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Create a UEFI variable at end of DXE indicating UEFI debugger is active #150

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Create a UEFI variable at end of DXE indicating UEFI debugger is active #150

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6 changes: 5 additions & 1 deletion DebuggerFeaturePkg/DebuggerFeaturePkg.dec
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Original file line number Diff line number Diff line change
Expand Up @@ -34,10 +34,14 @@
# { bf004bc2-da8c-4e44-b470-d69c286d712d }
DebuggerFeaturePkgTokenSpaceGuid = { 0xbf004bc2, 0xda8c, 0x4e44, {0xb4, 0x70, 0xd6, 0x9c, 0x28, 0x6d, 0x71, 0x2d }}

## Debugget Control Data Hob Guid
## Debugger Control Data Hob Guid
#
gDebuggerControlHobGuid = { 0x96ad286c, 0xac0f, 0x4ba1, {0x97, 0x34, 0x6c, 0x9f, 0x3c, 0xad, 0x90, 0x4c }}

## Debugger Variables
# { 9478feba-dfd5-4f7c-8ea2-fc51969e5416 }
gDebuggerVariableGuid = {0x9478feba, 0xdfd5, 0x4f7c, {0x8e, 0xa2, 0xfc, 0x51, 0x96, 0x9e, 0x54, 0x16}}

[PcdsFixedAtBuild]
## Controls the debug configuration flags.
# Bit 0 - Controls whether the debugger will break in on initialization.
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53 changes: 53 additions & 0 deletions DebuggerFeaturePkg/Library/DebugAgent/DebugAgentDxe.c
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Expand Up @@ -17,6 +17,7 @@
#include <Library/BaseMemoryLib.h>
#include <Library/DebugAgentLib.h>
#include <Library/UefiBootServicesTableLib.h>
#include <Library/UefiRuntimeServicesTableLib.h>
#include <Library/UefiLib.h>
#include <Library/PeCoffGetEntryPointLib.h>
#include <Library/CpuExceptionHandlerLib.h>
Expand Down Expand Up @@ -62,6 +63,7 @@ STATIC EFI_EVENT mTimerEvent;
STATIC EFI_EVENT mCpuArchEvent;
STATIC EFI_EVENT mLoadedImageEvent;
STATIC EFI_EVENT mExitBootServicesEvent;
STATIC EFI_EVENT mEndOfDxeEvent;
STATIC BOOLEAN mDebuggerInitialized;
STATIC BOOLEAN mDisablePolling;
STATIC CHAR8 mDbgBreakOnModuleLoadString[64] = { 0 };
Expand Down Expand Up @@ -153,6 +155,40 @@ OnExitBootServices (
return;
}

/**
This routine handles the END_OF_DXE notification, and terminates
the debugger

@param Event Not used.
@param Context Not used.

**/
VOID
EFIAPI
OnEndOfDxe (
EFI_EVENT Event,
VOID *Context
)
{
EFI_STATUS Status;
UINT8 Value;

Value = 1;
Status = gRT->SetVariable (
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@apop5 apop5 Feb 12, 2026

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This is might be a nit for the usecase:

Volatile variable that is Boot services access only, runs at end of dxe, before 3rd party code runs, and is un protected. Is it a problem that 3rd party code could change this value, and the boot loader wouldn't be aware of it anymore?

Is there a more standard way that presence of a debugger could be detected?

L"UefiDebuggerActive",
&gDebuggerVariableGuid,
EFI_VARIABLE_BOOTSERVICE_ACCESS,
sizeof (Value),
&Value
);

if (EFI_ERROR (Status)) {
DEBUG ((DEBUG_ERROR, "%a: Failed to set UefiDebuggerActive variable. %r\n", __FUNCTION__, Status));
}

return;
}

/**
Cpu Arch Protocol notification.

Expand Down Expand Up @@ -660,6 +696,23 @@ DxeDebugSetupCallbacks (
DEBUG ((DEBUG_ERROR, "%a: failed to create Exit Boot Services callback\n", __FUNCTION__));
}

//
// Register for READY_TO_BOOT notification, to set a variable for OS loaders to check for debugger enablement.
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@os-d os-d Feb 12, 2026

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You say RTB, but do EOD. I would have thought RTB would be the more natural place, right before we load a bootloader?

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@cfernald cfernald Feb 12, 2026

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It's somewhat arbitrary, and I have no strong preference. I can change to RTB.

//

Status = gBS->CreateEventEx (
EVT_NOTIFY_SIGNAL,
TPL_CALLBACK,
OnEndOfDxe,
NULL,
&gEfiEndOfDxeEventGroupGuid,
&mEndOfDxeEvent
);

if (EFI_ERROR (Status)) {
DEBUG ((DEBUG_ERROR, "%a: failed to create End of Dxe callback\n", __FUNCTION__));
}

return EFI_SUCCESS;
}

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2 changes: 2 additions & 0 deletions DebuggerFeaturePkg/Library/DebugAgent/DebugAgentDxe.inf
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Original file line number Diff line number Diff line change
Expand Up @@ -76,7 +76,9 @@

[Guids]
gEfiEventExitBootServicesGuid
gEfiEndOfDxeEventGroupGuid
gDebuggerControlHobGuid
gDebuggerVariableGuid

[Pcd.common]
DebuggerFeaturePkgTokenSpaceGuid.PcdForceEnableDebugger ## CONSUMES
Expand Down