parse: Start splitting out parsing helpers

As we add more complexity to what forwarding rules are allowed, our
existing ad-hoc C parsing is starting to become quite awkward.  We already
have some parts that resemble a very simple recursive[0] descent parser,
with composable subfunctions that consume as much input as they need,
rather than pre-splitting based on known delimiters.

Start extending this approach, by creating parse.[ch] with parsing helpers
with a uniform interface.  Initially we start with very simple cases: the
parse_keyword() function from fwd_rule.c and another helper to check that
we've reached the end of input.

Rename parse_keyword() to parse_literal(), because it's not just useful
for "keywords" as such.  We can use it in a bunch of additional places for
parsing delimiters and other symbols.  This doesn't gain us a lot now but
will make things clearer as we use more such parser helpers.

[0] Except that the grammars we have aren't actually recursive, so neither
    is the code.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

5 files changed, 102 insertions, 35 deletions

diff --git a/Makefile b/Makefile
index 5ed0f70..bd729c7 100644
--- a/Makefile
+++ b/Makefile
@@ -36,12 +36,13 @@ BASE_CFLAGS += -pedantic -Wall -Wextra -Wno-format-zero-length -Wformat-security
 PASST_SRCS = arch.c arp.c bitmap.c checksum.c conf.c dhcp.c dhcpv6.c \
 	epoll_ctl.c flow.c fwd.c fwd_rule.c icmp.c igmp.c inany.c iov.c ip.c \
 	isolation.c lineread.c log.c mld.c ndp.c netlink.c migrate.c packet.c \
-	passt.c pasta.c pcap.c pif.c repair.c serialise.c tap.c tcp.c \
+	parse.c passt.c pasta.c pcap.c pif.c repair.c serialise.c tap.c tcp.c \
 	tcp_buf.c tcp_splice.c tcp_vu.c udp.c udp_flow.c udp_vu.c util.c \
 	vhost_user.c virtio.c vu_common.c
 QRAP_SRCS = qrap.c
 PASST_REPAIR_SRCS = passt-repair.c
-PESTO_SRCS = pesto.c bitmap.c fwd_rule.c inany.c ip.c lineread.c serialise.c
+PESTO_SRCS = pesto.c bitmap.c fwd_rule.c inany.c ip.c lineread.c parse.c \
+	serialise.c
 SRCS = $(PASST_SRCS) $(QRAP_SRCS) $(PASST_REPAIR_SRCS) $(PESTO_SRCS)
 
 MANPAGES = passt.1 pasta.1 pesto.1 qrap.1 passt-repair.1
@@ -49,13 +50,14 @@ MANPAGES = passt.1 pasta.1 pesto.1 qrap.1 passt-repair.1
 PASST_HEADERS = arch.h arp.h bitmap.h checksum.h conf.h dhcp.h dhcpv6.h \
 	epoll_ctl.h flow.h fwd.h fwd_rule.h flow_table.h icmp.h icmp_flow.h \
 	inany.h iov.h ip.h isolation.h lineread.h log.h migrate.h ndp.h \
-	netlink.h packet.h passt.h pasta.h pcap.h pif.h repair.h serialise.h \
-	siphash.h tap.h tcp.h tcp_buf.h tcp_conn.h tcp_internal.h tcp_splice.h \
-	tcp_vu.h udp.h udp_flow.h udp_internal.h udp_vu.h util.h vhost_user.h \
-	virtio.h vu_common.h
+	netlink.h packet.h parse.h passt.h pasta.h pcap.h pif.h repair.h \
+	serialise.h siphash.h tap.h tcp.h tcp_buf.h tcp_conn.h tcp_internal.h \
+	tcp_splice.h tcp_vu.h udp.h udp_flow.h udp_internal.h udp_vu.h util.h \
+	vhost_user.h virtio.h vu_common.h
 QRAP_HEADERS = arp.h ip.h passt.h util.h
 PASST_REPAIR_HEADERS = linux_dep.h
-PESTO_HEADERS = bitmap.h common.h fwd_rule.h inany.h ip.h log.h pesto.h serialise.h
+PESTO_HEADERS = bitmap.h common.h fwd_rule.h inany.h ip.h log.h parse.h \
+	pesto.h serialise.h
 
