basic/xattr-util: do not cast ssize_t to int

gcc warns about unitialized memory access because it notices that ssize_t which
is < 0 could be cast to positive int value. We know that this can't really
happen because only -1 can be returned, but OTOH, in principle a large
*positive* value cannot be cast properly. This is unlikely too, since xattrs
cannot be too large, but it seems cleaner to just use a size_t to return the
value and avoid the cast altoghter. This makes the code simpler and gcc is
happy too.

The following warning goes away:
[113/1502] Compiling C object 'src/basic/basic@sta/xattr-util.c.o'.
In file included from ../src/basic/alloc-util.h:28:0,
                 from ../src/basic/xattr-util.c:30:
../src/basic/xattr-util.c: In function ‘fd_getcrtime_at’:
../src/basic/macro.h:207:60: warning: ‘b’ may be used uninitialized in this function [-Wmaybe-uninitialized]
                 UNIQ_T(A,aq) < UNIQ_T(B,bq) ? UNIQ_T(A,aq) : UNIQ_T(B,bq); \
                                                            ^
../src/basic/xattr-util.c:155:19: note: ‘b’ was declared here
         usec_t a, b;
                   ^

src/basic/xattr-util.c

@@ -107,7 +107,14 @@ int fgetxattr_malloc(int fd, const char *name, char **value) {
         }
 }
 
-ssize_t fgetxattrat_fake(int dirfd, const char *filename, const char *attribute, void *value, size_t size, int flags) {
+int fgetxattrat_fake(
+                int dirfd,
+                const char *filename,
+                const char *attribute,
+                void *value, size_t size,
+                int flags,
+                size_t *ret_size) {
+
         char fn[STRLEN("/proc/self/fd/") + DECIMAL_STR_MAX(int) + 1];
         _cleanup_close_ int fd = -1;
         ssize_t l;
@@ -134,7 +141,8 @@ ssize_t fgetxattrat_fake(int dirfd, const char *filename, const char *attribute,
         if (l < 0)
                 return -errno;
 
-        return l;
+        *ret_size = l;
+        return 0;
 }
 
 static int parse_crtime(le64_t le, usec_t *usec) {
@@ -154,7 +162,7 @@ int fd_getcrtime_at(int dirfd, const char *name, usec_t *ret, int flags) {
         struct_statx sx;
         usec_t a, b;
         le64_t le;
-        ssize_t n;
+        size_t n;
         int r;
 
         assert(ret);
@@ -180,13 +188,13 @@ int fd_getcrtime_at(int dirfd, const char *name, usec_t *ret, int flags) {
         else
                 a = USEC_INFINITY;
 
-        n = fgetxattrat_fake(dirfd, name, "user.crtime_usec", &le, sizeof(le), flags);
-        if (n < 0)
-                r = -errno;
-        else if (n != sizeof(le))
-                r = -EIO;
-        else
-                r = parse_crtime(le, &b);
+        r = fgetxattrat_fake(dirfd, name, "user.crtime_usec", &le, sizeof(le), flags, &n);
+        if (r >= 0) {
+                if (n != sizeof(le))
+                        r = -EIO;
+                else
+                        r = parse_crtime(le, &b);
+        }
         if (r < 0) {
                 if (a != USEC_INFINITY) {
                         *ret = a;

src/basic/xattr-util.h

@@ -29,7 +29,13 @@
 int getxattr_malloc(const char *path, const char *name, char **value, bool allow_symlink);
 int fgetxattr_malloc(int fd, const char *name, char **value);
 
-ssize_t fgetxattrat_fake(int dirfd, const char *filename, const char *attribute, void *value, size_t size, int flags);
+int fgetxattrat_fake(
+                int dirfd,
+                const char *filename,
+                const char *attribute,
+                void *value, size_t size,
+                int flags,
+                size_t *ret_size);
 
 int fd_setcrtime(int fd, usec_t usec);
 

src/test/test-xattr-util.c

@@ -36,8 +36,9 @@ static void test_fgetxattrat_fake(void) {
         char t[] = "/var/tmp/xattrtestXXXXXX";
         _cleanup_close_ int fd = -1;
         const char *x;
-        char v[3] = {};
+        char v[3];
         int r;
+        size_t size;
 
         assert_se(mkdtemp(t));
         x = strjoina(t, "/test");
@@ -51,13 +52,14 @@ static void test_fgetxattrat_fake(void) {
         fd = open(t, O_RDONLY|O_DIRECTORY|O_CLOEXEC|O_NOCTTY);
         assert_se(fd >= 0);
 
-        assert_se(fgetxattrat_fake(fd, "test", "user.foo", v, 3, 0) >= 0);
+        assert_se(fgetxattrat_fake(fd, "test", "user.foo", v, 3, 0, &size) >= 0);
+        assert_se(size == 3);
         assert_se(memcmp(v, "bar", 3) == 0);
 
         safe_close(fd);
         fd = open("/", O_RDONLY|O_DIRECTORY|O_CLOEXEC|O_NOCTTY);
         assert_se(fd >= 0);
-        assert_se(fgetxattrat_fake(fd, "usr", "user.idontexist", v, 3, 0) == -ENODATA);
+        assert_se(fgetxattrat_fake(fd, "usr", "user.idontexist", v, 3, 0, &size) == -ENODATA);
 
 cleanup:
         assert_se(unlink(x) >= 0);