fuzz: no longer skip empty files

Empty files and empty strings seem to have triggered various
issues in the past so it seems they shouldn't be ignore by the
fuzzers just because fmemopen can't handle them.

Prompted by https://github.com/systemd/systemd/pull/21939#issuecomment-1003113669

src/core/fuzz-unit-file.c

@@ -2,7 +2,6 @@
 
 #include "conf-parser.h"
 #include "fd-util.h"
-#include "fileio.h"
 #include "fuzz.h"
 #include "install.h"
 #include "load-fragment.h"
@@ -22,10 +21,7 @@ int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
         const char *name;
         long offset;
 
-        if (size == 0)
-                return 0;
-
-        f = fmemopen_unlocked((char*) data, size, "re");
+        f = data_to_file(data, size);
         assert_se(f);
 
         if (read_line(f, LINE_MAX, &p) < 0)

src/fuzz/fuzz-env-file.c

@@ -4,7 +4,6 @@
 
 #include "alloc-util.h"
 #include "env-file.h"
-#include "fileio.h"
 #include "fd-util.h"
 #include "fuzz.h"
 #include "strv.h"
@@ -13,10 +12,10 @@ int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
         _cleanup_fclose_ FILE *f = NULL;
         _cleanup_strv_free_ char **rl = NULL, **rlp =  NULL;
 
-        if (size == 0 || size > 65535)
+        if (size > 65535)
                 return 0;
 
-        f = fmemopen_unlocked((char*) data, size, "re");
+        f = data_to_file(data, size);
         assert_se(f);
 
         /* We don't want to fill the logs with messages about parse errors.

src/fuzz/fuzz-hostname-setup.c

@@ -2,7 +2,6 @@
 
 #include "alloc-util.h"
 #include "fd-util.h"
-#include "fileio.h"
 #include "fuzz.h"
 #include "hostname-setup.h"
 
@@ -10,10 +9,7 @@ int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
         _cleanup_fclose_ FILE *f = NULL;
         _cleanup_free_ char *ret = NULL;
 
-        if (size == 0)
-                return 0;
-
-        f = fmemopen_unlocked((char*) data, size, "re");
+        f = data_to_file(data, size);
         assert_se(f);
 
         /* We don't want to fill the logs with messages about parse errors.

src/fuzz/fuzz-json.c

@@ -1,7 +1,6 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 
 #include "alloc-util.h"
-#include "fileio.h"
 #include "fd-util.h"
 #include "fuzz.h"
 #include "json.h"
@@ -12,10 +11,7 @@ int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
         _cleanup_fclose_ FILE *f = NULL, *g = NULL;
         _cleanup_(json_variant_unrefp) JsonVariant *v = NULL;
 
-        if (size == 0)
-                return 0;
-
-        f = fmemopen_unlocked((char*) data, size, "re");
+        f = data_to_file(data, size);
         assert_se(f);
 
         if (json_parse_file(f, NULL, 0, &v, NULL, NULL) < 0)

src/fuzz/fuzz.h

@@ -4,5 +4,14 @@
 #include <stddef.h>
 #include <stdint.h>
 
+#include "fileio.h"
+
 /* The entry point into the fuzzer */
 int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size);
+
+static inline FILE* data_to_file(const uint8_t *data, size_t size) {
+        if (size == 0)
+                return fopen("/dev/null", "re");
+        else
+                return fmemopen_unlocked((char*) data, size, "re");
+}

src/nspawn/fuzz-nspawn-oci.c

@@ -2,7 +2,6 @@
 
 #include "alloc-util.h"
 #include "fd-util.h"
-#include "fileio.h"
 #include "fuzz.h"
 #include "nspawn-oci.h"
 
@@ -10,10 +9,7 @@ int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
         _cleanup_fclose_ FILE *f = NULL;
         _cleanup_(settings_freep) Settings *s = NULL;
 
-        if (size == 0)
-                return 0;
-
-        f = fmemopen_unlocked((char*) data, size, "re");
+        f = data_to_file(data, size);
         assert_se(f);
 
         /* We don't want to fill the logs with messages about parse errors.

src/nspawn/fuzz-nspawn-settings.c

@@ -2,7 +2,6 @@
 
 #include "alloc-util.h"
 #include "fd-util.h"
-#include "fileio.h"
 #include "fuzz.h"
 #include "nspawn-settings.h"
 
@@ -10,10 +9,7 @@ int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
         _cleanup_fclose_ FILE *f = NULL;
         _cleanup_(settings_freep) Settings *s = NULL;
 
-        if (size == 0)
-                return 0;
-
-        f = fmemopen_unlocked((char*) data, size, "re");
+        f = data_to_file(data, size);
         assert_se(f);
 
         /* We don't want to fill the logs with messages about parse errors.