 C := \#include <sys/random.h>\nint main(){int a=getrandom(0, 0, 0);}
 ifeq ($(shell printf "$(C)" | $(CC) -S -xc - -o - >/dev/null 2>&1; echo $$?),0)
@@ -223,6 +225,7 @@ pesto.cppcheck: CPPCHECK_FLAGS += --suppress=unusedFunction:bitmap.c
 pesto.cppcheck: CPPCHECK_FLAGS += --suppress=unusedFunction:inany.h
 pesto.cppcheck: CPPCHECK_FLAGS += --suppress=unusedFunction:inany.c
 pesto.cppcheck: CPPCHECK_FLAGS += --suppress=unusedFunction:ip.h
+pesto.cppcheck: CPPCHECK_FLAGS += --suppress=unusedFunction:parse.c
 pesto.cppcheck: CPPCHECK_FLAGS += --suppress=unusedFunction:serialise.c
 pesto.cppcheck: CPPCHECK_FLAGS += --suppress=staticFunction:fwd_rule.c
 pesto.cppcheck: $(PESTO_SRCS) $(PESTO_HEADERS) seccomp_pesto.h
diff --git a/conf.c b/conf.c
index 6ab8efe..ca6c859 100644
--- a/conf.c
+++ b/conf.c
@@ -52,6 +52,7 @@
 #include "conf.h"
 #include "pesto.h"
 #include "serialise.h"
+#include "parse.h"
 
 #define NETNS_RUN_DIR	"/run/netns"
 
@@ -1028,7 +1029,9 @@ static void conf_ugid(char *runas, uid_t *uid, gid_t *gid)
 static void conf_nat(const char *arg, struct in_addr *addr4,
 		     struct in6_addr *addr6, int *no_map_gw)
 {
-	if (strcmp(arg, "none") == 0) {
+	const char *p = arg;
+
+	if (parse_literal(&p, "none") && parse_eoi(p)) {
 		*addr4 = in4addr_any;
 		*addr6 = in6addr_any;
 		if (no_map_gw)
diff --git a/fwd_rule.c b/fwd_rule.c
index 04a0101..d0e9040 100644
--- a/fwd_rule.c
+++ b/fwd_rule.c
@@ -24,6 +24,7 @@
 #include "fwd_rule.h"
 #include "lineread.h"
 #include "log.h"
+#include "parse.h"
 #include "serialise.h"
 
 /* Ephemeral port range: values from RFC 6335 */
@@ -428,28 +429,6 @@ static int parse_port_range(const char *s, const char **endptr,
 }
 
 /**
- * parse_keyword() - Parse a literal keyword
- * @s:		String to parse
- * @endptr:	Update to the character after the keyword
- * @kw:		Keyword to accept
- *
- * Return: 0, if @s starts with @kw, -EINVAL if it does not
- */
-static int parse_keyword(const char *s, const char **endptr, const char *kw)
-{
-	size_t len = strlen(kw);
-
-	if (strlen(s) < len)
-		return -EINVAL;
-
-	if (memcmp(s, kw, len))
-		return -EINVAL;
-
-	*endptr = s + len;
-	return 0;
-}
-
-/**
  * fwd_rule_range_except() - Set up forwarding for a range of ports minus a
  *                           bitmap of exclusions
  * @fwd:	Forwarding table to be updated
@@ -568,7 +547,7 @@ static void fwd_rule_parse_ports(struct fwd_table *fwd, bool del, uint8_t proto,
 			continue;
 		}
 
