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kernel: fix rounding of bss size in exec (broke mntgen on arm64)

We must not round up bss size to pages, as this breaks sbrk()
thinking we are shrinkng the segment on first sbrk() call.

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

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

@@ -333,7 +333,7 @@

 	char *a, *e, *charp, *file;

 	int i, n, indir;

 	ulong magic, ssize, nargs, nbytes;

-	uintptr entry, text, data, bss, adata, abss, tstk, align;

+	uintptr entry, text, data, bss, adata, abss, ebss, tstk, align;

 	Segment *s, *ts;

 	Image *img;

 	Tos *tos;

@@ -432,8 +432,10 @@

 	data = beswal(u.ehdr.data);

 	bss = beswal(u.ehdr.bss);

 	align = BY2PG-1;

+	

 	abss = (adata + data + align) & ~align;

-	if(adata >= (USTKTOP-USTKSIZE) || abss >= (USTKTOP-USTKSIZE) || (abss+PGROUND(bss)) >= (USTKTOP-USTKSIZE))

+	ebss = (adata + data + bss + align) & ~align;

+	if(adata >= (USTKTOP-USTKSIZE) || abss >= (USTKTOP-USTKSIZE) || ebss >= (USTKTOP-USTKSIZE))

 		error(Ebadexec);

 	/*

@@ -616,7 +618,7 @@

 	up->seg[DSEG] = s;

 	/* BSS. Zero fill on demand */

-	up->seg[BSEG] = newseg(SG_BSS, abss, PGROUND(bss)>>PGSHIFT);

+	up->seg[BSEG] = newseg(SG_BSS, abss, (ebss - abss)>>PGSHIFT);

 	/*

 	 * Move the stack

-- 

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