linux-kernel - Re: [PATCH 27/32] aio: sanitize ki

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Message-ID: <x49lgh54d39.fsf@segfault.boston.devel.redhat.com>
Date:   Wed, 10 Jan 2018 16:29:30 -0500
From:   Jeff Moyer <jmoyer@...hat.com>
To:     Christoph Hellwig <hch@....de>
Cc:     viro@...iv.linux.org.uk, Avi Kivity <avi@...lladb.com>,
        linux-aio@...ck.org, linux-fsdevel@...r.kernel.org,
        netdev@...r.kernel.org, linux-api@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 27/32] aio: sanitize ki_list handling

Christoph Hellwig <hch@....de> writes:

> Instead of handcoded non-null checks always initialize ki_list to an
> empty list and use list_empty / list_empty_careful on it.

Yeah, who knows why list_empty wasn't used from the beginning.  In the
past, tricks were played by overwriting list pointers with non-null, but
that wasn't the case here.

> While we're at it also error out on a double call to
> kiocb_set_cancel_fn instead of ignoring it.
>
> Signed-off-by: Christoph Hellwig <hch@....de>
> ---
>  fs/aio.c | 13 ++++++-------
>  1 file changed, 6 insertions(+), 7 deletions(-)
>
> diff --git a/fs/aio.c b/fs/aio.c
> index 496b50f9e9b1..fe241b5b44b2 100644
> --- a/fs/aio.c
> +++ b/fs/aio.c
> @@ -555,13 +555,12 @@ void kiocb_set_cancel_fn(struct kiocb *iocb, kiocb_cancel_fn *cancel)
>  	struct kioctx *ctx = req->ki_ctx;
>  	unsigned long flags;
>  
> -	spin_lock_irqsave(&ctx->ctx_lock, flags);
> -
> -	if (!req->ki_list.next)
> -		list_add(&req->ki_list, &ctx->active_reqs);
> +	if (WARN_ON_ONCE(!list_empty(&req->ki_list)))
> +		return;
>  
> +	spin_lock_irqsave(&ctx->ctx_lock, flags);
> +	list_add_tail(&req->ki_list, &ctx->active_reqs);
>  	req->ki_cancel = cancel;

So, this changes behavior from quietly overwriting the prior cancel
function to not doing it.  I don't think it matters one bit, though.
Callers shouldn't do that.

Acked-by: Jeff Moyer <jmoyer@...hat.com>