__stack_start for ColdFire uClinux

I have been chasing what I suspect are stack overflows in NTP.  I spotted mention of that there was a gcc option, -fstack-check-symbol=__stack_start, for run-time stack checking on uClinux.  I tried that, but it caused an internal compiler error.  My development environment is the Sourcery (now Mentor Graphics) CodeBench Lite for ColdFire uClinux SDK.  I modified their GCC source to add support for -fstack-limit-symbol.  Unfortunately, using the __stack_start symbol to detect stack overflow does not seem to be working out.  For my test uClinux executable, stack-overflow.c:

#include <stdio.h>

void overflow( int i ) {

/* Allocate automatic array j[256] so stack limit checking works. */

   int j[256];

/* Stack limit checking tests only whether the stack is large enough for  */
/* the initial stack allocation: saved registers and automatic variables. */
/* Stack limit checking does not test for stack overflow in the body of a */
/* function.  That is what happens when j[] is not present.  When j[] is  */
/* too large, the program will abnormally terminate on its own (illegal   */
/* instruction trap?).  j[256] works.                                     */

   i++;
   printf( "i = %i\n", i );
   overflow( i );

}

int main() {

   overflow( 0 );

   return 0;

}

the stack is allocated 4K (0x1000).  When I hacked the .s file to print out the stack pointer and __stack start, it shows that there is more than 4K between the value in the stack pointer and __stack_start:

%sp = 0x405f3ee4
__stack_start = 0x405f08b4

That is not what I was expecting.

Can someone shed some light on why %sp is not roughly __stack_start + 0x1000?

__stack_start is defined in the linker script from the Sourcery SDK, m68k-uclinux/lib/elf2flt.ld, right after .bss and before .junk:

	.bss : {
		. = ALIGN(0x4) ;
		_sbss = ALIGN(0x4) ;
		__bss_start = . ;
		*(.dynsbss)
		*(.sbss)
		*(.sbss.*)
		*(.scommon)
		*(.dynbss)
		*(.bss)
		*(.bss.*)
		*(.bss*)
		*(.gnu.linkonce.b*)
		*(COMMON)
		. = ALIGN(0x10) ;
		_ebss = . ;
		_end = . ;
		end = . ;
	} > flatmem

	.stack : {
		. = ALIGN(0x4);
		__stack_start = .;
	}

	.junk 0 : { *(.rel*) *(.rela*) }
	/* Stabs debugging sections.    */
	.stab 0 : { *(.stab) }
	.stabstr 0 : { *(.stabstr) }
	.stab.excl 0 : { *(.stab.excl) }
	.stab.exclstr 0 : { *(.stab.exclstr) }
	.stab.index 0 : { *(.stab.index) }
	.stab.indexstr 0 : { *(.stab.indexstr) }
	.comment 0 : { *(.comment) }
	.debug_abbrev 0 : { *(.debug_abbrev) }
	.debug_info 0 : { *(.debug_info) }
	.debug_line 0 : { *(.debug_line) }
	.debug_pubnames 0 : { *(.debug_pubnames) }
	.debug_aranges 0 : { *(.debug_aranges) }

Is stack: in the wrong place?

Larry Baker
US Geological Survey
650-329-5608
[email protected]



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