[klibc] [PATCH] nfsmount: defeat dummypmap compiler warning

[Greg Thelen]

maximilian attems <max at stro.at> writes:
> On Wed, 11 Jan 2012, Greg Thelen wrote:
>
>> Without this patch gcc 4.6 warns:
>> KLIBCCC usr/kinit/nfsmount/dummypmap.o
>> usr/kinit/nfsmount/dummypmap.c: In function 'dummy_portmap':
>> usr/kinit/nfsmount/dummypmap.c:191:13: warning: array subscript is below array bounds [-Warray-bounds]
>> 
>> Signed-off-by: Greg Thelen <gthelen at google.com>
>> ---
>>  usr/kinit/nfsmount/dummypmap.c |    2 +-
>>  1 files changed, 1 insertions(+), 1 deletions(-)
>> 
>> diff --git a/usr/kinit/nfsmount/dummypmap.c b/usr/kinit/nfsmount/dummypmap.c
>> index 7c5b6bc..7c4da35 100644
>> --- a/usr/kinit/nfsmount/dummypmap.c
>> +++ b/usr/kinit/nfsmount/dummypmap.c
>> @@ -150,7 +150,7 @@ static int dummy_portmap(int sock, FILE *portmap_file)
>>  	int pktlen, addrlen;
>>  	unsigned char pkt[65536];	/* Max UDP packet size */
>>  	/* RPC UDP packets do not include TCP fragment size */
>> -	struct rpc_call *rpc = (struct rpc_call *) &pkt[-4];
>> +	struct rpc_call *rpc = (struct rpc_call *)((uintptr_t)pkt - 4);
>>  	struct rpc_auth *cred;
>>  	struct rpc_auth *vrf;
>>  	struct portmap_args *args;
>
> hmm suspect this looks like papering, please
> hpa has the following suggestion:
> "I would actually be happier making pkt a union/structure with a (dummy)
>  fragment size header
>  Wasting 4 bytes on the stack is not an issue here
>  I'd actually suggest we do:
>  union {
>    struct rpc_call rpc;
>    char pkt[65536+4];
>  } pkt;
>  And just load the packet at &rpc.hdr.udp" 
>
>
> thanks.

Is this what you are suggesting?

>From 87fae6051aa4b30643c964eb83f7d54a258f5a68 Mon Sep 17 00:00:00 2001
From: Greg Thelen <gthelen at google.com>
Date: Wed, 4 Jan 2012 17:02:09 -0800
Subject: [PATCH] [klibc] nfsmount: defeat dummypmap compiler warning

Without this patch gcc 4.6 warns:
KLIBCCC usr/kinit/nfsmount/dummypmap.o
usr/kinit/nfsmount/dummypmap.c: In function 'dummy_portmap':
usr/kinit/nfsmount/dummypmap.c:191:13: warning: array subscript is below array bounds [-Warray-bounds]

Signed-off-by: Greg Thelen <gthelen at google.com>
---
 usr/kinit/nfsmount/dummypmap.c |   20 +++++++++++++-------
 1 files changed, 13 insertions(+), 7 deletions(-)

diff --git a/usr/kinit/nfsmount/dummypmap.c b/usr/kinit/nfsmount/dummypmap.c
index 7c5b6bc..2cc889a 100644
--- a/usr/kinit/nfsmount/dummypmap.c
+++ b/usr/kinit/nfsmount/dummypmap.c
@@ -148,9 +148,13 @@ static int dummy_portmap(int sock, FILE *portmap_file)
 {
 	struct sockaddr_in sin;
 	int pktlen, addrlen;
-	unsigned char pkt[65536];	/* Max UDP packet size */
-	/* RPC UDP packets do not include TCP fragment size */
-	struct rpc_call *rpc = (struct rpc_call *) &pkt[-4];
+	enum { max_packet_sz = 65536 };
+	union {
+		struct rpc_call rpc;
+		/* Max UDP packet size + unused TCP fragment size */
+		char payload[max_packet_sz + offsetof(struct rpc_header, udp)];
+	} __attribute__ ((__packed__)) pkt;
+	struct rpc_call *rpc = &pkt.rpc;
 	struct rpc_auth *cred;
 	struct rpc_auth *vrf;
 	struct portmap_args *args;
@@ -158,8 +162,8 @@ static int dummy_portmap(int sock, FILE *portmap_file)
 
 	for (;;) {
 		addrlen = sizeof sin;
-		pktlen = recvfrom(sock, &pkt, sizeof pkt, 0,
-				  (struct sockaddr *)&sin, &addrlen);
+		pktlen = recvfrom(sock, &rpc->hdr.udp, max_packet_sz,
+				  0, (struct sockaddr *)&sin, &addrlen);
 
 		if (pktlen < 0) {
 			if (errno == EINTR)
@@ -189,9 +193,11 @@ static int dummy_portmap(int sock, FILE *portmap_file)
 		} else if (rpc->prog_vers != htonl(2)) {
 			rply.rpc.reply_state = htonl(PROG_MISMATCH);
 		} else if (!(vrf = get_auth(cred)) ||
-			   (char *)vrf > (char *)pkt + pktlen - 8 - sizeof(*args) ||
+			   (char *)vrf > ((char *)&rpc->hdr.udp + pktlen - 8 -
+					  sizeof(*args)) ||
 			   !(args = get_auth(vrf)) ||
-			   (char *)args > (char *)pkt + pktlen - sizeof(*args) ||
+			   (char *)args > ((char *)&rpc->hdr.udp + pktlen -
+					   sizeof(*args)) ||
 			   check_cred(cred) || check_vrf(vrf)) {
 			/* Can't deal with credentials data; the kernel
 			   won't send them */
-- 
1.7.7.3