the way to avoid C# memory freeing ! You're a master ;)

Arnaud

--------------------------------------------------
From: "Matthias Bolte" <matthias bolte googlemail com>
Sent: Sunday, October 17, 2010 3:21 PM
To: <arnaud champion devatom fr>
Cc: <libvir-list redhat com>
Subject: Re: [libvirt] alignment of data fields when compiling with mingwin

2010/10/17  <arnaud champion devatom fr>:
Hi Matthias,

I'm working on the virConnectOpenAuth method C# binding. And in fact, It
partially work. Let me explain you :



Now, I have made a little code to connect to an esx hypervisor like this :

... some init stuff...

          // Define the virConnectAuth struct
          virConnectAuth auth = new virConnectAuth
          {
              cbdata = authDataPtr,
              cb = AuthCallback,
              credtypes = credtypesPtr,
              ncredtype = (uint)credtypes.Length
          };

IntPtr conn = libVirt.virConnectOpenAuth(tbURI.Text, ref auth, 0);

I have the call back method "AuthCallback" defined like this :

      private int AuthCallback(IntPtr creds, uint ncred, IntPtr cbdata)
      {

          AuthData authData = (AuthData) Marshal.PtrToStructure(cbdata,
typeof (AuthData));
          int offset = 0;
          int credIndex = 0;

          while (credIndex < ncred)
          {
              IntPtr currentCred = new IntPtr(creds.ToInt32() + offset);

              virConnectCredential cred = (virConnectCredential)
Marshal.PtrToStructure(currentCred, typeof (virConnectCredential));
              offset += Marshal.SizeOf(cred);
              switch(cred.type)
              {
                  case virConnectCredentialType.VIR_CRED_AUTHNAME:
                      cred.result = authData.user_name;
                      cred.resultlen = (uint)authData.user_name.Length;
                      break;
                  case virConnectCredentialType.VIR_CRED_PASSPHRASE:
                      cred.result = authData.password;
                      cred.resultlen = (uint) authData.password.Length;
                      break;
                  default:
                      return -1;
              }
              Marshal.StructureToPtr(cred, currentCred, true);

              credIndex++;
          }
          return 0;
      }

When I test this code, all seems to work well, I mean the "AuthCallback"
method is called twice, once for authname and once for passphrase.

But when I run my code in a debugger of Visual Studio I have these messages
:

HEAP[Win32_virConnectOpenAuth.exe]: Invalid address specified to
RtlFreeHeap( 00700000, 00EE6D78 )
HEAP[Win32_virConnectOpenAuth.exe]: Invalid address specified to
RtlFreeHeap( 00700000, 00EE6A80 )
HEAP[Win32_virConnectOpenAuth.exe]: Invalid address specified to
RtlFreeHeap( 00700000, 00EE6D78 )
HEAP[Win32_virConnectOpenAuth.exe]: Invalid address specified to
RtlFreeHeap( 00700000, 00EE6A80 )
HEAP[Win32_virConnectOpenAuth.exe]: Invalid address specified to
RtlFreeHeap( 00EE0000, 007DE480 )

but the connection is done, and I am able to list domains of the hypervisor for example. The real problem is that this program only work inside the the
visual studio debugger, when I launch the executable without debugger, it
fails.
So I am searching why this doesn't work, I want to check the bindings. And I
also want to try to fill the "result" member of the virConnectCredential
structure directly to understanding what can be the problem.


Okay, you see an invalid free call either because free is called on an
initial invalid pointer of because free is called twice on the same
pointer (on the second call the pointer is invalid).

When you look at the libvirt codebase in src/util/authhelper.c in
virRequestUsername you'll see that the cred.result is returned to the
driver directly without making a copy of it. The driver is then
responsible for freeing this memory. So the ownership of the memory
cred.result points to is transferred from your callback to the driver.

I think you see a double-free here because libvirt and C# are both
trying to free the same memory, but C# should not do this. I'm not
familiar with C#, so I'm just guessing here based on what you have
described. This cannot be a problem with the alignment or struct
layout, otherwise authentication would not work.

You can test if it's double-free by applying the attached patch to
libvirt. This patch stops the virRequestUsername and
virRequestPassword functions from transferring ownership of the
cred.result by copying the string. This creates a memory leak in the
normal case, but should stop the double-free in case C# is freeing
cred.result here.

Matthias


Anyway, thanks ofr these informations, this can help.

Best regards,

Arnaud

--------------------------------------------------
From: "Matthias Bolte" <matthias bolte googlemail com>
Sent: Sunday, October 17, 2010 12:04 PM
To: <arnaud champion devatom fr>
Cc: <libvir-list redhat com>
Subject: Re: [libvirt] alignment of data fields when compiling with mingwin

2010/10/15  <arnaud champion devatom fr>:

Hi,

I'm currently working on libvirt csharp bindings, and I have some trouble
with virConnectOpenAuth marshaling.
I need to know what is the alignment of data fields in structure when
compiling with mingwin.
Anyone know that ?

cheers,

Arnaud Champion


Why do you need to know the alignment? Do you need to build or access
members of the virConnectCredential or virConnectAuth structs by
offset in memory?

The Wikipedia article about data structure alignment might be helpful
when you really need to care about alignment.

If you actually need the offset of the members in those structs you
can just use the offsetof macro and let the compiler tell you:

#include <stdio.h>
#include <stddef.h>
#include <libvirt/libvirt.h>

void main(void)
{
  printf("virConnectCredential\n");
  printf("  type      %2u\n", offsetof(virConnectCredential, type));
  printf("  prompt    %2u\n", offsetof(virConnectCredential, prompt));
printf(" challenge %2u\n", offsetof(virConnectCredential, challenge)); printf(" defresult %2u\n", offsetof(virConnectCredential, defresult));
  printf("  result    %2u\n", offsetof(virConnectCredential, result));
printf(" resultlen %2u\n", offsetof(virConnectCredential, resultlen));
  printf("\n");
  printf("virConnectAuth\n");
  printf("  credtype  %2u\n", offsetof(virConnectAuth, credtype));
  printf("  ncredtype %2u\n", offsetof(virConnectAuth, ncredtype));
  printf("  cb        %2u\n", offsetof(virConnectAuth, cb));
  printf("  cbdata    %2u\n", offsetof(virConnectAuth, cbdata));
}


Output on x86:

virConnectCredential
 type       0
 prompt     4
 challenge  8
 defresult 12
 result    16
 resultlen 20

virConnectAuth
 credtype   0
 ncredtype  4
 cb         8
 cbdata    12


Output on x86_64:

virConnectCredential
 type       0
 prompt     8
 challenge 16
 defresult 24
 result    32
 resultlen 40

virConnectAuth
 credtype   0
 ncredtype  8
 cb        16
 cbdata    24

[1]
http://en.wikipedia.org/wiki/Data_structure_alignment#Typical_alignment_of_C_structs_on_x86

Matthias






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