linux-kernel - Re: [PATCH] proc/base: Skip assignment to len when there is no error on d_path in do_proc

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Date:   Wed, 27 May 2020 09:41:53 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Kaitao Cheng <pilgrimtao@...il.com>
Cc:     adobriyan@...il.com, christian@...uner.io,
        akpm@...ux-foundation.org, gladkov.alexey@...il.com, guro@...com,
        walken@...gle.com, avagin@...il.com, khlebnikov@...dex-team.ru,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] proc/base: Skip assignment to len when there is no error on d_path in do_proc_readlink.

Kaitao Cheng <pilgrimtao@...il.com> writes:

> we don't need {len = PTR_ERR(pathname)} when IS_ERR(pathname) is false,
> it's better to move it into if(IS_ERR(pathname)){}.

Please look at the generated code.

I believe you will find that your change will generate worse assembly.

Eric


> Signed-off-by: Kaitao Cheng <pilgrimtao@...il.com>
> ---
>  fs/proc/base.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index d86c0afc8a85..9509e0d42610 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -1753,9 +1753,10 @@ static int do_proc_readlink(struct path *path, char __user *buffer, int buflen)
>  		return -ENOMEM;
>  
>  	pathname = d_path(path, tmp, PAGE_SIZE);
> -	len = PTR_ERR(pathname);
> -	if (IS_ERR(pathname))
> +	if (IS_ERR(pathname)) {
> +		len = PTR_ERR(pathname);
>  		goto out;
> +	}
>  	len = tmp + PAGE_SIZE - 1 - pathname;
>  
>  	if (len > buflen)