seg6: fool-proof the processing of SRv6 behavior attributes

The set of required attributes for a given SRv6 behavior is identified
using a bitmap stored in an unsigned long, since the initial design of SRv6
networking in Linux. Recently the same approach has been used for
identifying the optional attributes.

However, the number of attributes supported by SRv6 behaviors depends on
the size of the unsigned long type which changes with the architecture.
Indeed, on a 64-bit architecture, an SRv6 behavior can support up to 64
attributes while on a 32-bit architecture it can support at most 32
attributes.

To fool-proof the processing of SRv6 behaviors we verify, at compile time,
that the set of all supported SRv6 attributes can be encoded into a bitmap
stored in an unsigned long. Otherwise, kernel build fails forcing
developers to reconsider adding a new attribute or extend the total
number of supported attributes by the SRv6 behaviors.

Moreover, we replace all patterns (1 << i) with the macro SEG6_F_ATTR(i) in
order to address potential overflow issues caused by 32-bit signed
arithmetic.

Thanks to Colin Ian King for catching the overflow problem, providing a
solution and inspiring this patch.
Thanks to Jakub Kicinski for his useful suggestions during the design of
this patch.

v2:
 - remove the SEG6_LOCAL_MAX_SUPP which is not strictly needed: it can
   be derived from the unsigned long type. Thanks to David Ahern for
   pointing it out.
Signed-off-by: NAndrea Mayer <andrea.mayer@uniroma2.it>
Reviewed-by: NDavid Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20210206170934.5982-1-andrea.mayer@uniroma2.itSigned-off-by: NJakub Kicinski <kuba@kernel.org>

net/ipv6/seg6_local.c

@@ -31,6 +31,8 @@
 #include <linux/etherdevice.h>
 #include <linux/bpf.h>
 
+#define SEG6_F_ATTR(i)		BIT(i)
+
 struct seg6_local_lwt;
 
 /* callbacks used for customizing the creation and destruction of a behavior */
@@ -660,8 +662,8 @@ seg6_end_dt_mode seg6_end_dt6_parse_mode(struct seg6_local_lwt *slwt)
 	unsigned long parsed_optattrs = slwt->parsed_optattrs;
 	bool legacy, vrfmode;
 
-	legacy	= !!(parsed_optattrs & (1 << SEG6_LOCAL_TABLE));
-	vrfmode	= !!(parsed_optattrs & (1 << SEG6_LOCAL_VRFTABLE));
+	legacy	= !!(parsed_optattrs & SEG6_F_ATTR(SEG6_LOCAL_TABLE));
+	vrfmode	= !!(parsed_optattrs & SEG6_F_ATTR(SEG6_LOCAL_VRFTABLE));
 
