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kernel: prohibit changing cache attributes (SG_CACHED|SG_DEVICE) in segattach(), set SG_RONLY in data2txt()

the user should not be able to change the cache
attributes for a segment in segattach() as this
can cause the same memory to be mapped with
conflicting attributes in the cache.

SG_TEXT should always be mapped with SG_RONLY
attribute. so fix data2txt() to follow the rules.

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

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

@@ -177,8 +177,8 @@

 	case SG_DATA:		/* Copy on write plus demand load info */

 		if(segno == TSEG){

 			n = data2txt(s);

-			poperror();

 			qunlock(s);

+			poperror();

 			return n;

 		}

@@ -200,14 +200,14 @@

 	n->flushme = s->flushme;

 	if(s->ref > 1)

 		procflushseg(s);

-	poperror();

 	qunlock(s);

+	poperror();

 	return n;

 sameseg:

 	incref(s);

-	poperror();

 	qunlock(s);

+	poperror();

 	return s;

 }

@@ -680,9 +680,12 @@

 	if(len > ps->size)

 		error(Enovmem);

-	attr &= ~SG_TYPE;		/* Turn off what is not allowed */

-	attr |= ps->attr;		/* Copy in defaults */

+	/* Turn off what is not allowed */

+	attr &= ~(SG_TYPE | SG_CACHED | SG_DEVICE);

+	/* Copy in defaults */

+	attr |= ps->attr;

+

 	s = newseg(attr, va, len/BY2PG);

 	s->pseg = ps;

 	up->seg[sno] = s;

@@ -788,7 +791,7 @@

 {

 	Segment *ps;

-	ps = newseg(SG_TEXT, s->base, s->size);

+	ps = newseg(SG_TEXT | SG_RONLY, s->base, s->size);

 	ps->image = s->image;

 	incref(ps->image);

 	ps->fstart = s->fstart;

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

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

@@ -512,7 +512,7 @@

 	/* Text.  Shared. Attaches to cache image if possible */

 	/* attachimage returns a locked cache image */

-	img = attachimage(SG_TEXT|SG_RONLY, tc, UTZERO, (t-UTZERO)>>PGSHIFT);

+	img = attachimage(SG_TEXT | SG_RONLY, tc, UTZERO, (t-UTZERO)>>PGSHIFT);

 	ts = img->s;

 	up->seg[TSEG] = ts;

 	ts->flushme = 1;