-		if (parse_keyword(p, &p, "auto") == 0) {
+		if (parse_literal(&p, "auto")) {
 			if (p != ep) /* Garbage after the keyword */
 				goto bad;
 
@@ -582,9 +561,8 @@ static void fwd_rule_parse_ports(struct fwd_table *fwd, bool del, uint8_t proto,
 		}
 
 		/* Should be an exclude range */
-		if (*p != '~')
+		if (!parse_literal(&p, "~"))
 			goto bad;
-		p++;
 
 		if (parse_port_range(p, &p, &xrange))
 			goto bad;
@@ -616,8 +594,9 @@ static void fwd_rule_parse_ports(struct fwd_table *fwd, bool del, uint8_t proto,
 		if (parse_port_range(p, &p, &orig_range))
 			goto bad;
 
-		if (*p == ':') { /* There's a range to map to as well */
-			if (parse_port_range(p + 1, &p, &mapped_range))
+		if (parse_literal(&p, ":")) {
+			/* There's a range to map to as well */
+			if (parse_port_range(p, &p, &mapped_range))
 				goto bad;
 			if ((mapped_range.last - mapped_range.first) !=
 			    (orig_range.last - orig_range.first))
diff --git a/parse.c b/parse.c
new file mode 100644
index 0000000..4e3b527
--- /dev/null
+++ b/parse.c
@@ -0,0 +1,68 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+/* PASST - Plug A Simple Socket Transport
+ *  for qemu/UNIX domain socket mode
+ *
+ * PASTA - Pack A Subtle Tap Abstraction
+ *  for network namespace/tap device mode
+ *
+ * parse.c - Composable parsing helpers
+ *
+ * Copyright Red Hat
+ * Author: David Gibson <david@gibson.dropbear.id.au>
+ */
+
+#include <assert.h>
+#include <stddef.h>
+#include <fcntl.h>
+#include <string.h>
+#include <stdbool.h>
+#include <unistd.h>
+
+#include "common.h"
+
+/**
+ * DOC: Theory of Operation
+ *
+ * These are a number of primitives which can be combined into parsers for
+ * moderately complex input.  For simpler composability, they have common
+ * conventions.
+ *
+ * - Functions return a bool indicating whether they successfully parsed.
+ * - Any specific output from the parse is as output parameters
+ * - First argument is always @cursor, a const char **.
+ * - On entry, *@cursor has the point to start parsing.
+ * - On successful exit, *@cursor is updated to the next character after the
+ *    parsed portion of the input
+ * - On failure, *@cursor and any output arguments are not modified
+ *
+ * For brevity the common parameters and return information are omitted from the
+ * individual function documentation comments.
+ */
+
+/**
+ * parse_literal() - Parse a specified literal string
+ * @cursor:	Point to parse from, updated on success
+ * @lit:	Keyword to accept
+ */
+bool parse_literal(const char **cursor, const char *lit)
+{
+	size_t len = strlen(lit);
+
+	if (strlen(*cursor) < len || memcmp(*cursor, lit, len))
+		return false;
+
+	*cursor += len;
+	return true;
+}
+
+/**
+ * parse_eoi() - Parse end of input
+ * @cursor:	Point to parse from
+ *
+ * Return: true if @p is at End of Input (\0), false otherwise
+ */
+bool parse_eoi(const char *cursor)
+{
+	return !(*cursor);
+}
diff --git a/parse.h b/parse.h
new file mode 100644
index 0000000..eb51a46
--- /dev/null
+++ b/parse.h
@@ -0,0 +1,14 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later
+ * Copyright Red Hat
+ * Author: David Gibson <david@gibson.dropbear.id.au>
+ */
+
+#ifndef PARSE_H
+#define PARSE_H
+
+#include <stdbool.h>
+
+bool parse_literal(const char **cursor, const char *lit);
+bool parse_eoi(const char *cursor);
+
+#endif /* _PARSE_H */