linux-hardening - Re: RFC for a new string-copy function, using mixtures of strlcpy and strscpy

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Date:   Mon, 8 Nov 2021 14:16:16 +0530
From:   Ajay Garg <ajaygargnsit@...il.com>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
Cc:     "andy@...nel.org" <andy@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "adobriyan@...il.com" <adobriyan@...il.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "linux-hardening@...r.kernel.org" <linux-hardening@...r.kernel.org>
Subject: Re: RFC for a new string-copy function, using mixtures of strlcpy and strscpy

On Mon, Nov 8, 2021 at 2:04 PM Andy Shevchenko <andy.shevchenko
> >
> > For example, in file fs/kernfs/dir.c, there are methods like
> > "kernfs_name_locked", "kernfs_path_from_node_locked" which simply
> > consume the return-value without any checks.
> >
> > All the above functions have a simple motive : copy as much bytes as
> > possible in the destination buffer, and then consume/return the number
> > of bytes actually copied (minus the null-terminator byte of course).
>
> Nope. Read the comment WRT strscpy().

Seems there is a confusion.

I meant the functions "kernfs_name_locked" and others in
fs/kernfs/dir.c, that use strlcpy and then simply consume/propogate
the return-value without any checks.


>
> > If checks are to be put in-place, it would be too much code/churn,
> > adding if checks all over the place.
>
> Yep, that's why in some cases where we know that there can't be
> overflow the checks are not present. In some cases it's historically
> like this, in some cases checks might be useful and so on. But no, we
> do not need more chaos in the string functions.
>

If the client knows that overflow cannot be there, it is better to use
the simple vanilla strcpy.
Using strlcpy means that the client believes there might be case when
src-buffer might be bigger.

Again, functions like "kernfs_name_locked" and others in
fs/kernfs/dir.c demonstrate that the client cannot be sure whether the
src-buffer is small enough to be fit into the dest-buffer.


Thanks and Regards,
Ajay