[PATCH v2 i-g-t 1/2] lib/igt_fs: add igt_fs_check_root_perm() helper

[Peter Senna Tschudin <peter.senna@linux.intel.com>]

Add igt_fs_check_root_perm() to check that a file is owned by root
(uid=0, gid=0) and that only the owner has write permissions.

This will be used for validating the permissions and ownership of files
where strict root-only write access is required. Example usage:

 int result = igt_fs_check_root_perm(dirfd, pathname);
 if (result == -ENOENT) {
 	igt_skip("File does not exist\n");
 } else if (result < 0) {
 	igt_skip("Cannot stat file: %s\n", strerror(-result));
 } else {
 	igt_assert_f(result == 0, "Permission check failed\n");
 }

Signed-off-by: Peter Senna Tschudin <peter.senna@linux.intel.com>
---

Changes from v1:
 - changed return type from bool to int
 - Use errno code including for file not found

 lib/igt_fs.c | 32 ++++++++++++++++++++++++++++++++
 lib/igt_fs.h |  1 +
 2 files changed, 33 insertions(+)

diff --git a/lib/igt_fs.c b/lib/igt_fs.c
index 8f4d17546..660def4cd 100644
--- a/lib/igt_fs.c
+++ b/lib/igt_fs.c
@@ -25,6 +25,7 @@
 #include <fcntl.h>
 #include <stdlib.h>
 #include <sys/stat.h>
+#include <stdbool.h>
 #include <unistd.h>
 
 #include "igt_fs.h"
@@ -141,3 +142,34 @@ int igt_fs_remove_dir(int fd, const char *name)
 
 	return 0;
 }
+
+/**
+ * igt_fs_check_root_perm: Checks if the user and group are root and that
+ *                         only the user can write to the file.
+ * @dirfd: file descriptor of the directory containing the file
+ * @pathname:  name of the file to check
+ *
+ * Returns:
+ *  0 if the file is owned by root and only root can write to it (success),
+ *  1 if the file exists but fails permission checks (failure),
+ * -ENOENT if the file does not exist,
+ * -errno for other stat failures
+ */
+int igt_fs_check_root_perm(int dirfd, const char *pathname)
+{
+	struct stat st;
+
+	if (fstatat(dirfd, pathname, &st, 0))
+		return -errno;
+
+	if (st.st_uid != 0 || st.st_gid != 0)
+		return 1;
+
+	if (st.st_mode & S_IWGRP)
+		return 1;
+
+	if (st.st_mode & S_IWOTH)
+		return 1;
+
+	return 0;
+}
diff --git a/lib/igt_fs.h b/lib/igt_fs.h
index ee3e7593b..3a3f81d80 100644
--- a/lib/igt_fs.h
+++ b/lib/igt_fs.h
@@ -31,5 +31,6 @@ int igt_fs_create_dir(int fd, const char *name, mode_t mode);
 int igt_fs_remove_dir(int fd, const char *name);
 ssize_t igt_readn(int fd, char *buf, size_t len);
 ssize_t igt_writen(int fd, const char *buf, size_t len);
+int igt_fs_check_root_perm(int dirfd, const char *pathname);
 
 #endif /* __IGT_FS_H__ */
-- 
2.43.0