Re: [PATCH 2/2] ArmPkg/DefaultExceptionHandlerLib AARCH64: add minimal backtrace to crash dump

[Ard Biesheuvel <ard.biesheuvel@linaro.org>]

On 7 September 2016 at 12:25, Michael Zimmermann
<sigmaepsilon92@gmail.com> wrote:
>> However, looking at this
>> code, this is still not sufficient to find the *next* frame pointer on
>> the stack.
> are you sure about that? this code looks like it does just that:
> https://github.com/torvalds/linux/blob/master/arch/arm/kernel/stacktrace.c
>

First of all, that comment makes little sense:

* With framepointer enabled, a simple function prologue looks like this:
* mov ip, sp
* stmdb sp!, {fp, ip, lr, pc}

I assume this should be

stmdb sp!, {fp, ip, lr}

* sub fp, ip, #4
*
* A simple function epilogue looks like this:
* ldm sp, {fp, sp, pc}

But then, it looks like GCC in ARM mode happens to generate stack
frames that are easily unwound.

However, in EDK2 we use Thumb2 primarily, and the code I snipped is
actually from a -fno-omit-frame-pointer Thumb2 build, and it does not
look like whatever is pushed onto the stack can be decoded
unambiguously like that.


> On Wed, Sep 7, 2016 at 1:03 PM, Ard Biesheuvel <ard.biesheuvel@linaro.org>
> wrote:
>>
>> On 7 September 2016 at 10:48, Michael Zimmermann
>> <sigmaepsilon92@gmail.com> wrote:
>> > nice, can we do this for ARM too? I usually need to add DEBUG((...))'s
>> > all
>> > over the place for hours until I found the reason for a fault.
>> >
>>
>> This is going to be tricky. Unlike AARCH64, which unambiguously
>> describes in the AAPCS how to link the stack frames using x29, the
>> AAPCS for ARM does not standardise this, and so different compilers
>> may do different things, also depending on whether you are executing
>> in ARM or Thumb mode. For instance, the following prologue was snipped
>> from a BdsDxe.dll build using GCC5 in Thumb2 mode.
>>
>> 00000254 <_ModuleEntryPoint>:
>>      254:       e96d 7e02       strd    r7, lr, [sp, #-8]!
>>      258:       b084            sub     sp, #16
>>      25a:       af00            add     r7, sp, #0
>>
>> Googling around, i can indeed find sources that describe how GCC uses
>> r7 as the frame pointer for Thumb2 code. However, looking at this
>> code, this is still not sufficient to find the *next* frame pointer on
>> the stack. IOW, there is no standardised layout to find the next frame
>> pointer inside the stack frame.
>>
>> The only way to do this reliably is using unwind tables, but this is
>> *much* more complicated than the 20 line patch that enables it for
>> AArch64
>>
>> Ard.
>>
>>
>> > On Wed, Sep 7, 2016 at 10:21 AM, Ard Biesheuvel
>> > <ard.biesheuvel@linaro.org>
>> > wrote:
>> >>
>> >> When dumping the CPU state after an unhandled fault, walk the stack
>> >> frames and decode the return addresses so we can show a minimal
