linux-kernel - Re: [PATCH] fs/file: fix the check in find_next

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Message-ID: <20240529162235.b76ywqisawmcm22o@quack3>
Date: Wed, 29 May 2024 18:22:35 +0200
From: Jan Kara <jack@...e.cz>
To: Yuntao Wang <yuntao.wang@...ux.dev>
Cc: linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>
Subject: Re: [PATCH] fs/file: fix the check in find_next_fd()

On Thu 30-05-24 00:06:56, Yuntao Wang wrote:
> The maximum possible return value of find_next_zero_bit(fdt->full_fds_bits,
> maxbit, bitbit) is maxbit. This return value, multiplied by BITS_PER_LONG,
> gives the value of bitbit, which can never be greater than maxfd, it can
> only be equal to maxfd at most, so the following check 'if (bitbit > maxfd)'
> will never be true.
> 
> Moreover, when bitbit equals maxfd, it indicates that there are no unused
> fds, and the function can directly return.
> 
> Fix this check.
> 
> Signed-off-by: Yuntao Wang <yuntao.wang@...ux.dev>

Good point. Feel free to add:

Reviewed-by: Jan Kara <jack@...e.cz>

								Honza

> ---
>  fs/file.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/file.c b/fs/file.c
> index 8076aef9c210..7058901a2154 100644
> --- a/fs/file.c
> +++ b/fs/file.c
> @@ -491,7 +491,7 @@ static unsigned int find_next_fd(struct fdtable *fdt, unsigned int start)
>  	unsigned int bitbit = start / BITS_PER_LONG;
>  
>  	bitbit = find_next_zero_bit(fdt->full_fds_bits, maxbit, bitbit) * BITS_PER_LONG;
> -	if (bitbit > maxfd)
> +	if (bitbit >= maxfd)
>  		return maxfd;
>  	if (bitbit > start)
>  		start = bitbit;
> -- 
> 2.45.1
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR