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

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Date:   Wed, 12 Jul 2023 15:38:44 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Kent Overstreet <kent.overstreet@...ux.dev>
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 04:19:31PM -0400, Kent Overstreet 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.
> 
> ...All of which would be solved if we were converting code away from raw
> char * buffers to a proper string building type.
> 
> Which I tried to address when I tried to push printbufs upstream, but
> that turned into a giant exercise in frustration in dealing with
> maintainers.

Heh, yeah, I've been trying to aim people at using seq_buf instead of
a long series of snprintf/strlcat/etc calls. Where can I look at how
you wired this up to seq_buf/printbuf? I had trouble finding it when I
looked before. I'd really like to find a way to do it without leaving
around foot-guns for future callers of string_get_size(). :)

I found the printbuf series:
https://lore.kernel.org/lkml/20220808024128.3219082-1-willy@infradead.org/
It seems there are some nice improvements in there. It'd be really nice
if seq_buf could just grow those changes. Adding a static version of
seq_buf_init to be used like you have PRINTBUF_EXTERN would be nice
(or even a statically sized initializer). And much of the conversions
is just changing types and functions. If we can leave all that alone,
things become MUCH easier to review, etc, etc. I'd *love* to see an
incremental improvement for seq_buf, especially the heap-allocation
part.

-- 
Kees Cook