Re: [edk2-devel] [PATCH 08/10] OvmfPkg: suppress "Value stored to ... is never read" analyzer warnings

["Laszlo Ersek" <lersek@redhat.com>]

On 04/14/19 10:03, Jordan Justen wrote:
> On 2019-04-12 16:31:26, Laszlo Ersek wrote:
>> RH covscan warns about assignments that it can determine are never
>> "consumed" later ("dead stores"). The idea behind the warning is
>> presumably that the programmer forgot to implement a dependent check
>> somewhere.
>>
>> For each such warning that has been emitted for OvmfPkg,
>> the case is one of the following however:
>>
>> - We assign a variable a value for (re-)initialization's sake, in
>>   preparation for further or future uses. This practice is safe (sometimes
>>   even recommended!), hence we should suppress these warnings.
>>
>> - We capture a result or a computation in a variable, following a general
>>   programming pattern, but then decide to ignore the value in that
>>   particular case. This is again safe, and we should suppress these
>>   warnings too.
>>
>> According to the Clang documentation at
>>
>>   https://clang-analyzer.llvm.org/faq.html#dead_store
>>
>> we should use
>>
>>   (void)x;
>>
>> See the logs below (produced originally for edk2-stable201903).
>>
>>> Error: CLANG_WARNING:
>>> edk2-89910a39dcfd/OvmfPkg/AcpiPlatformDxe/Xen.c:156:3: warning: Value
>>> stored to 'NumberOfTableEntries' is never read
>>> #  NumberOfTableEntries = 0;
>>> #  ^                      ~
>>> edk2-89910a39dcfd/OvmfPkg/AcpiPlatformDxe/Xen.c:156:3: note: Value
>>> stored to 'NumberOfTableEntries' is never read
>>> #  NumberOfTableEntries = 0;
>>> #  ^                      ~
>>> #  154|     DsdtTable   = NULL;
>>> #  155|     TableHandle = 0;
>>> #  156|->   NumberOfTableEntries = 0;
>>> #  157|
>>> #  158|     //
>>>
>>> Error: CLANG_WARNING:
>>> edk2-89910a39dcfd/OvmfPkg/Library/PlatformBootManagerLib/QemuKernel.c:43:3:
>>> warning: Value stored to 'SetupSize' is never read
>>> #  SetupSize = 0;
>>> #  ^           ~
>>> edk2-89910a39dcfd/OvmfPkg/Library/PlatformBootManagerLib/QemuKernel.c:43:3:
>>> note: Value stored to 'SetupSize' is never read
>>> #  SetupSize = 0;
>>> #  ^           ~
>>> #   41|
>>> #   42|     SetupBuf = NULL;
>>> #   43|->   SetupSize = 0;
>>> #   44|     KernelBuf = NULL;
>>> #   45|     KernelInitialSize = 0;
>>>
>>> Error: CLANG_WARNING:
>>> edk2-89910a39dcfd/OvmfPkg/Library/SerializeVariablesLib/SerializeVariablesLib.c:609:9:
>>> warning: Value stored to 'VariableDataBufferSize' is never read
>>> #        VariableDataBufferSize = 0;
>>> #        ^                        ~
>>> edk2-89910a39dcfd/OvmfPkg/Library/SerializeVariablesLib/SerializeVariablesLib.c:609:9:
>>> note: Value stored to 'VariableDataBufferSize' is never read
>>> #        VariableDataBufferSize = 0;
>>> #        ^                        ~
>>> #  607|           FreePool (VariableData);
>>> #  608|           VariableData = NULL;
>>> #  609|->         VariableDataBufferSize = 0;
>>> #  610|         }
>>> #  611|         VariableData = AllocatePool (VariableDataSize);
>>>
>>> Error: CLANG_WARNING:
