linux-kernel - Re: [PATCH v2] bcachefs: Avoid a potential useless over memory allocation in bch2_prt

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Message-ID: <o3wahdwbcpktshcblnbbf2bkmf6omtmf42xzprjtg627vpcbf7@bagppe6ucens>
Date: Sun, 18 Feb 2024 22:42:45 -0500
From: Kent Overstreet <kent.overstreet@...ux.dev>
To: Christophe JAILLET <christophe.jaillet@...adoo.fr>
Cc: Brian Foster <bfoster@...hat.com>, linux-kernel@...r.kernel.org, 
	kernel-janitors@...r.kernel.org, linux-bcachefs@...r.kernel.org
Subject: Re: [PATCH v2] bcachefs: Avoid a potential useless over memory
 allocation in bch2_prt_[v]printf()

On Sun, Feb 18, 2024 at 05:12:28PM +0100, Christophe JAILLET wrote:
> 2 issues related to incorrect available space in the output buffer
> computation may lead to some extra memory allocation.
> 
> 
> First: vsnprintf() takes the size of the buffer, *including* the space for
> the trailing null.
> 
> But printbuf_remaining() returns the number of characters we can print
> to the output buffer, *excluding* the terminating null.
> 
> So, use printbuf_remaining_size(), which includes the space for the
> terminating null.
> 
> 
> Second: if the return value of vsnprintf() is greater than or equal to the
> passed size, the resulting string is truncated.
> So, in order to see if some extra space is needed, the check needs to be
> changed.

btw, the patch was suspect to begin with

in cases where off-by-one errors are difficult to avoid, but harmless in
one direction - just over allocate.