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Date: Tue, 4 Aug 2026 10:40:26 +0200
From: Greg KH <gregkh@linuxfoundation.org>
To: Yichong Chen <chenyichong@uniontech.com>
Cc: jirislaby@kernel.org, viro@zeniv.linux.org.uk, willy@infradead.org,
	kees@kernel.org, linux-serial@vger.kernel.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH] devpts: fix pty count limit off by one
Message-ID: <2026080419-chatting-curve-e49c@gregkh>
References: <20260804082321.884880-1-chenyichong@uniontech.com>
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In-Reply-To: <20260804082321.884880-1-chenyichong@uniontech.com>

On Tue, Aug 04, 2026 at 04:23:21PM +0800, Yichong Chen wrote:
> devpts_new_index() increments the global pty count before checking it
> against the effective global limit. The check currently rejects a new pty
> when the incremented count is equal to the limit.
> 
> This makes kernel.pty.max allow only max - 1 ptys. For example, if
> kernel.pty.nr is 3 and kernel.pty.max is set to 4, opening /dev/ptmx
> fails with -ENOSPC even though one more pty should be allowed.
> 
> Allow the incremented count to be equal to the effective limit and reject
> only counts above it.
> 
> Fixes: 0f0a0e54a2a1 ("devpts: Convert to new IDA API")

As this is a user-visible change, what is now going to break because of
this?  It's been this way for a very long time now.

thanks,

greg k-h