>>> edk2-89910a39dcfd/OvmfPkg/Library/VirtioLib/VirtioLib.c:125:3: warning:
>>> Value stored to 'RingPagesPtr' is never read
>>> #  RingPagesPtr += sizeof *Ring->Used.AvailEvent;
>>> #  ^               ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>> edk2-89910a39dcfd/OvmfPkg/Library/VirtioLib/VirtioLib.c:125:3: note:
>>> Value stored to 'RingPagesPtr' is never read
>>> #  RingPagesPtr += sizeof *Ring->Used.AvailEvent;
>>> #  ^               ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>> #  123|
>>> #  124|     Ring->Used.AvailEvent = (volatile VOID *) RingPagesPtr;
>>> #  125|->   RingPagesPtr += sizeof *Ring->Used.AvailEvent;
>>> #  126|
>>> #  127|     Ring->QueueSize = QueueSize;
>>>
>>> Error: CLANG_WARNING:
>>> edk2-89910a39dcfd/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/FwBlockService.c:1079:3:
>>> warning: Value stored to 'FwhInstance' is never read
>>> #  FwhInstance = (EFI_FW_VOL_INSTANCE *)
>>> #  ^             ~~~~~~~~~~~~~~~~~~~~~~~
>>> edk2-89910a39dcfd/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/FwBlockService.c:1079:3:
>>> note: Value stored to 'FwhInstance' is never read
>>> #  FwhInstance = (EFI_FW_VOL_INSTANCE *)
>>> #  ^             ~~~~~~~~~~~~~~~~~~~~~~~
>>> # 1077|     ASSERT_RETURN_ERROR (PcdStatus);
>>> # 1078|
>>> # 1079|->   FwhInstance = (EFI_FW_VOL_INSTANCE *)
>>> # 1080|       (
>>> # 1081|         (UINTN) ((UINT8 *) FwhInstance) + FwVolHeader->HeaderLength +
>>>
>>> Error: CLANG_WARNING:
>>> edk2-89910a39dcfd/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/FwBlockServiceDxe.c:173:3:
>>> warning: Value stored to 'Status' is never read
>>> #  Status = gDS->RemoveMemorySpace (
>>> #  ^        ~~~~~~~~~~~~~~~~~~~~~~~~
>>> edk2-89910a39dcfd/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/FwBlockServiceDxe.c:173:3:
>>> note: Value stored to 'Status' is never read
>>> #  Status = gDS->RemoveMemorySpace (
>>> #  ^        ~~~~~~~~~~~~~~~~~~~~~~~~
>>> #  171|     // Mark flash region as runtime memory
>>> #  172|     //
>>> #  173|->   Status = gDS->RemoveMemorySpace (
>>> #  174|                     BaseAddress,
>>> #  175|                     Length
>>>
>>> Error: CLANG_WARNING:
>>> edk2-89910a39dcfd/OvmfPkg/SataControllerDxe/SataController.c:231:3:
>>> warning: Value stored to 'DeviceUDmaMode' is never read
>>> #  DeviceUDmaMode = 0;
>>> #  ^                ~
>>> edk2-89910a39dcfd/OvmfPkg/SataControllerDxe/SataController.c:231:3:
>>> note: Value stored to 'DeviceUDmaMode' is never read
>>> #  DeviceUDmaMode = 0;
>>> #  ^                ~
>>> #  229|     UINT16    DeviceUDmaMode;
>>> #  230|
>>> #  231|->   DeviceUDmaMode = 0;
>>> #  232|
>>> #  233|     //
>>>
>>> Error: CLANG_WARNING:
>>> edk2-89910a39dcfd/OvmfPkg/VirtioGpuDxe/Gop.c:65:5: warning: Value stored
>>> to 'Status' is never read
>>> #    Status = VirtioGpuSetScanout (
>>> #    ^        ~~~~~~~~~~~~~~~~~~~~~
>>> edk2-89910a39dcfd/OvmfPkg/VirtioGpuDxe/Gop.c:65:5: note: Value stored to
>>> 'Status' is never read
>>> #    Status = VirtioGpuSetScanout (
>>> #    ^        ~~~~~~~~~~~~~~~~~~~~~
>>> #   63|       // by setting ResourceId=0 for it.
