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kernel: fill stack segments with 0xfefe... pattern

To aid detecting uninitialized variables (or missing
NUL termination of string buffers), fill the stack
segment with 0xfefe... pattern instead of zero.

--- a/sys/src/9/port/devsegment.c

+++ b/sys/src/9/port/devsegment.c

@@ -522,7 +522,7 @@

 			p->va = va;

 			va += BY2PG;

 			p->modref = 0;

-			zeropage(p);

+			fillpage(p, 0);

 			if(waserror()){

 				while(++p <= l)

 					freepages(p, p, 1);

--- a/sys/src/9/port/fault.c

+++ b/sys/src/9/port/fault.c

@@ -174,12 +174,13 @@

 		break;

 	case SG_BSS:

-	case SG_SHARED:			/* Zero fill on demand */

+	case SG_SHARED:			/* fill on demand */

 	case SG_STACK:

 		if(*pg == nil) {

-			new = newpage(1, &s, addr);

+			new = newpage(0, &s, addr);

 			if(s == nil)

 				return -1;

+			fillpage(new, (s->type&SG_TYPE)==SG_STACK? 0xfe: 0);

 			*pg = new;

 			s->used++;

 		}

--- a/sys/src/9/port/page.c

+++ b/sys/src/9/port/page.c

@@ -232,7 +232,7 @@

 	inittxtflush(p);

 	if(clear)

-		zeropage(p);

+		fillpage(p, 0);

 	return p;

 }

@@ -273,14 +273,17 @@

 	kunmap(kd);

 }

-void

-zeropage(Page *p)

+Page*

+fillpage(Page *p, int c)

 {

 	KMap *k;

-	k = kmap(p);

-	memset((void*)VA(k), 0, BY2PG);

-	kunmap(k);

+	if(p != nil){

+		k = kmap(p);

+		memset((void*)VA(k), c, BY2PG);

+		kunmap(k);

+	}

+	return p;

 }

 void

@@ -388,7 +391,7 @@

 	for(p = palloc.pages; p != pe; p++) {

 		if(p->modref & PG_PRIV){

 			incref(p);

-			zeropage(p);

+			fillpage(p, 0);

 			decref(p);

 		}

 	}

--- a/sys/src/9/port/portfns.h

+++ b/sys/src/9/port/portfns.h

@@ -416,7 +416,7 @@

 void*		xspanalloc(ulong, int, ulong);

 void		xsummary(void);

 void		yield(void);

-void		zeropage(Page*);

+Page*		fillpage(Page*, int);

 void		zeroprivatepages(void);

 Segment*	data2txt(Segment*);

 Segment*	dupseg(Segment**, int, int);

--- a/sys/src/9/port/sysproc.c

+++ b/sys/src/9/port/sysproc.c

@@ -541,6 +541,7 @@

 		memmove(charp, a, n);

 		charp += n;

 	}

+	*argv = nil;

 	/* copy args; easiest from new process's stack */

 	a = (char*)(tstk - nbytes);

-- 

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