linux-kernel - Re: [PATCH v2 1/4] fs/pipe: Limit the slots in pipe_resize

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Message-ID: <20250307223020.GA28762@redhat.com>
Date: Fri, 7 Mar 2025 23:30:20 +0100
From: Oleg Nesterov <oleg@...hat.com>
To: K Prateek Nayak <kprateek.nayak@....com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Christian Brauner <brauner@...nel.org>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	Jan Kara <jack@...e.cz>,
	"Matthew Wilcox (Oracle)" <willy@...radead.org>,
	Mateusz Guzik <mjguzik@...il.com>,
	Rasmus Villemoes <ravi@...vas.dk>,
	"Gautham R. Shenoy" <gautham.shenoy@....com>,
	Neeraj.Upadhyay@....com, Ananth.narayan@....com,
	Swapnil Sapkal <swapnil.sapkal@....com>
Subject: Re: [PATCH v2 1/4] fs/pipe: Limit the slots in pipe_resize_ring()

On 03/07, K Prateek Nayak wrote:
>
> On 3/7/2025 8:21 PM, Oleg Nesterov wrote:
> >On 03/07, K Prateek Nayak wrote:
> >>
> >>--- a/fs/pipe.c
> >>+++ b/fs/pipe.c
> >>@@ -1271,6 +1271,10 @@ int pipe_resize_ring(struct pipe_inode_info *pipe, unsigned int nr_slots)
> >>  	struct pipe_buffer *bufs;
> >>  	unsigned int head, tail, mask, n;
> >>
> >>+	/* nr_slots larger than limits of pipe->{head,tail} */
> >>+	if (unlikely(nr_slots > (pipe_index_t)-1u))
> >>+		return -EINVAL;
> >
> >The whole series look "obviously" good to me,
> >
> >Reviewed-by: Oleg Nesterov <oleg@...hat.com>

So, in case it wasn't clear, you could safely ignore everything else below ;)

> >pipe_resize_ring() has another caller, watch_queue_set_size(), but it has
> >its own hard limits...
>
> "nr_notes" for watch queues cannot cross 512 so we should be covered there.

Yes, yes, this is what I meant,

> As for round_pipe_size(), we can do:
>
> diff --git a/fs/pipe.c b/fs/pipe.c
> index ce1af7592780..f82098aaa510 100644
> --- a/fs/pipe.c
> +++ b/fs/pipe.c
> @@ -1253,6 +1253,8 @@ const struct file_operations pipefifo_fops = {
>   */
>  unsigned int round_pipe_size(unsigned int size)
>  {
> +	unsigned int max_slots;
> +
>  	if (size > (1U << 31))
>  		return 0;
> @@ -1260,7 +1262,14 @@ unsigned int round_pipe_size(unsigned int size)
>  	if (size < PAGE_SIZE)
>  		return PAGE_SIZE;
> -	return roundup_pow_of_two(size);
> +	size = roundup_pow_of_two(size);
> +	max_slots = size >> PAGE_SHIFT;
> +
> +	/* Max slots cannot be covered pipe->{head,tail} limits */
> +	if (max_slots > (pipe_index_t)-1U)
> +		return 0;

Sure, this will work, but still it doesn't look clear/clean to me.
But no, no, I don't blame your suggestion.

To me, round_pipe_size() looks confusing with or without the changes we
discuss. Why does it use "1U << 31" as a maximum size? OK, this is because
that "1 << 31" is the maximum power-of-2 which can fit into u32.

But, even if this code assumes that pipe->head/tail are u32, why this
restriction? Most probably I missed something, but I don't understand.

> Since pipe_resize_ring() can be called without actually looking at
> "pipe_max_size"

Again, only if the caller is watch_queue_set_size(), but it has its own
hard limit.

So. I won't argue either way. Whatever looks better to you. My ack
still stands.

Sorry for (yet another) confusing and almost off-topic email from me.

Oleg.