[PATCH] selftests/unprivileged-remount: Strip ST_VALID before flag sanity check

[Chris Gellermann]

Musl (and potentially other non-glibc C libraries) just copies the
statfs flags received from the kernel to the user [1]. This way,
ST_VALID may end up in the f_flags, a Linux-specific flag [2] that is
generally hidden from the user. The unprivileged-remout-test
fails when checking for unknown flags.

If we run on a non-glibc C library, strip ST_VALID before sanity
checking the flags.

Link: https://git.musl-libc.org/cgit/musl/tree/src/stat/statvfs.c#n40
Link: https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/sys_statvfs.h.html
Signed-off-by: Chris Gellermann <christian.gellermann@xxxxxxxxxxx>
---
 tools/testing/selftests/mount/unprivileged-remount-test.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/tools/testing/selftests/mount/unprivileged-remount-test.c b/tools/testing/selftests/mount/unprivileged-remount-test.c
index d2917054fe3a..c7bbed313495 100644
--- a/tools/testing/selftests/mount/unprivileged-remount-test.c
+++ b/tools/testing/selftests/mount/unprivileged-remount-test.c
@@ -45,6 +45,10 @@
 # define MS_STRICTATIME (1 << 24)
 #endif
 
+#ifndef ST_VALID
+# define ST_VALID (0x0020)
+#endif
+
 static void die(char *fmt, ...)
 {
 	va_list ap;
@@ -123,6 +127,9 @@ static int read_mnt_flags(const char *path)
 		die("statvfs of %s failed: %s\n",
 			path, strerror(errno));
 	}
+#ifndef __GLIBC__
+	stat.f_flag &= ~ST_VALID;
+#endif
 	if (stat.f_flag & ~(ST_RDONLY | ST_NOSUID | ST_NODEV | \
 			ST_NOEXEC | ST_NOATIME | ST_NODIRATIME | ST_RELATIME | \
 			ST_SYNCHRONOUS | ST_MANDLOCK)) {
-- 
2.47.3