diff --git a/meson.build b/meson.build
index c60c4ab873..bffa9c2d3e 100644
--- a/meson.build
+++ b/meson.build
@@ -29,6 +29,10 @@ substs.set('PROJECT_VERSION',      meson.project_version())
 # This is to be used instead of meson.source_root(), as the latter will return
 # the wrong result when systemd is being built as a meson subproject
 project_source_root = meson.current_source_dir()
+relative_source_path = run_command('realpath',
+                                   '--relative-to=@0@'.format(meson.current_build_dir()),
+                                   project_source_root).stdout().strip()
+conf.set_quoted('RELATIVE_SOURCE_PATH', relative_source_path)
 
 want_ossfuzz = get_option('oss-fuzz')
 want_libfuzzer = get_option('llvm-fuzz')
diff --git a/src/basic/log.h b/src/basic/log.h
index aa3d5b75f1..7f78a417cf 100644
--- a/src/basic/log.h
+++ b/src/basic/log.h
@@ -73,6 +73,9 @@ int log_get_max_level_realm(LogRealm realm) _pure_;
  * for the application itself.
  */
 
+assert_cc(STRLEN(__FILE__) > STRLEN(RELATIVE_SOURCE_PATH) + 1)
+#define PROJECT_FILE (__FILE__ + STRLEN(RELATIVE_SOURCE_PATH) + 1)
+
 int log_open(void);
 void log_close(void);
 void log_forget_fds(void);
diff --git a/src/test/test-log.c b/src/test/test-log.c
index 18ef56ac39..c1f2f8b09d 100644
--- a/src/test/test-log.c
+++ b/src/test/test-log.c
@@ -6,6 +6,7 @@
 #include "format-util.h"
 #include "log.h"
 #include "process-util.h"
+#include "string-util.h"
 #include "util.h"
 
 assert_cc(LOG_REALM_REMOVE_LEVEL(LOG_REALM_PLUS_LEVEL(LOG_REALM_SYSTEMD, LOG_FTP | LOG_DEBUG))
@@ -26,6 +27,14 @@ assert_cc(!IS_SYNTHETIC_ERRNO(0));
 #define X100(x) X10(X10(x))
 #define X1000(x) X100(X10(x))
 
+static void test_file(void) {
+        log_info("__FILE__: %s", __FILE__);
+        log_info("RELATIVE_SOURCE_PATH: %s", RELATIVE_SOURCE_PATH);
+        log_info("PROJECT_FILE: %s", PROJECT_FILE);
+
+        assert(startswith(__FILE__, RELATIVE_SOURCE_PATH "/"));
+}
+
 static void test_log_struct(void) {
         log_struct(LOG_INFO,
                    "MESSAGE=Waldo PID="PID_FMT" (no errno)", getpid_cached(),
@@ -68,6 +77,8 @@ static void test_log_syntax(void) {
 int main(int argc, char* argv[]) {
         int target;
 
+        test_file();
+
         for (target = 0; target < _LOG_TARGET_MAX; target++) {
                 log_set_target(target);
                 log_open();