linux-kernel - Re: [PATCH v2] unicode: don't write -1 after NUL terminator

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Message-ID: <Y2kfxmf3x5bF6tkR@zx2c4.com>
Date:   Mon, 7 Nov 2022 16:09:58 +0100
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Gabriel Krisman Bertazi <krisman@...e.de>
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        krisman@...labora.com, jirislaby@...nel.org, stable@...r.kernel.org
Subject: Re: [PATCH v2] unicode: don't write -1 after NUL terminator

Hi Gabriel,

On Mon, Nov 07, 2022 at 09:45:25AM -0500, Gabriel Krisman Bertazi wrote:
> "Jason A. Donenfeld" <Jason@...c4.com> writes:
> 
> > If the intention is to overwrite the first NUL with a -1, s[strlen(s)]
> > is the first NUL, not s[strlen(s)+1].
> 
> Hi Jason,
> 
> This code is part of the verification of the trie that done at the end
> of utf8data generation. It is making sure the tree is not corrupted, by
> ensuring that utf8byte doesn't see something past the correct end of the
> string (the first NULL byte).  Note it is not a bad memory access
> either, since we guarantee to have allocated enough space.
> 
> So I think the code is correct as is. if you apply your patch and
> regenerate utf8data.h_shipped, utf8byte will reach that -1 and fail the
> verification.

Ah, okay. "Replace NUL" would seem to be wrong/confusing comment text I
suppose. Thanks for the explanation anyhow, and sorry for the noise.

Jason

> 
> > Cc: Gabriel Krisman Bertazi <krisman@...labora.com>
> > Cc: stable@...r.kernel.org
> > Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>
> > ---
> >  fs/unicode/mkutf8data.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/fs/unicode/mkutf8data.c b/fs/unicode/mkutf8data.c
> > index bc1a7c8b5c8d..61800e0d3226 100644
> > --- a/fs/unicode/mkutf8data.c
> > +++ b/fs/unicode/mkutf8data.c
> > @@ -3194,7 +3194,7 @@ static int normalize_line(struct tree *tree)
> >  	/* Second test: length-limited string. */
> >  	s = buf2;
> >  	/* Replace NUL with a value that will cause an error if seen. */
> > -	s[strlen(s) + 1] = -1;
> > +	s[strlen(s)] = -1;
> >  	t = buf3;
> >  	if (utf8cursor(&u8c, tree, s))
> >  		return -1;
> 
> -- 
> Gabriel Krisman Bertazi