linux-kernel - Re: [PATCH 3/5] debugfs: Update debugfs_create_str() kerneldoc to warn about pointer race

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Message-ID: <2023051607-trial-worrisome-87df@gregkh>
Date:   Tue, 16 May 2023 18:35:25 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Richard Fitzgerald <rf@...nsource.cirrus.com>
Cc:     rafael@...nel.org, linux-kernel@...r.kernel.org,
        patches@...nsource.cirrus.com
Subject: Re: [PATCH 3/5] debugfs: Update debugfs_create_str() kerneldoc to
 warn about pointer race

On Tue, May 16, 2023 at 05:07:51PM +0100, Richard Fitzgerald wrote:
> Add a warning to the debugfs_create_str() kerneldoc that the char * pointer
> value must not change after the function returns, because of a race with
> debugfs_read_file_str() accessing the pointer.
> 
> The only safe case is a change from NULL to non-NULL because in that case
> debugfs_read_file_str() will see either the NULL or the valid pointer.
> 
> Signed-off-by: Richard Fitzgerald <rf@...nsource.cirrus.com>
> ---
>  fs/debugfs/file.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/debugfs/file.c b/fs/debugfs/file.c
> index 0c039a3d9a42..77794871f26d 100644
> --- a/fs/debugfs/file.c
> +++ b/fs/debugfs/file.c
> @@ -938,11 +938,18 @@ static const struct file_operations fops_str_wo = {
>   *          directory dentry if set.  If this parameter is %NULL, then the
>   *          file will be created in the root of the debugfs filesystem.
>   * @value: a pointer to the variable that the file should read to and write
> - *         from.
> + *         from. The char* pointer must not change, except from NULL to
> + *         non-NULL.

This feels odd.  Why wouldn't you want to change the string value?  Or
why would you?

And why is this one-way transition ok?

Given that this is only used internally, why is it exported?

thanks,

greg k-h