use HOST_NAME_MAX to size buffers for gethostname(2)

POSIX limits the length of host names to HOST_NAME_MAX.  Export the
fallback definition from daemon.c and use this constant to make all
buffers used with gethostname(2) big enough for any possible result
and a terminating NUL.

Inspired-by: David Turner <dturner@twosigma.com>
Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: David Turner <dturner@twosigma.com>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: rscharfe

builtin/gc.c

@@ -220,7 +220,7 @@ static int need_to_gc(void)
 static const char *lock_repo_for_gc(int force, pid_t* ret_pid)
 {
 	static struct lock_file lock;
-	char my_host[128];
+	char my_host[HOST_NAME_MAX + 1];
 	struct strbuf sb = STRBUF_INIT;
 	struct stat st;
 	uintmax_t pid;
@@ -239,8 +239,12 @@ static const char *lock_repo_for_gc(int force, pid_t* ret_pid)
 	fd = hold_lock_file_for_update(&lock, pidfile_path,
 				       LOCK_DIE_ON_ERROR);
 	if (!force) {
-		static char locking_host[128];
+		static char locking_host[HOST_NAME_MAX + 1];
+		static char *scan_fmt;
 		int should_exit;
+
+		if (!scan_fmt)
+			scan_fmt = xstrfmt("%s %%%dc", "%"SCNuMAX, HOST_NAME_MAX);
 		fp = fopen(pidfile_path, "r");
 		memset(locking_host, 0, sizeof(locking_host));
 		should_exit =
@@ -256,7 +260,7 @@ static const char *lock_repo_for_gc(int force, pid_t* ret_pid)
 			 * running.
 			 */
 			time(NULL) - st.st_mtime <= 12 * 3600 &&
-			fscanf(fp, "%"SCNuMAX" %127c", &pid, locking_host) == 2 &&
+			fscanf(fp, scan_fmt, &pid, locking_host) == 2 &&
 			/* be gentle to concurrent "gc" on remote hosts */
 			(strcmp(locking_host, my_host) || !kill(pid, 0) || errno == EPERM);
 		if (fp != NULL)

builtin/receive-pack.c

@@ -1655,7 +1655,7 @@ static const char *unpack(int err_fd, struct shallow_info *si)
 		if (status)
 			return "unpack-objects abnormal exit";
 	} else {
-		char hostname[256];
+		char hostname[HOST_NAME_MAX + 1];
 
 		argv_array_pushl(&child.args, "index-pack",
 				 "--stdin", hdr_arg, NULL);

daemon.c

@@ -4,10 +4,6 @@
 #include "strbuf.h"
 #include "string-list.h"
 
-#ifndef HOST_NAME_MAX
-#define HOST_NAME_MAX 256
-#endif
-
 #ifdef NO_INITGROUPS
 #define initgroups(x, y) (0) /* nothing */
 #endif

fetch-pack.c

@@ -745,7 +745,7 @@ static int get_pack(struct fetch_pack_args *args,
 		if (args->use_thin_pack)
 			argv_array_push(&cmd.args, "--fix-thin");
 		if (args->lock_pack || unpack_limit) {
-			char hostname[256];
+			char hostname[HOST_NAME_MAX + 1];
 			if (gethostname(hostname, sizeof(hostname)))
 				xsnprintf(hostname, sizeof(hostname), "localhost");
 			argv_array_pushf(&cmd.args,

git-compat-util.h

@@ -878,6 +878,10 @@ static inline size_t xsize_t(off_t len)
 __attribute__((format (printf, 3, 4)))
 extern int xsnprintf(char *dst, size_t max, const char *fmt, ...);
 
+#ifndef HOST_NAME_MAX
+#define HOST_NAME_MAX 256
+#endif
+
 /* in ctype.c, for kwset users */
 extern const unsigned char tolower_trans_tbl[256];
 

ident.c

@@ -120,7 +120,7 @@ static int canonical_name(const char *host, struct strbuf *out)
 
 static void add_domainname(struct strbuf *out, int *is_bogus)
 {
-	char buf[1024];
+	char buf[HOST_NAME_MAX + 1];
 
 	if (gethostname(buf, sizeof(buf))) {
 		warning_errno("cannot get host name");