Merge pull request #17079 from keszybz/late-exec-resolution

Resolve executable paths before execution, use fexecve()

src/basic/missing_syscall.h

@@ -749,6 +749,25 @@ static inline int missing_rt_sigqueueinfo(pid_t tgid, int sig, siginfo_t *info)
 #  define rt_sigqueueinfo missing_rt_sigqueueinfo
 #endif
 
+/* ======================================================================= */
+ 
+#if !HAVE_EXECVEAT
+static inline int missing_execveat(int dirfd, const char *pathname,
+                                   char *const argv[], char *const envp[],
+                                   int flags) {
+#  if defined __NR_execveat && __NR_execveat >= 0
+        return syscall(__NR_execveat, dirfd, pathname, argv, envp, flags);
+#  else
+        errno = ENOSYS;
+        return -1;
+#  endif
+}
+ 
+#  undef AT_EMPTY_PATH
+#  define AT_EMPTY_PATH 0x1000
+#  define execveat missing_execveat
+#endif
+
 /* ======================================================================= */
 
 #define systemd_NR_close_range systemd_SC_arch_bias(436)

src/basic/path-util.c

@@ -585,22 +585,53 @@ char* path_join_internal(const char *first, ...) {
         return joined;
 }
 
-int find_executable_full(const char *name, bool use_path_envvar, char **ret) {
+static int check_x_access(const char *path, int *ret_fd) {
+        if (ret_fd) {
+                _cleanup_close_ int fd = -1;
+                int r;
+
+                /* We need to use O_PATH because there may be executables for which we have only exec
+                 * permissions, but not read (usually suid executables). */
+                fd = open(path, O_PATH|O_CLOEXEC);
+                if (fd < 0)
+                        return -errno;
+
+                r = access_fd(fd, X_OK);
+                if (r < 0)
+                        return r;
+
+                *ret_fd = TAKE_FD(fd);
+        } else {
+                /* Let's optimize things a bit by not opening the file if we don't need the fd. */
+                if (access(path, X_OK) < 0)
+                        return -errno;
+        }
+
+        return 0;
+}
+
+int find_executable_full(const char *name, bool use_path_envvar, char **ret_filename, int *ret_fd) {
         int last_error, r;
         const char *p = NULL;
 
         assert(name);
 
         if (is_path(name)) {
-                if (access(name, X_OK) < 0)
-                        return -errno;
+                _cleanup_close_ int fd = -1;
 
-                if (ret) {
-                        r = path_make_absolute_cwd(name, ret);
+                r = check_x_access(name, ret_fd ? &fd : NULL);
+                if (r < 0)
+                        return r;
+
+                if (ret_filename) {
+                        r = path_make_absolute_cwd(name, ret_filename);
                         if (r < 0)
                                 return r;
                 }
 
+                if (ret_fd)
+                        *ret_fd = TAKE_FD(fd);
+
                 return 0;
         }
 
@@ -613,8 +644,10 @@ int find_executable_full(const char *name, bool use_path_envvar, char **ret) {
 
         last_error = -ENOENT;
 
+        /* Resolve a single-component name to a full path */
         for (;;) {
                 _cleanup_free_ char *j = NULL, *element = NULL;
+                _cleanup_close_ int fd = -1;
 
                 r = extract_first_word(&p, &element, ":", EXTRACT_RELAX|EXTRACT_DONT_COALESCE_SEPARATORS);
                 if (r < 0)
@@ -629,7 +662,8 @@ int find_executable_full(const char *name, bool use_path_envvar, char **ret) {
                 if (!j)
                         return -ENOMEM;
 
-                if (access(j, X_OK) >= 0) {
+                r = check_x_access(j, ret_fd ? &fd : NULL);
+                if (r >= 0) {
                         _cleanup_free_ char *with_dash;
 
                         with_dash = strjoin(j, "/");
@@ -643,8 +677,10 @@ int find_executable_full(const char *name, bool use_path_envvar, char **ret) {
                         /* We can't just `continue` inverting this case, since we need to update last_error. */
                         if (errno == ENOTDIR) {
                                 /* Found it! */
-                                if (ret)
-                                        *ret = path_simplify(TAKE_PTR(j), false);
+                                if (ret_filename)
+                                        *ret_filename = path_simplify(TAKE_PTR(j), false);
+                                if (ret_fd)
+                                        *ret_fd = TAKE_FD(fd);
 
                                 return 0;
                         }

src/basic/path-util.h

@@ -88,9 +88,9 @@ int path_strv_make_absolute_cwd(char **l);
 char** path_strv_resolve(char **l, const char *root);
 char** path_strv_resolve_uniq(char **l, const char *root);
 
-int find_executable_full(const char *name, bool use_path_envvar, char **ret);
-static inline int find_executable(const char *name, char **ret) {
-        return find_executable_full(name, true, ret);
+int find_executable_full(const char *name, bool use_path_envvar, char **ret_filename, int *ret_fd);
+static inline int find_executable(const char *name, char **ret_filename) {
+        return find_executable_full(name, true, ret_filename, NULL);
 }
 
 bool paths_check_timestamp(const char* const* paths, usec_t *paths_ts_usec, bool update);