linux-hardening - Re: [PATCH v2] kdb: replace deprecated strncpy

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Message-ID: <CAFhGd8q99emm1SFJSs=L7Pn0A79vCtDHtTD5XUbVCNh=DDadaQ@mail.gmail.com>
Date: Mon, 8 Apr 2024 17:46:42 -0700
From: Justin Stitt <justinstitt@...gle.com>
To: Daniel Thompson <daniel.thompson@...aro.org>
Cc: Jason Wessel <jason.wessel@...driver.com>, Douglas Anderson <dianders@...omium.org>, 
	kgdb-bugreport@...ts.sourceforge.net, linux-kernel@...r.kernel.org, 
	linux-hardening@...r.kernel.org
Subject: Re: [PATCH v2] kdb: replace deprecated strncpy

On Fri, Apr 5, 2024 at 2:51 AM Daniel Thompson
<daniel.thompson@...aro.org> wrote:
>
> >                       len_tmp = strlen(p_tmp);
> > -                     strncpy(cp, p_tmp+len, len_tmp-len + 1);
> > +                     strscpy(cp, p_tmp+len, len_tmp-len + 1);
>
> Again, I really don't think the third argument provides the number of
> characters in the destination buffer.
>

Right, the third argument is the length of the "remaining" characters
from the completion point.

if you type "tes" and press tab then kallsyms_symbol_complete() will
populate p_tmp with "test". Prior to rendering to the user, @cp points
to "s", we need to catch the user up and print the rest of the symbol
name since they've already typed "tes" we only need to print out "t".

len_tmp is the length of the entire symbol part as returned by
kallsyms_symbol_complete() and len is the length of only the
user-typed symbol. Therefore, the amount of remaining characters to
print is given by len_tmp-len (and +1 for a NUL-byte).

So, yeah, you're right. This isn't the length of the destination but I
don't see why we can't use memcpy() (or strscpy()) and have this not
be considered "broken". The pointer arithmetic checks out.

I tested out strcat and other alternatives and they all seem less readable.

>
> Daniel.