linux-kernel - Re: [PATCH 29/32] lib/string_helpers: string_get

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Message-ID: <20230712202309.xqtggydzf65p2cd7@moria.home.lan>
Date:   Wed, 12 Jul 2023 16:23:09 -0400
From:   Kent Overstreet <kent.overstreet@...ux.dev>
To:     Kees Cook <keescook@...omium.org>
Cc:     linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-bcachefs@...r.kernel.org,
        Kent Overstreet <kent.overstreet@...il.com>,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH 29/32] lib/string_helpers: string_get_size() now returns
 characters wrote

On Wed, Jul 12, 2023 at 12:58:54PM -0700, Kees Cook wrote:
> On Tue, May 09, 2023 at 12:56:54PM -0400, Kent Overstreet wrote:
> > From: Kent Overstreet <kent.overstreet@...il.com>
> > 
> > printbuf now needs to know the number of characters that would have been
> > written if the buffer was too small, like snprintf(); this changes
> > string_get_size() to return the the return value of snprintf().
> 
> Unfortunately, snprintf doesn't return characters written, it return
> what it TRIED to write, and can cause a lot of problems[1]. This patch
> would be fine with me if the snprintf was also replaced by scnprintf,
> which will return the actual string length copied (or 0) *not* including
> the trailing %NUL.

Anyways, I can't use scnprintf here, printbufs/seq_buf both need the
number of characters that would have been written, but I'll update the
comment.