DLL Load Error and Diamond Dependencies29 Apr 2016
Recently the host OS in Azure is being upgraded to a new version. As a part of this, a DLL of networking agent called WVCDLL has to be upgraded too. My team member tried and got a werid error. Attaching debugger to the program, the IP stopped at a logging entry, which did not make much sense. Then I looked closely at the stack trace:
... 0d 00000015`2b44dff0 00007ffe`2736fdbf wvcdll!__delayLoadHelper2+0x258 0e 00000015`2b44e0b0 00007ffe`27321177 wvcdll!_tailMerge_OsLogger_dll+0x3f 0f 00000015`2b44e120 00007ffe`273213a8 wvcdll!VirtualSwitchMgmtServiceClassFactory::GetInstance+0x87 10 00000015`2b44e290 00007ff7`1ad6d26d wvcdll!VirtualSwitchMgmtServiceClassFactory::GetInstance+0x18 11 00000015`2b44e2d0 00007ff7`1ab8713a NMAgent!IVirtualSwitchManagementService::~IVirtualSwitchManagementService+0x5dd ...
__delayLoadHelper caught my attention. Once I saw the assembly it looked clear to me:
0:019> uf 00007ffe`273210f0 wvcdll!VirtualSwitchMgmtServiceClassFactory::GetInstance 34 00007ffe`273210f0 4055 push rbp 34 00007ffe`273210f2 53 push rbx 34 00007ffe`273210f3 56 push rsi 34 00007ffe`273210f4 57 push rdi ... 37 00007ffe`2732114e 488d05fb960500 lea rax,[wvcdll!`string' (00007ffe`2737a850)] 37 00007ffe`27321155 4889442420 mov qword ptr [rsp+20h],rax 37 00007ffe`2732115a 4c8d0d5f970500 lea r9,[wvcdll!`string' (00007ffe`2737a8c0)] 37 00007ffe`27321161 4c8d442440 lea r8,[rsp+40h] 37 00007ffe`27321166 488b15a31b0800 mov rdx,qword ptr [wvcdll!wvcLogger (00007ffe`273a2d10)] 37 00007ffe`2732116d 488d4d90 lea rcx,[rbp-70h] 37 00007ffe`27321171 ff1599110800 call qword ptr [wvcdll!_imp_??0FunctionLoggerOsLoggerQEAAPEAVILogger (00007ffe`273a2310)] 37 00007ffe`27321177 90 nop
From the stack trace the problem occurred at
00007ffe 27321171, the call instruction. The symbol name indicates that
the code in WVCDLL is attempting to call a function defined in OsLogger via import table. Therefore the error is caused
by either the dependency does not exist or it is a wrong version.
In native code, a dependency DLL can be set to delay loaded for two reasons: * The main program may not call any function in the dependency at all. * Even if it does, the function may be called late in the execution.
Delay-loading can improve the program load speed for obvious reason. The downside is we may not see the dependency load failure when the program is loaded, as previous failure shows.
In this case, a closer look turned out that the problem was caused by wrong version was used during the build. This is a typical scenario of diamond dependencies: * Program depends on DLL A and B. * DLL A and B are developed and distributed independently. * Both A and B depend on DLL C.
Unlike managed code, native code does not check the version of DLL being loaded. Suppose DLL A version 1 uses DLL C version 1.1, DLL B version 2 uses DLL C version 1.2 which has a different import table, then we link A and B into the same program, although the build process is successful, at runtime it will fail to resolve function entries.
Most teams in Microsoft have compoentized the code base and migrated the source repository from centralized SD to distributed Git – I may write another post to talk about how the engineering system looks like. Since the release cadence of different components are completely different, this problem may not be rare to see.
Lastly, in case anyone does not know one can check the version of loaded module in windbg using
0:019> lmvm wvcdll Browse full module list start end module name 00007ffe`27310000 00007ffe`273ad000 wvcdll (deferred) Image path: xxxxxxxxxxxxxxxxxxxxxxxxxxxx\wvcdll.dll Image name: wvcdll.dll Browse all global symbols functions data Timestamp: Tue Mar 8 11:36:39 2016 (56DF29C7) ... File version: 188.8.131.52 Product version: 184.108.40.206 ... PrivateBuild: (by corecet on CY1AZRBLD15) FileDescription: xxxxxxxxxxxxxxxxxxxxxxxx r_mar_2016 (6f6baea) Microsoft Azure®
Based on the version and other info, one can load matching PDB for ease of debugging.