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From:   David Laight <David.Laight@ACULAB.COM>
To:     'Rasmus Villemoes' <linux@rasmusvillemoes.dk>,
        Joe Perches <joe@perches.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
CC:     Jonathan Corbet <corbet@lwn.net>, Stephen Kitt <steve@sk2.org>,
        Kees Cook <keescook@chromium.org>,
        Nitin Gote <nitin.r.gote@intel.com>,
        "jannh@google.com" <jannh@google.com>,
        "kernel-hardening@lists.openwall.com" 
        <kernel-hardening@lists.openwall.com>,
        Andrew Morton <akpm@linux-foundation.org>
Subject: RE: [PATCH 1/2] string: Add stracpy and stracpy_pad mechanisms
Thread-Topic: [PATCH 1/2] string: Add stracpy and stracpy_pad mechanisms
Thread-Index: AQHVQSOiPIn99bSetkamp+/6YQTlwKbYVhaA
Date:   Tue, 23 Jul 2019 15:41:27 +0000
Message-ID: <5ffdbf4f87054b47a2daf23a6afabecf@AcuMS.aculab.com>
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 <eec901c6-ca51-89e4-1887-1ccab0288bee@rasmusvillemoes.dk>
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From: Rasmus Villemoes
> Sent: 23 July 2019 07:56
...
> > +/**
> > + * stracpy - Copy a C-string into an array of char
> > + * @to: Where to copy the string, must be an array of char and not a pointer
> > + * @from: String to copy, may be a pointer or const char array
> > + *
> > + * Helper for strscpy.
> > + * Copies a maximum of sizeof(@to) bytes of @from with %NUL termination.
> > + *
> > + * Returns:
> > + * * The number of characters copied (not including the trailing %NUL)
> > + * * -E2BIG if @to is a zero size array.
> 
> Well, yes, but more importantly and generally: -E2BIG if the copy
> including %NUL didn't fit. [The zero size array thing could be made into
> a build bug for these stra* variants if one thinks that might actually
> occur in real code.]

Probably better is to return the size of the destination if the copy didn't fit
(zero if the buffer is zero length).
This allows code to do repeated:
	offset += str*cpy(buf + offset, src, sizeof buf - offset);
and do a final check for overflow after all the copies.

The same is true for a snprintf()like function

	David

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