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snap: be more conservative taking stack snapshot

include up to 8k below the stack to get some more context.

don't assume stack is at least 16K.

--- a/sys/src/cmd/snap/take.c

+++ b/sys/src/cmd/snap/take.c

@@ -20,7 +20,7 @@

 	return sum;

 }

-static int npage;

+static ulong npage;

 static Page *pgtab[1<<10];

 Page*

@@ -32,7 +32,7 @@

 	int iszero;

 	if(len > Pagesize) {

-		fprint(2, "datapage cannot handle pages > 1024\n");

+		fprint(2, "datapage cannot handle pages > %d\n", Pagesize);

 		exits("datapage");

 	}

@@ -277,14 +277,14 @@

 	if(stacklen) {

 		sp = stackptr(proc, fd);

 		if(stackoff <= sp && sp < stackoff+stacklen) {

-			off = (sp - Pagesize) & ~(Pagesize - 1);

-			if(off < stackoff)

-				off = stackoff;

-			len = stacklen - (off - stackoff);

+			off = sp - 8*1024;

 		} else {	/* stack pointer not in segment.  thread library? */

 			off = stackoff + stacklen - 16*1024;

-			len = 16*1024;

 		}

+		off &= ~((uvlong)Pagesize-1);

+		if(off < stackoff)

+			off = stackoff;

+		len = stacklen - (off - stackoff);

 		s[stacki] = readseg(fd, off, len, "Stack");

 	}