ArmPkg: DefaultExceptionHandlerLib: Do Not Allocate Memory

If gST->ConOut is available when Arm's DefaultExceptionHandler is
running, AsciiPrint will get called to attempt to print to ConOut, in
addition to the serial output.

AsciiPrint calls AsciiInternalPrint in UefiLibPrint.c which in turn
calls AllocatePool to allocate a buffer to convert the Ascii input
string to a Unicode string to pass to ConOut->OutputString.

Per the comment on DefaultExceptionHandler, we should not be allocating
memory in the exception handler, as this can cause the exception handler
to fail if we had a memory exception or the system state is such that we
cannot allocate memory.

It has been observed on ArmVirtQemu that exceptions generated in the
memory handling code will fail to output the stack dump and CPU state
that is critical to debugging because the AllocatePool will fail.

This patch fixes the Arm and AARCH64 DefaultExceptionHandlers to not
allocate memory when ConOut is available and instead use stack memory to
convert the Ascii string needed for SerialPortWrite to the Unicode
string needed for ConOut->OutputString. Correspondingly, ArmVirtQemu can
now output the stack dump and CPU state when hitting an exception in
memory code.

Signed-off-by: Oliver Smith-Denny <osde@linux.microsoft.com>

2 files changed, 41 insertions, 15 deletions

diff --git a/ArmPkg/Library/DefaultExceptionHandlerLib/AArch64/DefaultExceptionHandler.c b/ArmPkg/Library/DefaultExceptionHandlerLib/AArch64/DefaultExceptionHandler.c
index f2bca5d740..a39896d576 100644
--- a/ArmPkg/Library/DefaultExceptionHandlerLib/AArch64/DefaultExceptionHandler.c
+++ b/ArmPkg/Library/DefaultExceptionHandlerLib/AArch64/DefaultExceptionHandler.c
@@ -22,6 +22,12 @@
 #include <Protocol/DebugSupport.h>

 #include <Protocol/LoadedImage.h>

 

+//

+// Maximum number of characters to print to serial (UINT8s) and to console if

+// available (as UINT16s)

+//

+#define MAX_PRINT_CHARS  100

+

 STATIC CHAR8  *gExceptionTypeString[] = {

   "Synchronous",

   "IRQ",

@@ -188,18 +194,14 @@ DefaultExceptionHandler (
   IN OUT EFI_SYSTEM_CONTEXT  SystemContext

   )

 {

-  CHAR8  Buffer[100];

-  UINTN  CharCount;

-  INT32  Offset;

+  CHAR8   Buffer[MAX_PRINT_CHARS];

+  CHAR16  UnicodeBuffer[MAX_PRINT_CHARS];

+  UINTN   CharCount;

+  INT32   Offset;

 

   if (mRecursiveException) {

     STATIC CHAR8 CONST  Message[] = "\nRecursive exception occurred while dumping the CPU state\n";

-

     SerialPortWrite ((UINT8 *)Message, sizeof Message - 1);

-    if (gST->ConOut != NULL) {

-      AsciiPrint (Message);

-    }

-

     CpuDeadLoop ();

   }

 

@@ -207,9 +209,10 @@ DefaultExceptionHandler (
 

   CharCount = AsciiSPrint (Buffer, sizeof (Buffer), "\n\n%a Exception at 0x%016lx\n", gExceptionTypeString[ExceptionType], SystemContext.SystemContextAArch64->ELR);

   SerialPortWrite ((UINT8 *)Buffer, CharCount);

-  if (gST->ConOut != NULL) {

-    AsciiPrint (Buffer);

-  }

+

+  // Prepare a unicode buffer for ConOut, if applicable, in case the buffer

+  // gets reused.

+  UnicodeSPrintAsciiFormat (UnicodeBuffer, MAX_PRINT_CHARS, Buffer);

 

   DEBUG_CODE_BEGIN ();

   CHAR8   *Pdb, *PrevPdb;

@@ -330,6 +333,13 @@ DefaultExceptionHandler (
       ));

   }

 

+  // Attempt to print that we had a synchronous exception to ConOut.  We do

+  // this after the serial logging as ConOut's logging is more complex and we

+  // aren't guaranteed to succeed.

+  if (gST->ConOut != NULL) {

+    gST->ConOut->OutputString (gST->ConOut, UnicodeBuffer);

+  }

+

   ASSERT (FALSE);

   CpuDeadLoop ();

 }

diff --git a/ArmPkg/Library/DefaultExceptionHandlerLib/Arm/DefaultExceptionHandler.c b/ArmPkg/Library/DefaultExceptionHandlerLib/Arm/DefaultExceptionHandler.c
index 13b321e456..accad647d6 100644
--- a/ArmPkg/Library/DefaultExceptionHandlerLib/Arm/DefaultExceptionHandler.c
+++ b/ArmPkg/Library/DefaultExceptionHandlerLib/Arm/DefaultExceptionHandler.c
@@ -24,6 +24,12 @@
 #include <Library/DefaultExceptionHandlerLib.h>

 

 //

+// Maximum number of characters to print to serial (UINT8s) and to console if

+// available (as UINT16s)

+//

+#define MAX_PRINT_CHARS  100

+

+//

 // The number of elements in a CHAR8 array, including the terminating NUL, that

 // is meant to hold the string rendering of the CPSR.

 //

@@ -198,7 +204,8 @@ DefaultExceptionHandler (
   IN OUT EFI_SYSTEM_CONTEXT  SystemContext

   )

 {

-  CHAR8    Buffer[100];

+  CHAR8    Buffer[MAX_PRINT_CHARS];

+  CHAR16   UnicodeBuffer[MAX_PRINT_CHARS];

   UINTN    CharCount;

   UINT32   DfsrStatus;

   UINT32   IfsrStatus;

@@ -216,9 +223,10 @@ DefaultExceptionHandler (
                 SystemContext.SystemContextArm->CPSR

                 );

   SerialPortWrite ((UINT8 *)Buffer, CharCount);

-  if (gST->ConOut != NULL) {

-    AsciiPrint (Buffer);

-  }

+

+  // Prepare a unicode buffer for ConOut, if applicable, as Buffer is used

+  // below.

+  UnicodeSPrintAsciiFormat (UnicodeBuffer, MAX_PRINT_CHARS, Buffer);

 

   DEBUG_CODE_BEGIN ();

   CHAR8   *Pdb;

@@ -289,6 +297,14 @@ DefaultExceptionHandler (
   }

 

   DEBUG ((DEBUG_ERROR, "\n"));

+

+  // Attempt to print that we had a synchronous exception to ConOut.  We do

+  // this after the serial logging as ConOut's logging is more complex and we

+  // aren't guaranteed to succeed.

+  if (gST->ConOut != NULL) {

+    gST->ConOut->OutputString (gST->ConOut, UnicodeBuffer);

+  }

+

   ASSERT (FALSE);

 

   CpuDeadLoop ();   // may return if executing under a debugger