udev-spawn: search executed command in build directory (#36985)

This makes pin_callout_binary() optionally provides the path of the pinned
binary, and makes it used in udev-spawn.c, to allow easy debugging of
program invocations requested by RUN{program} and friends.

src/basic/build-path.c

@@ -7,6 +7,7 @@
 #include "build-path.h"
 #include "errno-list.h"
 #include "errno-util.h"
+#include "fd-util.h"
 #include "macro.h"
 #include "path-util.h"
 #include "process-util.h"
@@ -245,12 +246,39 @@ int invoke_callout_binary(const char *path, char *const argv[]) {
         return -errno;
 }
 
-int pin_callout_binary(const char *path) {
+static int open_executable(const char *path, char **ret_path) {
         int r;
 
         assert(path);
 
-        /* Similar to invoke_callout_binary(), but pins (i.e. O_PATH opens) the binary instead of executing it. */
+        _cleanup_close_ int fd = RET_NERRNO(open(path, O_CLOEXEC|O_PATH));
+        if (fd < 0)
+                return fd;
+
+        r = fd_verify_regular(fd);
+        if (r < 0)
+                return r;
+
+        r = access_fd(fd, X_OK);
+        if (r < 0)
+                return r;
+
+        if (ret_path) {
+                r = fd_get_path(fd, ret_path);
+                if (r < 0)
+                        return r;
+        }
+
+        return TAKE_FD(fd);
+}
+
+int pin_callout_binary(const char *path, char **ret_path) {
+        int r;
+
+        assert(path);
+
+        /* Similar to invoke_callout_binary(), but pins (i.e. O_PATH opens) the binary instead of executing
+         * it, also optionally provides the path to the binary. */
 
         _cleanup_free_ char *fn = NULL;
         r = path_extract_filename(path, &fn);
@@ -261,14 +289,14 @@ int pin_callout_binary(const char *path) {
 
         const char *e;
         if (find_environment_binary(fn, &e) >= 0)
-                return RET_NERRNO(open(e, O_CLOEXEC|O_PATH));
+                return open_executable(e, ret_path);
 
         _cleanup_free_ char *np = NULL;
         if (find_build_dir_binary(fn, &np) >= 0) {
-                r = RET_NERRNO(open(np, O_CLOEXEC|O_PATH));
+                r = open_executable(np, ret_path);
                 if (r >= 0)
                         return r;
         }
 
-        return RET_NERRNO(open(path, O_CLOEXEC|O_PATH));
+        return open_executable(path, ret_path);
 }

src/basic/build-path.h

@@ -5,4 +5,4 @@ int get_build_exec_dir(char **ret);
 
 int invoke_callout_binary(const char *path, char *const argv[]);
 
-int pin_callout_binary(const char *path);
+int pin_callout_binary(const char *path, char **ret_path);

src/core/manager.c

@@ -1048,7 +1048,7 @@ int manager_new(RuntimeScope runtime_scope, ManagerTestRunFlags test_run_flags,
         }
 
         if (!FLAGS_SET(test_run_flags, MANAGER_TEST_DONT_OPEN_EXECUTOR)) {
-                m->executor_fd = pin_callout_binary(SYSTEMD_EXECUTOR_BINARY_PATH);
+                m->executor_fd = pin_callout_binary(SYSTEMD_EXECUTOR_BINARY_PATH, /* ret_path = */ NULL);
                 if (m->executor_fd < 0)
                         return log_debug_errno(m->executor_fd, "Failed to pin executor binary: %m");
 

src/udev/udev-spawn.c

@@ -2,9 +2,11 @@
 
 #include "sd-event.h"
 
+#include "build-path.h"
 #include "device-private.h"
 #include "device-util.h"
 #include "event-util.h"
+#include "exec-util.h"
 #include "fd-util.h"
 #include "path-util.h"
 #include "process-util.h"
@@ -279,6 +281,14 @@ int udev_event_spawn(
                 free_and_replace(argv[0], program);
         }
 
+        char *found;
+        _cleanup_close_ int fd_executable = r = pin_callout_binary(argv[0], &found);
+        if (r < 0)
+                return log_device_error_errno(event->dev, r, "Failed to find and pin callout binary \"%s\": %m", argv[0]);
+
+        log_device_debug(event->dev, "Found callout binary: \"%s\".", found);
+        free_and_replace(argv[0], found);
+
         char **envp;
         r = device_get_properties_strv(event->dev, &envp);
         if (r < 0)
@@ -290,7 +300,7 @@ int udev_event_spawn(
         r = pidref_safe_fork_full(
                         "(spawn)",
                         (int[]) { -EBADF, outpipe[WRITE_END], errpipe[WRITE_END] },
-                        NULL, 0,
+                        &fd_executable, 1,
                         FORK_RESET_SIGNALS|FORK_CLOSE_ALL_FDS|FORK_DEATHSIG_SIGTERM|FORK_REARRANGE_STDIO|FORK_LOG|FORK_RLIMIT_NOFILE_SAFE,
                         &pidref);
         if (r < 0)
@@ -298,7 +308,7 @@ int udev_event_spawn(
                                               "Failed to fork() to execute command '%s': %m", cmd);
         if (r == 0) {
                 DEVICE_TRACE_POINT(spawn_exec, event->dev, cmd);
-                execve(argv[0], argv, envp);
+                (void) fexecve_or_execve(fd_executable, argv[0], argv, envp);
                 _exit(EXIT_FAILURE);
         }