>>> #   64|       //
>>> #   65|->     Status = VirtioGpuSetScanout (
>>> #   66|                  VgpuGop->ParentBus, // VgpuDev
>>> #   67|                  0, 0, 0, 0,         // X, Y, Width, Height
>>>
>>> Error: CLANG_WARNING:
>>> edk2-89910a39dcfd/OvmfPkg/VirtioNetDxe/SnpReceive.c:165:3: warning:
>>> Value stored to 'RxPtr' is never read
>>> #  RxPtr += sizeof (UINT16);
>>> #  ^        ~~~~~~~~~~~~~~~
>>> edk2-89910a39dcfd/OvmfPkg/VirtioNetDxe/SnpReceive.c:165:3: note: Value
>>> stored to 'RxPtr' is never read
>>> #  RxPtr += sizeof (UINT16);
>>> #  ^        ~~~~~~~~~~~~~~~
>>> #  163|       *Protocol = (UINT16) ((RxPtr[0] << 8) | RxPtr[1]);
>>> #  164|     }
>>> #  165|->   RxPtr += sizeof (UINT16);
>>> #  166|
>>> #  167|     Status = EFI_SUCCESS;
>>
>> Cc: Anthony Perard <anthony.perard@citrix.com>
>> Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
>> Cc: Jordan Justen <jordan.l.justen@intel.com>
>> Cc: Julien Grall <julien.grall@arm.com>
>> Bugzilla: https://bugzilla.tianocore.org/show_bug.cgi?id=1710
>> Issue: scan-0992.txt
>> Issue: scan-0996.txt
>> Issue: scan-0997.txt
>> Issue: scan-0998.txt
>> Issue: scan-1000.txt
>> Issue: scan-1001.txt
>> Issue: scan-1006.txt
>> Issue: scan-1011.txt
>> Issue: scan-1012.txt
>> Signed-off-by: Laszlo Ersek <lersek@redhat.com>
>> ---
>>  OvmfPkg/AcpiPlatformDxe/Xen.c                                 | 5 +++++
>>  OvmfPkg/Library/PlatformBootManagerLib/QemuKernel.c           | 5 +++++
>>  OvmfPkg/Library/SerializeVariablesLib/SerializeVariablesLib.c | 4 ++++
>>  OvmfPkg/Library/VirtioLib/VirtioLib.c                         | 5 +++++
>>  OvmfPkg/QemuFlashFvbServicesRuntimeDxe/FwBlockService.c       | 5 +++++
>>  OvmfPkg/QemuFlashFvbServicesRuntimeDxe/FwBlockServiceDxe.c    | 5 +++++
>>  OvmfPkg/SataControllerDxe/SataController.c                    | 5 +++++
>>  OvmfPkg/VirtioGpuDxe/Gop.c                                    | 6 ++++++
>>  OvmfPkg/VirtioNetDxe/SnpReceive.c                             | 5 +++++
>>  9 files changed, 45 insertions(+)
>>
>> diff --git a/OvmfPkg/AcpiPlatformDxe/Xen.c b/OvmfPkg/AcpiPlatformDxe/Xen.c
>> index 357c60d23f4e..c8f275a8ee84 100644
>> --- a/OvmfPkg/AcpiPlatformDxe/Xen.c
>> +++ b/OvmfPkg/AcpiPlatformDxe/Xen.c
>> @@ -144,16 +144,21 @@ InstallXenTables (
>>    Fadt2Table  = NULL;
>>    Fadt1Table  = NULL;
>>    Facs2Table  = NULL;
>>    Facs1Table  = NULL;
>>    DsdtTable   = NULL;
>>    TableHandle = 0;
>>    NumberOfTableEntries = 0;
>>  
>> +  //
>> +  // Suppress "Value stored to ... is never read" analyzer warnings.
