[edk2-devel] [PATCH 1/1] ArmPkg/ExceptionSupport: Support backtrace through an exception

[Ard Biesheuvel]

Laszlo reports that the efi_gdb.py script fails to produce a full
backtrace when attaching it to an ARM firmware build that has halted on
an unhandled exception.

The reason is that the asm code that processes the exception was not
implemented with this in mind, and therefore lacks any handling of it.

So let's add this: create a dummy frame record suitable for chasing the
frame pointer, and add the CFI metadata to describe where the return
value can be found on the stack.

When using a GCC5 build, this produces a stack trace such as

  (gdb) bt
  #0  0x000000007fd4537c in CpuDeadLoop () at /home/ardb/build/edk2/MdePkg/Library/BaseLib/CpuDeadLoop.c:30
  #1  0x000000007fd454f8 in DebugAssert (
      FileName=FileName at entry=0x7fd4a8a8 <MmioWrite64Internal+3604> "/home/ardb/build/edk2/ArmPkg/Library/DefaultExceptionHandlerLib/AArch64/DefaultExceptionHandler.c",
      LineNumber=LineNumber at entry=343, Description=Description at entry=0x7fd4a896 <MmioWrite64Internal+3586> "((BOOLEAN)(0==1))")
      at /home/ardb/build/edk2/MdePkg/Library/BaseDebugLibSerialPort/DebugLib.c:235
  #2  0x000000007fd479ec in DefaultExceptionHandler (ExceptionType=<optimized out>, SystemContext=...)
      at /home/ardb/build/edk2/ArmPkg/Library/DefaultExceptionHandlerLib/AArch64/DefaultExceptionHandler.c:343
  #3  0x000000007fd48eb8 in ExceptionHandlersEnd ()
  #4  0x000000007fcde944 in QemuLoadKernelImage (ImageHandle=<synthetic pointer>) at /home/ardb/build/edk2/OvmfPkg/Library/GenericQemuLoadImageLib/GenericQemuLoadImageLib.c:201
  #5  TryRunningQemuKernel () at /home/ardb/build/edk2/ArmVirtPkg/Library/PlatformBootManagerLib/QemuKernel.c:46
  #6  PlatformBootManagerAfterConsole () at /home/ardb/build/edk2/ArmVirtPkg/Library/PlatformBootManagerLib/PlatformBm.c:1139
  #7  BdsEntry (This=<optimized out>) at /home/ardb/build/edk2/MdeModulePkg/Universal/BdsDxe/BdsEntry.c:931
  #8  0x000000007ffd0018 in ?? ()
  Backtrace stopped: previous frame inner to this frame (corrupt stack?)

when QemuLoadKernelImage() has been tweaked to trigger an exception, as
is shown by GDB when walking the call stack:

|    0x7fcde938 <BdsEntry+3292>      b.ne    0x7fcdf134 <BdsEntry+5336>  // b.any
|    0x7fcde93c <BdsEntry+3296>      mov     x0, #0x40                       // #64
|    0x7fcde940 <BdsEntry+3300>      bl      0x7fcd7aec <DebugPrint>
|  > 0x7fcde944 <BdsEntry+3304>      brk     #0x4d2
|    0x7fcde948 <BdsEntry+3308>      bl      0x7fce4354 <ConnectDevicesFromQemu>
|    0x7fcde94c <BdsEntry+3312>      tbz     x0, #63, 0x7fcde954 <BdsEntry+3320>
|    0x7fcde950 <BdsEntry+3316>      bl      0x7fcd844c <EfiBootManagerConnectAll>
|    0x7fcde954 <BdsEntry+3320>      bl      0x7fcd990c <EfiBootManagerRefreshAllBootOption

Unfortunately, CLANGDWARF does not seem entirely happy with this
arrangement: it identifies the call frame where the exception
originated, but does not show any frames above that. (This could be
related to the fact that the exception code uses a separate exception
stack for handling synchronous exceptions)

Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
---
 ArmPkg/Library/ArmExceptionLib/AArch64/ExceptionSupport.S | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/ArmPkg/Library/ArmExceptionLib/AArch64/ExceptionSupport.S b/ArmPkg/Library/ArmExceptionLib/AArch64/ExceptionSupport.S
index cd9437b6aab8..345b566932bb 100644
--- a/ArmPkg/Library/ArmExceptionLib/AArch64/ExceptionSupport.S
+++ b/ArmPkg/Library/ArmExceptionLib/AArch64/ExceptionSupport.S
@@ -259,6 +259,8 @@ ASM_PFX(ExceptionHandlersEnd):
 
 
 ASM_PFX(CommonExceptionEntry):
+  .cfi_sections .debug_frame
+  .cfi_startproc
 
   EL1_OR_EL2_OR_EL3(x1)
 1:mrs      x2, elr_el1   // Exception Link Register
@@ -280,6 +282,13 @@ ASM_PFX(CommonExceptionEntry):
 
 4:mrs      x4, fpsr      // Floating point Status Register  32bit
 
+  // Create a dummy frame record using the ELR as the return address
+  stp      x29, x2, [sp, #-16]!
+  .cfi_def_cfa_offset (GP_CONTEXT_SIZE + FP_CONTEXT_SIZE + SYS_CONTEXT_SIZE + 16)
+  .cfi_rel_offset x29, 0
+  .cfi_rel_offset x30, 8
+  mov      x29, sp
+
   // Save the SYS regs
   stp      x2,  x3,  [x28, #-SYS_CONTEXT_SIZE]!
   stp      x4,  x5,  [x28, #0x10]
@@ -305,7 +314,7 @@ ASM_PFX(CommonExceptionEntry):
 
   // x0 still holds the exception type.
   // Set x1 to point to the top of our struct on the Stack
-  mov      x1, sp
+  add      x1, sp, #16
 
 // CommonCExceptionHandler (
 //   IN     EFI_EXCEPTION_TYPE           ExceptionType,   R0
@@ -318,6 +327,9 @@ ASM_PFX(CommonExceptionEntry):
   // We do not try to recover.
   bl       ASM_PFX(CommonCExceptionHandler) // Call exception handler
 
+  // Pop dummy frame record
+  add      sp, sp, #16
+
   // Pop as many GP regs as we can before entering the critical section below
   ldp      x2,  x3,  [sp, #0x10]
   ldp      x4,  x5,  [sp, #0x20]
@@ -378,13 +390,17 @@ ASM_PFX(CommonExceptionEntry):
 
   // pop remaining GP regs and return from exception.
   ldr      x30, [sp, #0xf0 - 0xe0]
+  .cfi_restore 30
   ldp      x28, x29, [sp], #GP_CONTEXT_SIZE - 0xe0
+  .cfi_restore 29
 
   // Adjust SP to be where we started from when we came into the handler.
   // The handler can not change the SP.
   add      sp, sp, #FP_CONTEXT_SIZE + SYS_CONTEXT_SIZE
+  .cfi_def_cfa_offset 0
 
   eret
+  .cfi_endproc
 
 ASM_FUNC(RegisterEl0Stack)
   msr     sp_el0, x0
-- 
2.42.0.rc2.253.gd59a3bf2b4-goog



-=-=-=-=-=-=-=-=-=-=-=-
Groups.io Links: You receive all messages sent to this group.
View/Reply Online (#108092): https://edk2.groups.io/g/devel/message/108092
Mute This Topic: https://groups.io/mt/101030910/1813853
Group Owner: devel+owner at edk2.groups.io
Unsubscribe: https://edk2.groups.io/g/devel/unsub [edk2-devel-archive at redhat.com]
-=-=-=-=-=-=-=-=-=-=-=-