Merge pull request #926 from brauner/2016-03-27/mmap_file_to_str

src/lxc/lxccontainer.c

@@ -2163,13 +2163,8 @@ static bool mod_rdep(struct lxc_container *c0, struct lxc_container *c, bool inc
 			}
 
 			if (fbuf.st_size != 0) {
-				/* write terminating \0-byte to file */
-				if (pwrite(fd, "", 1, fbuf.st_size) <= 0) {
-					close(fd);
-					goto out;
-				}
 
-				buf = mmap(NULL, fbuf.st_size + 1, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
+				buf = lxc_strmmap(NULL, fbuf.st_size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
 				if (buf == MAP_FAILED) {
 					SYSERROR("Failed to create mapping %s", path);
 					close(fd);
@@ -2182,7 +2177,7 @@ static bool mod_rdep(struct lxc_container *c0, struct lxc_container *c, bool inc
 					bytes += len;
 				}
 
-				munmap(buf, fbuf.st_size + 1);
+				lxc_strmunmap(buf, fbuf.st_size);
 				if (ftruncate(fd, fbuf.st_size - bytes) < 0) {
 					SYSERROR("Failed to truncate file %s", path);
 					close(fd);

src/lxc/utils.c

@@ -1811,3 +1811,31 @@ int lxc_count_file_lines(const char *fn)
 	fclose(f);
 	return n;
 }
+
+void *lxc_strmmap(void *addr, size_t length, int prot, int flags, int fd,
+		  off_t offset)
+{
+	void *tmp = NULL, *overlap = NULL;
+
+	/* We establish an anonymous mapping that is one byte larger than the
+	 * underlying file. The pages handed to us are zero filled. */
+	tmp = mmap(addr, length + 1, PROT_READ, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+	if (tmp == MAP_FAILED)
+		goto out;
+
+	/* Now we establish a fixed-address mapping starting at the address we
+	 * received from our anonymous mapping and replace all bytes excluding
+	 * the additional \0-byte with the file. This allows us to use normal
+	 * string-handling function. */
+	overlap = mmap(tmp, length, prot, MAP_FIXED | flags, fd, offset);
+	if (overlap == MAP_FAILED)
+		goto out;
+
+out:
+	return overlap;
+}
+
+int lxc_strmunmap(void *addr, size_t length)
+{
+	return munmap(addr, length + 1);
+}

src/lxc/utils.h

@@ -252,6 +252,14 @@ extern void lxc_free_array(void **array, lxc_free_fn element_free_fn);
 extern size_t lxc_array_len(void **array);
 
 extern void **lxc_append_null_to_array(void **array, size_t count);
+
+/* mmap() wrapper. lxc_mmap() will take care to \0-terminate files so that
+ * normal string-handling functions can be used on the buffer. */
+extern void *lxc_strmmap(void *addr, size_t length, int prot, int flags, int fd,
+			 off_t offset);
+/* munmap() wrapper. Use it to free memory mmap()ed with lxc_mmap(). */
+extern int lxc_strmunmap(void *addr, size_t length);
+
 //initialize rand with urandom
 extern int randseed(bool);