log: Enable format warnings

logmsg() takes printf like arguments, but because it's not a built in, the
compiler won't generate warnings if the format string and parameters don't
match.  Enable those by using the format attribute.

Strictly speaking this is a gcc extension, but I believe it is also
supported by some other common compilers.  We already use some other
attributes in various places.  For now, just use it and we can worry about
compilers that don't support it if it comes up.

This exposes some warnings from existing callers, both in gcc and in
clang-tidy:
 - Some are straight out bugs, which we correct
 - It's occasionally useful to invoke the logging functions with an empty
   string, which gcc objects to, so disable that specific warning in the
   Makefile
 - Strictly speaking the C standard requires that the parameter for a %p
   be a (void *), not some other pointer type.  That's only likely to cause
   problems in practice on weird architectures with different sized
   representations for pointers to different types.  Nonetheless add the
   casts to make it happy.

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

Makefile

@@ -33,7 +33,8 @@ AUDIT_ARCH := $(shell echo $(AUDIT_ARCH) | sed 's/MIPS64EL/MIPSEL64/')
 AUDIT_ARCH := $(shell echo $(AUDIT_ARCH) | sed 's/HPPA/PARISC/')
 AUDIT_ARCH := $(shell echo $(AUDIT_ARCH) | sed 's/SH4/SH/')
 
-FLAGS := -Wall -Wextra -pedantic -std=c11 -D_XOPEN_SOURCE=700 -D_GNU_SOURCE
+FLAGS := -Wall -Wextra -Wno-format-zero-length
+FLAGS += -pedantic -std=c11 -D_XOPEN_SOURCE=700 -D_GNU_SOURCE
 FLAGS += -D_FORTIFY_SOURCE=2 -O2 -pie -fPIE
 FLAGS += -DPAGE_SIZE=$(shell getconf PAGE_SIZE)
 FLAGS += -DNETNS_RUN_DIR=\"/run/netns\"

conf.c

@@ -493,7 +493,7 @@ static void conf_netns_opt(char *netns, const char *arg)
 	}
 
 	if (ret <= 0 || ret > PATH_MAX)
-		die("Network namespace name/path %s too long");
+		die("Network namespace name/path %s too long", arg);
 }
 
 /**

log.h

@@ -12,7 +12,8 @@
 #define LOGFILE_CUT_RATIO		30	/* When full, cut ~30% size */
 #define LOGFILE_SIZE_MIN		(5UL * MAX(BUFSIZ, PAGE_SIZE))
 
-void logmsg(int pri, const char *format, ...);
+void logmsg(int pri, const char *format, ...)
+	__attribute__((format(printf, 2, 3)));
 
 #define err(...)	logmsg(LOG_ERR, __VA_ARGS__)
 #define warn(...)	logmsg(LOG_WARNING, __VA_ARGS__)

packet.c

@@ -43,13 +43,14 @@ void packet_add_do(struct pool *p, size_t len, const char *start,
 
 	if (start < p->buf) {
 		trace("add packet start %p before buffer start %p, %s:%i",
-		      start, p->buf, func, line);
+		      (void *)start, (void *)p->buf, func, line);
 		return;
 	}
 
 	if (start + len > p->buf + p->buf_size) {
 		trace("add packet start %p, length: %lu, buffer end %p, %s:%i",
-		      start, len, p->buf + p->buf_size, func, line);
+		      (void *)start, len, (void *)(p->buf + p->buf_size),
+		      func, line);
 		return;
 	}
 
@@ -61,7 +62,7 @@ void packet_add_do(struct pool *p, size_t len, const char *start,
 #if UINTPTR_MAX == UINT64_MAX
 	if ((uintptr_t)start - (uintptr_t)p->buf > UINT32_MAX) {
 		trace("add packet start %p, buffer start %p, %s:%i",
-		      start, p->buf, func, line);
+		      (void *)start, (void *)p->buf, func, line);
 		return;
 	}
 #endif

pasta.c

@@ -239,7 +239,7 @@ void pasta_start_ns(struct ctx *c, uid_t uid, gid_t gid,
 
 		if ((size_t)snprintf(sh_arg0, sizeof(sh_arg0),
 				     "-%s", arg.exe) >= sizeof(sh_arg0))
-			die("$SHELL is too long (%u bytes)", strlen(arg.exe));
+			die("$SHELL is too long (%zu bytes)", strlen(arg.exe));
 
 		sh_argv[0] = sh_arg0;
 		arg.argv = sh_argv;

tcp.c

@@ -1212,7 +1212,8 @@ static void tcp_hash_insert(const struct ctx *c, struct tcp_tap_conn *conn)
 	tc_hash[b] = conn;
 
 	debug("TCP: hash table insert: index %li, sock %i, bucket: %i, next: "
-	      "%p", CONN_IDX(conn), conn->sock, b, conn_at_idx(conn->next_index));
+	      "%p", CONN_IDX(conn), conn->sock, b,
+	      (void *)conn_at_idx(conn->next_index));
 }
 
 /**
@@ -1239,7 +1240,7 @@ static void tcp_hash_remove(const struct ctx *c,
 
 	debug("TCP: hash table remove: index %li, sock %i, bucket: %i, new: %p",
 	      CONN_IDX(conn), conn->sock, b,
-	      prev ? conn_at_idx(prev->next_index) : tc_hash[b]);
+	      (void *)(prev ? conn_at_idx(prev->next_index) : tc_hash[b]));
 }
 
 /**
@@ -1267,7 +1268,8 @@ static void tcp_tap_conn_update(const struct ctx *c, struct tcp_tap_conn *old,
 
 	debug("TCP: hash table update: old index %li, new index %li, sock %i, "
 	      "bucket: %i, old: %p, new: %p",
-	      CONN_IDX(old), CONN_IDX(new), new->sock, b, old, new);
+	      CONN_IDX(old), CONN_IDX(new), new->sock, b,
+	      (void *)old, (void *)new);
 
 	tcp_epoll_ctl(c, new);
 }
@@ -1311,7 +1313,7 @@ void tcp_table_compact(struct ctx *c, union tcp_conn *hole)
 
 	if (CONN_IDX(hole) == --c->tcp.conn_count) {
 		debug("TCP: table compaction: maximum index was %li (%p)",
-		      CONN_IDX(hole), hole);
+		      CONN_IDX(hole), (void *)hole);
 		memset(hole, 0, sizeof(*hole));
 		return;
 	}
@@ -1326,7 +1328,8 @@ void tcp_table_compact(struct ctx *c, union tcp_conn *hole)
 
 	debug("TCP: table compaction (spliced=%d): old index %li, new index %li, "
 	      "from: %p, to: %p",
-	      from->c.spliced, CONN_IDX(from), CONN_IDX(hole), from, hole);
+	      from->c.spliced, CONN_IDX(from), CONN_IDX(hole),
+	      (void *)from, (void *)hole);
 
 	memset(from, 0, sizeof(*from));
 }