>> +  //
>> +  (VOID)NumberOfTableEntries;
>> +
> 
> I've seen this solution to shut up the compiler for similar reasons,
> and it kind of bugs me.
> 
> It looks like both paths of the if & else initialize
> NumberOfTableEntries, so maybe we can just drop setting it to 0?

It might trigger *other* compilers to whine about "variable read without
initialization" :)

> I looked at FwhInstance too, and it doesn't look like it is used after
> being set. So, maybe that should just be dropped?
> 
> I guess both of your bullet points give arguments allowing the unused
> set.
> 
> Rather than adding "Suppress..." comments everywhere, maybe a global
> macro could be defined for similar uses in EDK II?
> 
> //
> // Suppress "Value stored to ... is never read" analyzer warnings.
> //
> #define IGNORE_UNUSED_ASSIGNMENT(var) ((VOID)var)
> 
> Then again, I guess we decided to suppress this in EDK II compiler
> warnings with -Wno-unused-but-set-variable.
> 
> So, can you adjust the RH covscan tool settings to similarly ignore
> these warnings?

I think that should be possible, yes. AIUI it is supposed to have
location-sensitive permanent overrides (maintained outside of the source
code), and it should be doing incremental / differential scanning --
report only "new" issues.

Admittedly, I hate most of the warning suppression patches myself,
littering our code -- and that applies to all the *historical* spurious
assignments that we already have in place --, so I'm 100% fine with
dropping this patch (and other suppression patches too, if that is the
upstream edk2 project's preference!)

Thanks!
Laszlo



>>    //
>>    // Try to find Xen ACPI tables
>>    //
>>    Status = GetXenAcpiRsdp (&XenAcpiRsdpStructurePtr);
>>    if (EFI_ERROR (Status)) {
>>      return Status;
>>    }
>>  
>> diff --git a/OvmfPkg/Library/PlatformBootManagerLib/QemuKernel.c b/OvmfPkg/Library/PlatformBootManagerLib/QemuKernel.c
>> index ddfef925edd3..fde8c40218a4 100644
>> --- a/OvmfPkg/Library/PlatformBootManagerLib/QemuKernel.c
>> +++ b/OvmfPkg/Library/PlatformBootManagerLib/QemuKernel.c
>> @@ -37,16 +37,21 @@ TryRunningQemuKernel (
>>    SetupSize = 0;
>>    KernelBuf = NULL;
>>    KernelInitialSize = 0;
>>    CommandLine = NULL;
>>    CommandLineSize = 0;
>>    InitrdData = NULL;
>>    InitrdSize = 0;
>>  
>> +  //
>> +  // Suppress "Value stored to ... is never read" analyzer warnings.
>> +  //
>> +  (VOID)SetupSize;
>> +
>>    if (!QemuFwCfgIsAvailable ()) {
>>      return EFI_NOT_FOUND;
>>    }
>>  
>>    QemuFwCfgSelectItem (QemuFwCfgItemKernelSize);
>>    KernelSize = (UINTN) QemuFwCfgRead64 ();
>>  
>>    QemuFwCfgSelectItem (QemuFwCfgItemKernelSetupSize);
>> diff --git a/OvmfPkg/Library/SerializeVariablesLib/SerializeVariablesLib.c b/OvmfPkg/Library/SerializeVariablesLib/SerializeVariablesLib.c
>> index a6cebcb54f6b..64fe9d9be543 100644
>> --- a/OvmfPkg/Library/SerializeVariablesLib/SerializeVariablesLib.c
>> +++ b/OvmfPkg/Library/SerializeVariablesLib/SerializeVariablesLib.c
>> @@ -596,16 +596,20 @@ SerializeVariablesIterateSystemVariables (
>>        //
>>        // The currently allocated VariableData buffer is too small,
>>        // so we allocate a larger buffer.
