Re: [PATCH 1/1] lib/vsprintf: Implement ssprintf() to catch truncated strings

[Lee Jones <lee@kernel.org>]

On Tue, 30 Jan 2024, Kees Cook wrote:

> On Tue, Jan 30, 2024 at 04:18:42PM +0100, Rasmus Villemoes wrote:
> > So here scnprint() would have returned 1, leaving size at 1. scnprintf()
> > has the invariant that, for non-zero size, the return value is strictly
> > less than that size, so when passed a size of 1, all subsequent calls
> > return 0 (corresponding to the fact that all it could do was to write
> > the '\0' terminator).
> > 
> > This pattern already exists, and is really the reason scnprint exists.
> > Yes, scnprintf() cannot distinguish overflow from
> > it-just-exactly-fitted. Maybe it would have been better to make it work
> > like this, but I don't think there's a real use - and we do have
> > seq_buf() if one really wants an interface that can build a string
> > piece-meal while keeping track of whether it ever caused overflow.
> 
> Yeah, I think we can take the handful of places that really need to know
> about the overflow and can't reliably use scnprintf() and migrate them
> to the seq_buf API. It should be much easier to use now[1] too.
> 
> That way we won't add a new string API, and we can continue to remove
> snprintf.

This looks promising.  I'll have a look and get back to you.

> [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/include/linux/seq_buf.h?id=dcc4e5728eeaeda84878ca0018758cff1abfca21
>     https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/include/linux/seq_buf.h?id=7a8e9cdf9405819105ae7405cd91e482bf574b01

-- 
Lee Jones [李琼斯]