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auth/keyfs: support -r flag to mount read-only

--- a/sys/man/4/keyfs

+++ b/sys/man/4/keyfs

@@ -14,6 +14,9 @@

 .BI -m mntpt

 ]

 [

+.B -r

+]

+[

 .I keyfile

 ]

 .PP

@@ -83,6 +86,10 @@

 If any changes are made to the database that affect the information stored in

 .IR keyfile ,

 a new version of the file is written.

+.PP

+If the

+.B -r

+option is given, the database is mounted `read-only' and no changes are permitted.

 .PP

 There are two authentication databases,

 one for Plan 9 user information,

--- a/sys/src/cmd/auth/keyfs.c

+++ b/sys/src/cmd/auth/keyfs.c

@@ -92,6 +92,7 @@

 ulong	uniq = 1;

 Fcall	rhdr, thdr;

 int	usepass;

+int	readonly;

 char	*warnarg;

 uchar	mdata[8192 + IOHDRSZ];

 int	messagesize = sizeof mdata;

@@ -137,7 +138,7 @@

 static void

 usage(void)

 {

-	fprint(2, "usage: %s [-p] [-m mtpt] [-w warn] [keyfile]\n", argv0);

+	fprint(2, "usage: %s [-p] [-r] [-m mtpt] [-w warn] [keyfile]\n", argv0);

 	exits("usage");

 }

@@ -165,6 +166,9 @@

 	case 'w':

 		warnarg = EARGF(usage());

 		break;

+	case 'r':

+		readonly = 1;

+		break;

 	default:

 		usage();

 		break;

@@ -390,6 +394,8 @@

 	if(!f->busy)

 		return "create of unused fid";

+	if(readonly)

+		return "mounted read-only";

 	name = rhdr.name;

 	if(f->user != nil){

 		return "permission denied";

@@ -531,6 +537,8 @@

 	if(!f->busy)

 		return "permission denied";

+	if(readonly)

+		return "mounted read-only";

 	n = rhdr.count;

 	data = rhdr.data;

 	switch(f->qtype){

@@ -613,6 +621,10 @@

 {

 	if(!f->busy)

 		return "permission denied";

+	if(readonly){

+		Clunk(f);

+		return "mounted read-only";

+	}

 	if(f->qtype == Qwarnings)

 		f->user->warnings = 0;

 	else if(f->qtype == Quser)

@@ -649,6 +661,8 @@

 	if(!f->busy || f->qtype != Quser)

 		return "permission denied";

+	if(readonly)

+		return "mounted read-only";

 	if(rhdr.nstat > sizeof buf)

 		return "wstat buffer too big";

 	if(convM2D(rhdr.stat, rhdr.nstat, &d, buf) == 0)

@@ -711,6 +725,11 @@

 	User *u;

 	uchar *p, *buf;

 	ulong expire;

+

+	if(readonly){

+		fprint(2, "writeusers called while read-only; shouldn't happen\n");

+		return;

+	}

 	/* what format to use */

 	keydblen = KEYDBLEN;