>>        //
>>        if (VariableDataBufferSize != 0) {
>>          FreePool (VariableData);
>>          VariableData = NULL;
>>          VariableDataBufferSize = 0;
>> +        //
>> +        // Suppress "Value stored to ... is never read" analyzer warnings.
>> +        //
>> +        (VOID)VariableDataBufferSize;
>>        }
>>        VariableData = AllocatePool (VariableDataSize);
>>        if (VariableData == NULL) {
>>          Status = EFI_OUT_OF_RESOURCES;
>>          break;
>>        }
>>        VariableDataBufferSize = VariableDataSize;
>>  
>> diff --git a/OvmfPkg/Library/VirtioLib/VirtioLib.c b/OvmfPkg/Library/VirtioLib/VirtioLib.c
>> index 555d2a5ce7c9..0e4b8b6b265e 100644
>> --- a/OvmfPkg/Library/VirtioLib/VirtioLib.c
>> +++ b/OvmfPkg/Library/VirtioLib/VirtioLib.c
>> @@ -113,16 +113,21 @@ VirtioRingInit (
>>    RingPagesPtr += sizeof *Ring->Used.Idx;
>>  
>>    Ring->Used.UsedElem = (volatile VOID *) RingPagesPtr;
>>    RingPagesPtr += sizeof *Ring->Used.UsedElem * QueueSize;
>>  
>>    Ring->Used.AvailEvent = (volatile VOID *) RingPagesPtr;
>>    RingPagesPtr += sizeof *Ring->Used.AvailEvent;
>>  
>> +  //
>> +  // Suppress "Value stored to ... is never read" analyzer warnings.
>> +  //
>> +  (VOID)RingPagesPtr;
>> +
>>    Ring->QueueSize = QueueSize;
>>    return EFI_SUCCESS;
>>  }
>>  
>>  
>>  /**
>>  
>>    Tear down the internal resources of a configured virtio ring.
>> diff --git a/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/FwBlockService.c b/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/FwBlockService.c
>> index edf438a422fa..ff6565865846 100644
>> --- a/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/FwBlockService.c
>> +++ b/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/FwBlockService.c
>> @@ -1071,16 +1071,21 @@ FvbInitialize (
>>    ASSERT_RETURN_ERROR (PcdStatus);
>>  
>>    FwhInstance = (EFI_FW_VOL_INSTANCE *)
>>      (
>>        (UINTN) ((UINT8 *) FwhInstance) + FwVolHeader->HeaderLength +
>>        (sizeof (EFI_FW_VOL_INSTANCE) - sizeof (EFI_FIRMWARE_VOLUME_HEADER))
>>      );
>>  
>> +  //
>> +  // Suppress "Value stored to ... is never read" analyzer warnings.
>> +  //
>> +  (VOID)FwhInstance;
>> +
>>    //
>>    // Module type specific hook.
>>    //
>>    InstallVirtualAddressChangeHandler ();
>>  
>>    PcdStatus = PcdSetBoolS (PcdOvmfFlashVariablesEnable, TRUE);
>>    ASSERT_RETURN_ERROR (PcdStatus);
>>    return EFI_SUCCESS;
>> diff --git a/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/FwBlockServiceDxe.c b/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/FwBlockServiceDxe.c
>> index 69b20916bc7c..979122f2e2fd 100644
>> --- a/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/FwBlockServiceDxe.c
>> +++ b/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/FwBlockServiceDxe.c
>> @@ -164,16 +164,21 @@ MarkIoMemoryRangeForRuntimeAccess (
>>    //
>>    // Mark flash region as runtime memory
>>    //
>>    Status = gDS->RemoveMemorySpace (
>>                    BaseAddress,
>>                    Length
>>                    );
>>  
>> +  //
>> +  // Suppress "Value stored to ... is never read" analyzer warnings.
