linux-kernel - Re: [PATCH v2 10/16] overlayfs/file: Convert to cred

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Message-ID: <CAJfpegsq5NruDeL6HRgkpj=QvdOKdnqOwZiRS0VY092=h0RSkg@mail.gmail.com>
Date: Thu, 22 Aug 2024 10:41:55 +0200
From: Miklos Szeredi <miklos@...redi.hu>
To: Vinicius Costa Gomes <vinicius.gomes@...el.com>
Cc: brauner@...nel.org, amir73il@...il.com, hu1.chen@...el.com, 
	malini.bhandaru@...el.com, tim.c.chen@...el.com, mikko.ylinen@...el.com, 
	linux-unionfs@...r.kernel.org, linux-fsdevel@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 10/16] overlayfs/file: Convert to cred_guard()

On Thu, 22 Aug 2024 at 03:25, Vinicius Costa Gomes
<vinicius.gomes@...el.com> wrote:
>
> Replace the override_creds_light()/revert_creds_light() pairs of
> operations with cred_guard()/cred_scoped_guard().
>
> Only ovl_copyfile() and ovl_fallocate() use cred_scoped_guard(),
> because of 'goto', which can cause the cleanup flow to run on garbage
> memory.

This doesn't sound good.  Is this a compiler bug or a limitation of guards?

> @@ -211,9 +208,8 @@ static loff_t ovl_llseek(struct file *file, loff_t offset, int whence)
>         ovl_inode_lock(inode);
>         real.file->f_pos = file->f_pos;
>
> -       old_cred = ovl_override_creds_light(inode->i_sb);
> +       cred_guard(ovl_creds(inode->i_sb));
>         ret = vfs_llseek(real.file, offset, whence);
> -       revert_creds_light(old_cred);

Why not use scoped guard, like in fallocate?

> @@ -398,9 +393,8 @@ static int ovl_fsync(struct file *file, loff_t start, loff_t end, int datasync)
>
>         /* Don't sync lower file for fear of receiving EROFS error */
>         if (file_inode(real.file) == ovl_inode_upper(file_inode(file))) {
> -               old_cred = ovl_override_creds_light(file_inode(file)->i_sb);
> +               cred_guard(ovl_creds(file_inode(file)->i_sb));
>                 ret = vfs_fsync_range(real.file, start, end, datasync);
> -               revert_creds_light(old_cred);

Same here.

> @@ -584,9 +571,8 @@ static int ovl_flush(struct file *file, fl_owner_t id)
>                 return err;
>
>         if (real.file->f_op->flush) {
> -               old_cred = ovl_override_creds_light(file_inode(file)->i_sb);
> +               cred_guard(ovl_creds(file_inode(file)->i_sb));

What's the scope of this?  The function or the inner block?

Thanks,
Miklos