Introduce free_and_strndup and use it in bus-message.c

v2: fix error in free_and_strndup()

When the orignal and copied message were the same, but shorter than specified
length l, memory read past the end of the buffer would be performed. A test
case is included: a string that had an embedded NUL ("q\0") is used to replace
"q".

v3: Fix one more bug in free_and_strndup and add tests.

v4: Some style fixed based on review, one more use of free_and_replace, and
make the tests more comprehensive.

(cherry picked from commit 7f546026abbdc56c453a577e52d57159458c3e9c)

2 files changed, 28 insertions, 1 deletions

diff --git a/src/basic/string-util.c b/src/basic/string-util.c
index d7ba43541..38adaaf45 100644
--- a/src/basic/string-util.c
+++ b/src/basic/string-util.c
@@ -1006,7 +1006,7 @@ int free_and_strdup(char **p, const char *s) {
 
         assert(p);
 
-        /* Replaces a string pointer with an strdup()ed new string,
+        /* Replaces a string pointer with a strdup()ed new string,
          * possibly freeing the old one. */
 
         if (streq_ptr(*p, s))
@@ -1025,6 +1025,32 @@ int free_and_strdup(char **p, const char *s) {
         return 1;
 }
 
+int free_and_strndup(char **p, const char *s, size_t l) {
+        char *t;
+
+        assert(p);
+        assert(s || l == 0);
+
+        /* Replaces a string pointer with a strndup()ed new string,
+         * freeing the old one. */
+
+        if (!*p && !s)
+                return 0;
+
+        if (*p && s && strneq(*p, s, l) && (l > strlen(*p) || (*p)[l] == '\0'))
+                return 0;
+
+        if (s) {
+                t = strndup(s, l);
+                if (!t)
+                        return -ENOMEM;
+        } else
+                t = NULL;
+
+        free_and_replace(*p, t);
+        return 1;
+}
+
 #if !HAVE_EXPLICIT_BZERO
 /*
  * Pointer to memset is volatile so that compiler must de-reference
diff --git a/src/basic/string-util.h b/src/basic/string-util.h
index 3cada129e..46760cbc1 100644
--- a/src/basic/string-util.h
+++ b/src/basic/string-util.h
@@ -182,6 +182,7 @@ char *strrep(const char *s, unsigned n);
 int split_pair(const char *s, const char *sep, char **l, char **r);
 
 int free_and_strdup(char **p, const char *s);
+int free_and_strndup(char **p, const char *s, size_t l);
 
 /* Normal memmem() requires haystack to be nonnull, which is annoying for zero-length buffers */
 static inline void *memmem_safe(const void *haystack, size_t haystacklen, const void *needle, size_t needlelen) {