>> +  //
>> +  (VOID)Status;
>> +
>>    Status = gDS->AddMemorySpace (
>>                    EfiGcdMemoryTypeMemoryMappedIo,
>>                    BaseAddress,
>>                    Length,
>>                    EFI_MEMORY_UC | EFI_MEMORY_RUNTIME
>>                    );
>>    ASSERT_EFI_ERROR (Status);
>>  
>> diff --git a/OvmfPkg/SataControllerDxe/SataController.c b/OvmfPkg/SataControllerDxe/SataController.c
>> index 8d6a6bbb2286..0cf6b42c886a 100644
>> --- a/OvmfPkg/SataControllerDxe/SataController.c
>> +++ b/OvmfPkg/SataControllerDxe/SataController.c
>> @@ -219,16 +219,21 @@ CalculateBestUdmaMode (
>>    OUT UINT16            *SelectedMode
>>    )
>>  {
>>    UINT16    TempMode;
>>    UINT16    DeviceUDmaMode;
>>  
>>    DeviceUDmaMode = 0;
>>  
>> +  //
>> +  // Suppress "Value stored to ... is never read" analyzer warnings.
>> +  //
>> +  (VOID)DeviceUDmaMode;
>> +
>>    //
>>    // Check whether the WORD 88 (supported UltraDMA by drive) is valid
>>    //
>>    if ((IdentifyData->AtaData.field_validity & 0x04) == 0x00) {
>>      return EFI_UNSUPPORTED;
>>    }
>>  
>>    DeviceUDmaMode = IdentifyData->AtaData.ultra_dma_mode;
>> diff --git a/OvmfPkg/VirtioGpuDxe/Gop.c b/OvmfPkg/VirtioGpuDxe/Gop.c
>> index 0b2659d1d2b9..d0f81c349f73 100644
>> --- a/OvmfPkg/VirtioGpuDxe/Gop.c
>> +++ b/OvmfPkg/VirtioGpuDxe/Gop.c
>> @@ -57,16 +57,22 @@ ReleaseGopResources (
>>      // by setting ResourceId=0 for it.
>>      //
>>      Status = VirtioGpuSetScanout (
>>                 VgpuGop->ParentBus, // VgpuDev
>>                 0, 0, 0, 0,         // X, Y, Width, Height
>>                 0,                  // ScanoutId
>>                 0                   // ResourceId
>>                 );
>> +
>> +    //
>> +    // Suppress "Value stored to ... is never read" analyzer warnings.
>> +    //
>> +    (VOID)Status;
>> +
>>      //
>>      // HACK BEGINS HERE
>>      //
>>      // According to the GPU Device section of the VirtIo specification, the
>>      // above operation is valid:
>>      //
>>      // "The driver can use resource_id = 0 to disable a scanout."
>>      //
>> diff --git a/OvmfPkg/VirtioNetDxe/SnpReceive.c b/OvmfPkg/VirtioNetDxe/SnpReceive.c
>> index cdee9a2aee47..51b2f8a4e370 100644
>> --- a/OvmfPkg/VirtioNetDxe/SnpReceive.c
>> +++ b/OvmfPkg/VirtioNetDxe/SnpReceive.c
>> @@ -153,16 +153,21 @@ VirtioNetReceive (
>>    }
>>    RxPtr += SIZE_OF_VNET (Mac);
>>  
>>    if (Protocol != NULL) {
>>      *Protocol = (UINT16) ((RxPtr[0] << 8) | RxPtr[1]);
>>    }
>>    RxPtr += sizeof (UINT16);
>>  
>> +  //
>> +  // Suppress "Value stored to ... is never read" analyzer warnings.
>> +  //
>> +  (VOID)RxPtr;
>> +
>>    Status = EFI_SUCCESS;
>>  
>>  RecycleDesc:
>>    ++Dev->RxLastUsed;
>>  
>>    //
>>    // virtio-0.9.5, 2.4.1 Supplying Buffers to The Device
>>    //
>> -- 
>> 2.19.1.3.g30247aa5d201
>>
>>
>>
>> 
>>