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From:   Azeem Shaikh <azeemshaikh38@gmail.com>
Date:   Wed, 10 May 2023 16:48:23 -0400
Message-ID: <CADmuW3VngW3ineOjUNLYQT1UXsW-s46HjLKx0H3JyexKMtUhQA@mail.gmail.com>
Subject: Re: kernfs: Prefer strscpy over strlcpy calls
To:     Linus Torvalds <torvalds@linuxfoundation.org>
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Tejun Heo <tj@kernel.org>, security@kernel.org,
        linux-kernel@vger.kernel.org
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> Absolutely not.
>
> This makes the whole exercise pointless.
>
> The reason to use strscpy() is to *avoid* doing the strlen() on the
> source, and limit things to the limited size.
>
> If you need to do the strlen(), then use strlcpy(). It's a broken
> interface, but creating this kind of horror wrapper that does the same
> thing as strlcpy() is worse than just using the regular version.
>
> So the strscpy() conversion should *only* be done if the caller
> doesn't care about the difference in return values (or done *together*
> with changing the caller to use the nicer strscpy() return value).
>
> It's also worth noting that 'strscpy()' not only returns a negative
> error value when the string doesn't fit - it will also possibly do the
> copy one word at a time, and may write extra zeroes at the end of the
> destination (all within the given size, of course).
>
> So strscpy() is _different_ from strlcpy(), and the conversion should
> not be done unless those differences are ok.

Thanks Linus, that helps clarify a lot. I traced the usage of these
functions across the kernel and plan to do direct replacement only
where it's safe (see thread here:
https://lore.kernel.org/all/CADmuW3XiYpGK7BessXJWjGnnxZti_3mawDSX7QPawsfmATxCng@mail.gmail.com/).
Let me know if that works for you.