pxelinux: complete but disable debugging code, not for production

Disable some debugging code that was never intended for production
use.  The intent is to track down specific memory-clobbering bugs.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>

diff --git a/core/pxelinux.asm b/core/pxelinux.asm
index ff0f36c..929ab67 100644 (file)
--- a/core/pxelinux.asm
+++ b/core/pxelinux.asm
@@ -255,12 +255,21 @@ _start1:
                mov ds,ax
                mov es,ax
 
+%if 0 ; debugging code only... not intended for production use
                ; Clobber the stack segment, to test for specific pathologies
                mov di,STACK_BASE
                mov cx,STACK_LEN >> 1
                mov ax,0xf4f4
                rep stosw
 
+               ; Clobber the tail of the 64K segment, too
+               extern __bss1_end
+               mov di,__bss1_end
+               sub cx,di               ; CX = 0 previously
+               shr cx,1
+               rep stosw
+%endif
+
                ; That is all pushed onto the PXE stack.  Save the pointer
                ; to it and switch to an internal stack.
                mov [InitStack],sp
@@ -2258,11 +2267,11 @@ do_reset_pxe:
                TRACER 'F'
 
 ;
-; Look to see if we are on an EFI CSM system.  Some EFI
-; CSM systems put the BEV stack in low memory, which means
-; a return to the PXE stack will crash the system.  However,
-; INT 18h works reliably, so in that case hack the stack and
-; point the "return address" to an INT 18h instruction.
+; Look to see if we are on an EFI CSM system.  Some EFI CSM systems
+; (AMI CSM) put the BEV stack in low memory (just below 64K), which
+; means a return to the PXE stack will crash the system.  However, INT
+; 18h works reliably, so in that case hack the stack and point the
+; "return address" to an INT 18h instruction.
 ;
 ; Hack the stack instead of the much simpler "just invoke INT 18h
 ; if we want to reset", so that chainloading other NBPs will work.