pidfs: add pidfs_dentry_open() helper

Both pidfs_alloc_file() and pidfs_export_open() need to force O_RDWR
and reapply the pidfd flags that do_dentry_open() strips. Move the
common logic into a helper.

PIDFD_AUTOKILL is now part of the restore mask in the file handle path
as well, but pidfs_export_permission() rejects O_TRUNC, so this is a
no-op there. But warn nonetheless.

Link: https://patch.msgid.link/20260722-esszimmer-umsetzen-nennt-ed5fc604300a@brauner
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

diff --git a/fs/pidfs.c b/fs/pidfs.c
index c20ffd747ff5119cfbcb159da28b6b12ff8a1577..695215aa2a5856e85854ab2aa90137182ebb6ddb 100644 (file)
--- a/fs/pidfs.c
+++ b/fs/pidfs.c
@@ -915,6 +915,20 @@ static struct dentry *pidfs_fh_to_dentry(struct super_block *sb,
        return path.dentry;
 }
 
+static struct file *pidfs_dentry_open(const struct path *path,
+                                     unsigned int flags,
+                                     const struct cred *cred)
+{
+       struct file *file;
+
+       /* pidfds are always O_RDWR. */
+       file = dentry_open(path, flags | O_RDWR, cred);
+       /* do_dentry_open() strips O_EXCL and O_TRUNC. */
+       if (!IS_ERR(file))
+               file->f_flags |= flags & (PIDFD_THREAD | PIDFD_AUTOKILL);
+       return file;
+}
+
 /*
  * Make sure that we reject any nonsensical flags that users pass via
  * open_by_handle_at(). Note that PIDFD_THREAD is defined as O_EXCL, and
@@ -939,18 +953,14 @@ 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.
+        * Opening via file handle may never raise PIDFD_AUTOKILL. That can
+        * only be done at task creation!
         */
-       oflags &= ~O_LARGEFILE;
-       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;
+       if (WARN_ON_ONCE(oflags & PIDFD_AUTOKILL))
+               return ERR_PTR(-EINVAL);
+       /* Clear O_LARGEFILE as open_by_handle_at() forces it. */
+       return pidfs_dentry_open(path, oflags & ~O_LARGEFILE, current_cred());
 }
 
 static const struct export_operations pidfs_export_operations = {
@@ -1114,7 +1124,6 @@ static struct file_system_type pidfs_type = {
 
 struct file *pidfs_alloc_file(struct pid *pid, unsigned int flags)
 {
-       struct file *pidfd_file;
        struct path path __free(path_put) = {};
        int ret;
 
@@ -1132,16 +1141,7 @@ struct file *pidfs_alloc_file(struct pid *pid, unsigned int flags)
        VFS_WARN_ON_ONCE(!pid->attr);
 
        flags &= ~PIDFD_STALE;
-       flags |= O_RDWR;
-       pidfd_file = dentry_open(&path, flags, current_cred());
-       /*
-        * Raise PIDFD_THREAD and PIDFD_AUTOKILL explicitly as
-        * do_dentry_open() strips O_EXCL and O_TRUNC.
-        */
-       if (!IS_ERR(pidfd_file))
-               pidfd_file->f_flags |= (flags & (PIDFD_THREAD | PIDFD_AUTOKILL));
-
-       return pidfd_file;
+       return pidfs_dentry_open(&path, flags, current_cred());
 }
 
 void __init pidfs_init(void)