linux-kernel - Re: [PATCH bpf-next v5 3/5] bpf: Add bpf_fget

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Message-ID: <20241210-elastisch-flamingo-9271dc82c3a0@brauner>
Date: Tue, 10 Dec 2024 15:28:46 +0100
From: Christian Brauner <brauner@...nel.org>
To: Juntong Deng <juntong.deng@...look.com>
Cc: ast@...nel.org, daniel@...earbox.net, john.fastabend@...il.com, 
	andrii@...nel.org, martin.lau@...ux.dev, eddyz87@...il.com, song@...nel.org, 
	yonghong.song@...ux.dev, kpsingh@...nel.org, sdf@...ichev.me, haoluo@...gle.com, 
	jolsa@...nel.org, memxor@...il.com, snorcht@...il.com, bpf@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH bpf-next v5 3/5] bpf: Add bpf_fget_task() kfunc

On Tue, Dec 10, 2024 at 02:03:52PM +0000, Juntong Deng wrote:
> This patch adds bpf_fget_task() kfunc.
> 
> bpf_fget_task() is used to get a pointer to the struct file
> corresponding to the task file descriptor. Note that this function
> acquires a reference to struct file.
> 
> Signed-off-by: Juntong Deng <juntong.deng@...look.com>
> ---
>  fs/bpf_fs_kfuncs.c | 21 +++++++++++++++++++++
>  1 file changed, 21 insertions(+)
> 
> diff --git a/fs/bpf_fs_kfuncs.c b/fs/bpf_fs_kfuncs.c
> index 3fe9f59ef867..19a9d45c47f9 100644
> --- a/fs/bpf_fs_kfuncs.c
> +++ b/fs/bpf_fs_kfuncs.c
> @@ -152,6 +152,26 @@ __bpf_kfunc int bpf_get_file_xattr(struct file *file, const char *name__str,
>  	return bpf_get_dentry_xattr(dentry, name__str, value_p);
>  }
>  
> +/**
> + * bpf_fget_task() - Get a pointer to the struct file corresponding to
> + * the task file descriptor
> + *
> + * Note that this function acquires a reference to struct file.
> + *
> + * @task: the specified struct task_struct
> + * @fd: the file descriptor
> + *
> + * @returns the corresponding struct file pointer if found,
> + * otherwise returns NULL
> + */
> +__bpf_kfunc struct file *bpf_fget_task(struct task_struct *task, unsigned int fd)
> +{
> +	struct file *file;
> +
> +	file = fget_task(task, fd);
> +	return file;

Why the local variable?