u-a: Fix short-lived memory leaks

These interfaces were bad, as requiring to pass pre-allocated strings,
means we cannot sanely recover and the call sites do not know when the
function took ownership of the pointers or not, and as such subsequent
calls might or might not be able to reuse the pointers or free them.

Reported-by: Helmut Grohne <helmut@subdivi.de>

debian/changelog

@@ -30,6 +30,8 @@ dpkg (1.18.11) UNRELEASED; urgency=medium
     output substvar prefixed with “S:”.
   * Make dpkg-source generate reproducible source packages when run
     standalone, by honoring SOURCE_DATE_EPOCH.
+  * Fix several short-lived memory leaks in update-alternatives.
+    Reported by Helmut Grohne <helmut@subdivi.de>.
   * Portability:
     - Cast off_t variables to intmax_t when printing them with "%jd".
     - Add missing <string.h> include in libdpkg.

utils/update-alternatives.c

@@ -513,9 +513,8 @@ fileset_free(struct fileset *fs)
 	free(fs);
 }
 
-/* name and file must be allocated with malloc */
 static void
-fileset_add_slave(struct fileset *fs, char *name, char *file)
+fileset_add_slave(struct fileset *fs, const char *name, const char *file)
 {
 	struct slave_file *sl, *cur, *prev = NULL;
 
@@ -532,8 +531,8 @@ fileset_add_slave(struct fileset *fs, char *name, char *file)
 	/* Otherwise add new at the end */
 	sl = xmalloc(sizeof(*sl));
 	sl->next = NULL;
-	sl->name = name;
-	sl->file = file;
+	sl->name = xstrdup(name);
+	sl->file = xstrdup(file);
 	if (prev)
 		prev->next = sl;
 	else
@@ -904,10 +903,9 @@ alternative_add_choice(struct alternative *a, struct fileset *fs)
 	a->modified = true;
 }
 
-/* slave_name and slave_link must be allocated with malloc */
 static struct slave_link *
-alternative_add_slave(struct alternative *a, char *slave_name,
-                      char *slave_link)
+alternative_add_slave(struct alternative *a,
+                      const char *slave_name, const char *slave_link)
 {
 	struct slave_link *sl, *new;
 
@@ -924,8 +922,8 @@ alternative_add_slave(struct alternative *a, char *slave_name,
 
 	/* Otherwise create new and add at the end */
 	new = xmalloc(sizeof(*new));
-	new->name = slave_name;
-	new->link = slave_link;
+	new->name = xstrdup(slave_name);
+	new->link = xstrdup(slave_link);
 	new->updated = false;
 	new->next = NULL;
 	if (sl)
@@ -941,7 +939,7 @@ alternative_copy_slave(struct alternative *a, struct slave_link *sl)
 {
 	struct slave_link *sl_new;
 
-	sl_new = alternative_add_slave(a, xstrdup(sl->name), xstrdup(sl->link));
+	sl_new = alternative_add_slave(a, sl->name, sl->link);
 	sl_new->updated = sl->updated;
 }
 
@@ -970,15 +968,14 @@ alternative_set_status(struct alternative *a, enum alternative_status status)
 	a->status = status;
 }
 
-/* link must be allocated with malloc */
 static void
-alternative_set_link(struct alternative *a, char *linkname)
+alternative_set_link(struct alternative *a, const char *linkname)
 {
 	if (a->master_link == NULL || strcmp(linkname, a->master_link) != 0)
 		a->modified = true;
 
 	free(a->master_link);
-	a->master_link = linkname;
+	a->master_link = xstrdup(linkname);
 }
 
 static bool
@@ -1158,6 +1155,8 @@ alternative_parse_slave(struct alternative *a, struct altdb_context *ctx)
 	}
 
 	alternative_add_slave(a, name, linkname);
+	free(linkname);
+	free(name);
 
 	return true;
 }
@@ -1217,8 +1216,9 @@ alternative_parse_fileset(struct alternative *a, struct altdb_context *ctx)
 
 		fs = fileset_new(master_file, prio);
 		for (sl = a->slaves; sl; sl = sl->next) {
-			fileset_add_slave(fs, xstrdup(sl->name),
-			                  altdb_get_line(ctx, _("slave file")));
+			char *slave_file = altdb_get_line(ctx, _("slave file"));
+			fileset_add_slave(fs, sl->name, slave_file);
+			free(slave_file);
 		}
 		alternative_add_choice(a, fs);
 	}
@@ -1233,6 +1233,7 @@ alternative_load(struct alternative *a, enum altdb_flags flags)
 	struct altdb_context ctx;
 	struct stat st;
 	char *status;
+	char *master_link;
 
 	/* Initialize parse context */
 	if (setjmp(ctx.on_error)) {
@@ -1274,7 +1275,9 @@ alternative_load(struct alternative *a, enum altdb_flags flags)
 	                       ALT_ST_AUTO : ALT_ST_MANUAL);
 	free(status);
 
-	alternative_set_link(a, altdb_get_line(&ctx, _("master link")));
+	master_link = altdb_get_line(&ctx, _("master link"));
+	alternative_set_link(a, master_link);
+	free(master_link);
 
 	/* Parse the description of the slaves links of the alternative */
 	while (alternative_parse_slave(a, &ctx));
@@ -2208,7 +2211,7 @@ alternative_evolve(struct alternative *a, struct alternative *b,
 		     a->master_link, b->master_link);
 		checked_mv(a->master_link, b->master_link);
 	}
-	alternative_set_link(a, xstrdup(b->master_link));
+	alternative_set_link(a, b->master_link);
 
 	/* Check if new slaves have been added, or existing
 	 * ones renamed. */
@@ -2621,7 +2624,7 @@ main(int argc, char **argv)
 			a = alternative_new(argv[i + 2]);
 			inst_alt = alternative_new(argv[i + 2]);
 			alternative_set_status(inst_alt, ALT_ST_AUTO);
-			alternative_set_link(inst_alt, xstrdup(argv[i + 1]));
+			alternative_set_link(inst_alt, argv[i + 1]);
 			fileset = fileset_new(argv[i + 3], prio);
 
 			i += 4;
@@ -2657,7 +2660,7 @@ main(int argc, char **argv)
 			   strcmp("--set-selections", argv[i]) == 0) {
 			set_action(argv[i] + 2);
 		} else if (strcmp("--slave", argv[i]) == 0) {
-			char *slink, *sname, *spath;
+			const char *slink, *sname, *spath;
 			struct slave_link *sl;
 
 			if (action == NULL ||
@@ -2666,9 +2669,9 @@ main(int argc, char **argv)
 			if (MISSING_ARGS(3))
 				badusage(_("--slave needs <link> <name> <path>"));
 
-			slink = xstrdup(argv[i + 1]);
-			sname = xstrdup(argv[i + 2]);
-			spath = xstrdup(argv[i + 3]);
+			slink = argv[i + 1];
+			sname = argv[i + 2];
+			spath = argv[i + 3];
 
 			if (strcmp(slink, spath) == 0)
 				badusage(_("<link> and <path> can't be the same"));
@@ -2691,7 +2694,7 @@ main(int argc, char **argv)
 			}
 
 			alternative_add_slave(inst_alt, sname, slink);
-			fileset_add_slave(fileset, xstrdup(sname), spath);
+			fileset_add_slave(fileset, sname, spath);
 
 			i+= 3;
 		} else if (strcmp("--log", argv[i]) == 0) {