>> >> backtrace. Unfortunately, we do not have sufficient information to
>> >> show the function names, but at least we can see the modules and the
>> >> return addresses inside the modules.
>> >>
>> >> Contributed-under: TianoCore Contribution Agreement 1.0
>> >> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
>> >> ---
>> >>
>> >> Example output:
>> >>
>> >> Synchronous Exception at 0x00000000F5EA4C8C
>> >>
>> >>
>> >> /home/ard/build/edk2/Build/ArmVExpress-FVP-AArch64/DEBUG_GCC5/AARCH64/ArmPlatformPkg/Drivers/NorFlashDxe/NorFlashDxe/DEBUG/ArmVeNorFlashDxe.dll
>> >> loaded at 0x00000000F5E90000
>> >> called from
>> >>
>> >> /home/ard/build/edk2/Build/ArmVExpress-FVP-AArch64/DEBUG_GCC5/AARCH64/ArmPlatformPkg/Drivers/NorFlashDxe/NorFlashDxe/DEBUG/ArmVeNorFlashDxe.dll
>> >> (0x00000000F5EA4AE8) loaded at 0x00000000F5E90000
>> >> called from
>> >>
>> >> /home/ard/build/edk2/Build/ArmVExpress-FVP-AArch64/DEBUG_GCC5/AARCH64/ArmPlatformPkg/Drivers/NorFlashDxe/NorFlashDxe/DEBUG/ArmVeNorFlashDxe.dll
>> >> (0x00000000F5EA1BFC) loaded at 0x00000000F5E90000
>> >> called from
>> >>
>> >> /home/ard/build/edk2/Build/ArmVExpress-FVP-AArch64/DEBUG_GCC5/AARCH64/ArmPlatformPkg/Drivers/NorFlashDxe/NorFlashDxe/DEBUG/ArmVeNorFlashDxe.dll
>> >> (0x00000000F5EA23F4) loaded at 0x00000000F5E90000
>> >> called from
>> >>
>> >> /work/jenkins/workspace/ap-uefi-bin/EDK2_ARCH/ARM/EDK2_BINARY/FatPkg/label/sas-sw/Build/Fat/RELEASE_GCC49/AARCH64/FatPkg/EnhancedFatDxe/Fat/DEBUG/Fat.dll
>> >> (0x00000000FADC454C) loaded at 0x00000000FADC3000
>> >> called from
>> >>
>> >> /home/ard/build/edk2/Build/ArmVExpress-FVP-AArch64/DEBUG_GCC5/AARCH64/MdeModulePkg/Core/Dxe/DxeMain/DEBUG/DxeCore.dll
>> >> (0x00000000FE47C868) loaded at 0x00000000FE471000
>> >> called from
>> >>
>> >> /home/ard/build/edk2/Build/ArmVExpress-FVP-AArch64/DEBUG_GCC5/AARCH64/IntelFrameworkModulePkg/Universal/BdsDxe/BdsDxe/DEBUG/BdsDxe.dll
>> >> (0x00000000FAE2C50C) loaded at 0x00000000FAE01000
>> >> called from
>> >>
>> >> /home/ard/build/edk2/Build/ArmVExpress-FVP-AArch64/DEBUG_GCC5/AARCH64/IntelFrameworkModulePkg/Universal/BdsDxe/BdsDxe/DEBUG/BdsDxe.dll
>> >> (0x00000000FAE2C674) loaded at 0x00000000FAE01000
>> >> called from
>> >>
>> >> /home/ard/build/edk2/Build/ArmVExpress-FVP-AArch64/DEBUG_GCC5/AARCH64/IntelFrameworkModulePkg/Universal/BdsDxe/BdsDxe/DEBUG/BdsDxe.dll
>> >> (0x00000000FAE2C264) loaded at 0x00000000FAE01000
>> >> called from
>> >>
>> >> /home/ard/build/edk2/Build/ArmVExpress-FVP-AArch64/DEBUG_GCC5/AARCH64/IntelFrameworkModulePkg/Universal/BdsDxe/BdsDxe/DEBUG/BdsDxe.dll
>> >> (0x00000000FAE36998) loaded at 0x00000000FAE01000
>> >> called from
>> >>
>> >> /home/ard/build/edk2/Build/ArmVExpress-FVP-AArch64/DEBUG_GCC5/AARCH64/IntelFrameworkModulePkg/Universal/BdsDxe/BdsDxe/DEBUG/BdsDxe.dll