 	if (!(legacy ^ vrfmode))
 		/* both are absent or present: invalid DT6 mode */
@@ -883,32 +885,32 @@ static struct seg6_action_desc seg6_action_table[] = {
 	},
 	{
 		.action		= SEG6_LOCAL_ACTION_END_X,
-		.attrs		= (1 << SEG6_LOCAL_NH6),
+		.attrs		= SEG6_F_ATTR(SEG6_LOCAL_NH6),
 		.input		= input_action_end_x,
 	},
 	{
 		.action		= SEG6_LOCAL_ACTION_END_T,
-		.attrs		= (1 << SEG6_LOCAL_TABLE),
+		.attrs		= SEG6_F_ATTR(SEG6_LOCAL_TABLE),
 		.input		= input_action_end_t,
 	},
 	{
 		.action		= SEG6_LOCAL_ACTION_END_DX2,
-		.attrs		= (1 << SEG6_LOCAL_OIF),
+		.attrs		= SEG6_F_ATTR(SEG6_LOCAL_OIF),
 		.input		= input_action_end_dx2,
 	},
 	{
 		.action		= SEG6_LOCAL_ACTION_END_DX6,
-		.attrs		= (1 << SEG6_LOCAL_NH6),
+		.attrs		= SEG6_F_ATTR(SEG6_LOCAL_NH6),
 		.input		= input_action_end_dx6,
 	},
 	{
 		.action		= SEG6_LOCAL_ACTION_END_DX4,
-		.attrs		= (1 << SEG6_LOCAL_NH4),
+		.attrs		= SEG6_F_ATTR(SEG6_LOCAL_NH4),
 		.input		= input_action_end_dx4,
 	},
 	{
 		.action		= SEG6_LOCAL_ACTION_END_DT4,
-		.attrs		= (1 << SEG6_LOCAL_VRFTABLE),
+		.attrs		= SEG6_F_ATTR(SEG6_LOCAL_VRFTABLE),
 #ifdef CONFIG_NET_L3_MASTER_DEV
 		.input		= input_action_end_dt4,
 		.slwt_ops	= {
@@ -920,30 +922,30 @@ static struct seg6_action_desc seg6_action_table[] = {
 		.action		= SEG6_LOCAL_ACTION_END_DT6,
 #ifdef CONFIG_NET_L3_MASTER_DEV
 		.attrs		= 0,
-		.optattrs	= (1 << SEG6_LOCAL_TABLE) |
-				  (1 << SEG6_LOCAL_VRFTABLE),
+		.optattrs	= SEG6_F_ATTR(SEG6_LOCAL_TABLE) |
+				  SEG6_F_ATTR(SEG6_LOCAL_VRFTABLE),
 		.slwt_ops	= {
 					.build_state = seg6_end_dt6_build,
 				  },
 #else
-		.attrs		= (1 << SEG6_LOCAL_TABLE),
+		.attrs		= SEG6_F_ATTR(SEG6_LOCAL_TABLE),
 #endif
 		.input		= input_action_end_dt6,
 	},
 	{
 		.action		= SEG6_LOCAL_ACTION_END_B6,
-		.attrs		= (1 << SEG6_LOCAL_SRH),
+		.attrs		= SEG6_F_ATTR(SEG6_LOCAL_SRH),
 		.input		= input_action_end_b6,
 	},
 	{
 		.action		= SEG6_LOCAL_ACTION_END_B6_ENCAP,
-		.attrs		= (1 << SEG6_LOCAL_SRH),
+		.attrs		= SEG6_F_ATTR(SEG6_LOCAL_SRH),
 		.input		= input_action_end_b6_encap,
 		.static_headroom	= sizeof(struct ipv6hdr),
 	},
 	{
 		.action		= SEG6_LOCAL_ACTION_END_BPF,
-		.attrs		= (1 << SEG6_LOCAL_BPF),
+		.attrs		= SEG6_F_ATTR(SEG6_LOCAL_BPF),
 		.input		= input_action_end_bpf,
 	},
 
@@ -1366,7 +1368,7 @@ static void __destroy_attrs(unsigned long parsed_attrs, int max_parsed,
 	 * attribute; otherwise, we call the destroy() callback.
 	 */
 	for (i = 0; i < max_parsed; ++i) {
-		if (!(parsed_attrs & (1 << i)))
+		if (!(parsed_attrs & SEG6_F_ATTR(i)))
 			continue;
 
 		param = &seg6_action_params[i];
@@ -1395,7 +1397,7 @@ static int parse_nla_optional_attrs(struct nlattr **attrs,
 	int err, i;
 
 	for (i = 0; i < SEG6_LOCAL_MAX + 1; ++i) {
-		if (!(desc->optattrs & (1 << i)) || !attrs[i])
+		if (!(desc->optattrs & SEG6_F_ATTR(i)) || !attrs[i])
 			continue;
 
 		/* once here, the i-th attribute is provided by the
@@ -1408,7 +1410,7 @@ static int parse_nla_optional_attrs(struct nlattr **attrs,
 			goto parse_optattrs_err;
 
 		/* current attribute has been correctly parsed */
-		parsed_optattrs |= (1 << i);
+		parsed_optattrs |= SEG6_F_ATTR(i);
 	}
 
 	/* store in the tunnel state all the optional attributed successfully
@@ -1494,7 +1496,7 @@ static int parse_nla_action(struct nlattr **attrs, struct seg6_local_lwt *slwt)
 
