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lib9p: use Emsize for "message size too small"

Use sizeD2M() instead of calling convD2M() twice.
Use common Emsize = "message size too small"
error string for Tversion and Rstat.

--- a/sys/src/lib9p/srv.c

+++ b/sys/src/lib9p/srv.c

@@ -23,6 +23,7 @@

 static char Eunknownfid[] = "unknown fid";

 static char Ebaddir[] = "bad directory in wstat";

 static char Ewalknodir[] = "walk in non-directory";

+static char Emsize[] = "message size too small";

 static void

 setfcallerror(Fcall *f, char *err)

@@ -175,7 +176,7 @@

 	}

 	r->ofcall.version = "9P2000";

 	if(r->ifcall.msize < 256){

-		respond(r, "version: message size too small");

+		respond(r, Emsize);

 		return;

 	}

 	if(r->ifcall.msize < 1024*1024)

@@ -660,31 +661,24 @@

 	else

 		respond(r, Enostat);

 }

+

 static void

 rstat(Req *r, char *error)

 {

 	int n;

 	uchar *statbuf;

-	uchar tmp[BIT16SZ];

 	if(error)

 		return;

-	if(convD2M(&r->d, tmp, BIT16SZ) != BIT16SZ){

-		r->error = "convD2M(_,_,BIT16SZ) did not return BIT16SZ";

-		return;

-	}

-	n = GBIT16(tmp)+BIT16SZ;

-	if(n+BIT32SZ+BIT8SZ+BIT16SZ+BIT16SZ > r->srv->msize){

-		r->error = "rstat: message size too small";

-		return;

-	}

+	n = sizeD2M(&r->d);

 	statbuf = emalloc9p(n);

-	r->ofcall.nstat = convD2M(&r->d, statbuf, n);

-	if(r->ofcall.nstat <= BIT16SZ){

-		r->error = "convD2M fails";

+	n = convD2M(&r->d, statbuf, n);

+	if(n <= BIT16SZ || n > r->srv->msize-(BIT32SZ+BIT8SZ+BIT16SZ+BIT16SZ)){

+		r->error = Emsize;

 		free(statbuf);

 		return;

 	}

+	r->ofcall.nstat = n;

 	r->ofcall.stat = statbuf;	/* freed in closereq */

 }

-- 

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