>> >> (0x00000000FAE03668) loaded at 0x00000000FAE01000
>> >> called from
>> >>
>> >> /home/ard/build/edk2/Build/ArmVExpress-FVP-AArch64/DEBUG_GCC5/AARCH64/MdeModulePkg/Core/Dxe/DxeMain/DEBUG/DxeCore.dll
>> >> (0x00000000FE4733F0) loaded at 0x00000000FE471000
>> >> called from
>> >>
>> >> /home/ard/build/edk2/Build/ArmVExpress-FVP-AArch64/DEBUG_GCC5/AARCH64/MdeModulePkg/Core/Dxe/DxeMain/DEBUG/DxeCore.dll
>> >> (0x00000000FE4724A0) loaded at 0x00000000FE471000
>> >> called from
>> >>
>> >> /home/ard/build/edk2/Build/ArmVExpress-FVP-AArch64/DEBUG_GCC5/AARCH64/MdeModulePkg/Core/Dxe/DxeMain/DEBUG/DxeCore.dll
>> >> (0x00000000FE472024) loaded at 0x00000000FE471000
>> >> ---
>> >>  .../AArch64/DefaultExceptionHandler.c               | 21
>> >> +++++++++++++++++++++
>> >>  1 file changed, 21 insertions(+)
>> >>
>> >> diff --git
>> >>
>> >> a/ArmPkg/Library/DefaultExceptionHandlerLib/AArch64/DefaultExceptionHandler.c
>> >>
>> >> b/ArmPkg/Library/DefaultExceptionHandlerLib/AArch64/DefaultExceptionHandler.c
>> >> index 31fc936b21ff..84b442f2b6f4 100644
>> >> ---
>> >>
>> >> a/ArmPkg/Library/DefaultExceptionHandlerLib/AArch64/DefaultExceptionHandler.c
>> >> +++
>> >>
>> >> b/ArmPkg/Library/DefaultExceptionHandlerLib/AArch64/DefaultExceptionHandler.c
>> >> @@ -152,9 +152,30 @@ DefaultExceptionHandler (
>> >>      CHAR8  *Pdb;
>> >>      UINTN  ImageBase;
>> >>      UINTN  PeCoffSizeOfHeader;
>> >> +    UINT64 *Fp;
>> >> +
>> >>      Pdb = GetImageName (SystemContext.SystemContextAArch64->ELR,
>> >> &ImageBase, &PeCoffSizeOfHeader);
>> >>      if (Pdb != NULL) {
>> >>        DEBUG ((EFI_D_ERROR, "%a loaded at 0x%016lx \n", Pdb,
>> >> ImageBase));
>> >> +
>> >> +      Pdb = GetImageName (SystemContext.SystemContextAArch64->LR,
>> >> &ImageBase,
>> >> +              &PeCoffSizeOfHeader);
>> >> +      if (Pdb != NULL) {
>> >> +        DEBUG ((EFI_D_ERROR, "called from %a (0x%016lx) loaded at
>> >> 0x%016lx \n",
>> >> +          Pdb, SystemContext.SystemContextAArch64->LR, ImageBase));
>> >> +      }
>> >> +      for (Fp = (UINT64 *)SystemContext.SystemContextAArch64->FP;
>> >> +           *Fp != 0;
>> >> +           Fp = (UINT64 *)Fp[0]) {
>> >> +        if (Fp[1] == SystemContext.SystemContextAArch64->LR) {
>> >> +         continue;
>> >> +        }
>> >> +        Pdb = GetImageName (Fp[1], &ImageBase, &PeCoffSizeOfHeader);
>> >> +        if (Pdb != NULL) {
>> >> +          DEBUG ((EFI_D_ERROR, "called from %a (0x%016lx) loaded at
>> >> 0x%016lx \n",
>> >> +            Pdb, Fp[1], ImageBase));
>> >> +        }
>> >> +      }
>> >>      }
>> >>    DEBUG_CODE_END ();
>> >>
>> >> --
>> >> 2.7.4
>> >>
>> >> _______________________________________________
>> >> edk2-devel mailing list
>> >> edk2-devel@lists.01.org
>> >> https://lists.01.org/mailman/listinfo/edk2-devel
>> >
>> >
>
>