 	/* parse the required attributes */
 	for (i = 0; i < SEG6_LOCAL_MAX + 1; i++) {
-		if (desc->attrs & (1 << i)) {
+		if (desc->attrs & SEG6_F_ATTR(i)) {
 			if (!attrs[i])
 				return -EINVAL;
 
@@ -1599,7 +1601,7 @@ static int seg6_local_fill_encap(struct sk_buff *skb,
 	attrs = slwt->desc->attrs | slwt->parsed_optattrs;
 
 	for (i = 0; i < SEG6_LOCAL_MAX + 1; i++) {
-		if (attrs & (1 << i)) {
+		if (attrs & SEG6_F_ATTR(i)) {
 			param = &seg6_action_params[i];
 			err = param->put(skb, slwt);
 			if (err < 0)
@@ -1620,30 +1622,30 @@ static int seg6_local_get_encap_size(struct lwtunnel_state *lwt)
 
 	attrs = slwt->desc->attrs | slwt->parsed_optattrs;
 
-	if (attrs & (1 << SEG6_LOCAL_SRH))
+	if (attrs & SEG6_F_ATTR(SEG6_LOCAL_SRH))
 		nlsize += nla_total_size((slwt->srh->hdrlen + 1) << 3);
 
-	if (attrs & (1 << SEG6_LOCAL_TABLE))
+	if (attrs & SEG6_F_ATTR(SEG6_LOCAL_TABLE))
 		nlsize += nla_total_size(4);
 
-	if (attrs & (1 << SEG6_LOCAL_NH4))
+	if (attrs & SEG6_F_ATTR(SEG6_LOCAL_NH4))
 		nlsize += nla_total_size(4);
 
-	if (attrs & (1 << SEG6_LOCAL_NH6))
+	if (attrs & SEG6_F_ATTR(SEG6_LOCAL_NH6))
 		nlsize += nla_total_size(16);
 
-	if (attrs & (1 << SEG6_LOCAL_IIF))
+	if (attrs & SEG6_F_ATTR(SEG6_LOCAL_IIF))
 		nlsize += nla_total_size(4);
 
-	if (attrs & (1 << SEG6_LOCAL_OIF))
+	if (attrs & SEG6_F_ATTR(SEG6_LOCAL_OIF))
 		nlsize += nla_total_size(4);
 
-	if (attrs & (1 << SEG6_LOCAL_BPF))
+	if (attrs & SEG6_F_ATTR(SEG6_LOCAL_BPF))
 		nlsize += nla_total_size(sizeof(struct nlattr)) +
 		       nla_total_size(MAX_PROG_NAME) +
 		       nla_total_size(4);
 
-	if (attrs & (1 << SEG6_LOCAL_VRFTABLE))
+	if (attrs & SEG6_F_ATTR(SEG6_LOCAL_VRFTABLE))
 		nlsize += nla_total_size(4);
 
 	return nlsize;
@@ -1670,7 +1672,7 @@ static int seg6_local_cmp_encap(struct lwtunnel_state *a,
 		return 1;
 
 	for (i = 0; i < SEG6_LOCAL_MAX + 1; i++) {
-		if (attrs_a & (1 << i)) {
+		if (attrs_a & SEG6_F_ATTR(i)) {
 			param = &seg6_action_params[i];
 			if (param->cmp(slwt_a, slwt_b))
 				return 1;
@@ -1692,6 +1694,15 @@ static const struct lwtunnel_encap_ops seg6_local_ops = {
 
 int __init seg6_local_init(void)
 {
+	/* If the max total number of defined attributes is reached, then your
+	 * kernel build stops here.
+	 *
+	 * This check is required to avoid arithmetic overflows when processing
+	 * behavior attributes and the maximum number of defined attributes
+	 * exceeds the allowed value.
+	 */
+	BUILD_BUG_ON(SEG6_LOCAL_MAX + 1 > BITS_PER_TYPE(unsigned long));
+
 	return lwtunnel_encap_add_ops(&seg6_local_ops,
 				      LWTUNNEL_ENCAP_SEG6_LOCAL);
 }