[PATCH] pidfs: preserve thread pidfds reopened by file handle

[Li Chen <me@linux.beauty>]

PIDFD_THREAD shares O_EXCL. do_dentry_open() clears O_EXCL after
pidfs_export_open() validates the flags, so open_by_handle_at()
silently turns a thread pidfd into a process pidfd.

Restore PIDFD_THREAD on the opened file, matching pidfs_alloc_file(),
and cover the restored flag with F_GETFL.

Signed-off-by: Li Chen <me@linux.beauty>
---
 fs/pidfs.c                                             | 8 +++++++-
 tools/testing/selftests/pidfd/pidfd_file_handle_test.c | 1 +
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/fs/pidfs.c b/fs/pidfs.c
index aaa609ddab044..c20ffd747ff51 100644
--- a/fs/pidfs.c
+++ b/fs/pidfs.c
@@ -939,12 +939,18 @@ static int pidfs_export_permission(struct handle_to_path_ctx *ctx,
 
 static struct file *pidfs_export_open(const struct path *path, unsigned int oflags)
 {
+	struct file *file;
+
 	/*
 	 * Clear O_LARGEFILE as open_by_handle_at() forces it and raise
 	 * O_RDWR as pidfds always are.
 	 */
 	oflags &= ~O_LARGEFILE;
-	return dentry_open(path, oflags | O_RDWR, current_cred());
+	file = dentry_open(path, oflags | O_RDWR, current_cred());
+	/* do_dentry_open() strips O_EXCL, which encodes PIDFD_THREAD. */
+	if (!IS_ERR(file))
+		file->f_flags |= oflags & PIDFD_THREAD;
+	return file;
 }
 
 static const struct export_operations pidfs_export_operations = {
diff --git a/tools/testing/selftests/pidfd/pidfd_file_handle_test.c b/tools/testing/selftests/pidfd/pidfd_file_handle_test.c
index 68918734dcf32..1e03ae9575fe6 100644
--- a/tools/testing/selftests/pidfd/pidfd_file_handle_test.c
+++ b/tools/testing/selftests/pidfd/pidfd_file_handle_test.c
@@ -373,6 +373,7 @@ TEST_F(file_handle, open_by_handle_at_valid_flags)
 				  O_CLOEXEC |
 				  O_EXCL);
 	ASSERT_GE(pidfd, 0);
+	ASSERT_NE(fcntl(pidfd, F_GETFL) & PIDFD_THREAD, 0);
 
 	ASSERT_EQ(fstat(pidfd, &st2), 0);
 	ASSERT_TRUE(st1.st_dev == st2.st_dev && st1.st_ino == st2.st_ino);
-- 
2.52.0