linux-ext4 - Re: [PATCH] ext4: avoid utf8_strncasecmp() with unstable name

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Date:   Sun, 31 May 2020 23:45:14 -0700
From:   Eric Biggers <ebiggers@...nel.org>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Matthew Wilcox <willy@...radead.org>, linux-ext4@...r.kernel.org,
        linux-fsdevel@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net, stable@...r.kernel.org,
        Daniel Rosenberg <drosen@...gle.com>,
        Gabriel Krisman Bertazi <krisman@...labora.co.uk>
Subject: Re: [PATCH] ext4: avoid utf8_strncasecmp() with unstable name

On Sat, May 30, 2020 at 06:59:07PM +0100, Al Viro wrote:
> On Sat, May 30, 2020 at 10:35:47AM -0700, Eric Biggers wrote:
> > On Sat, May 30, 2020 at 10:18:14AM -0700, Matthew Wilcox wrote:
> > > On Fri, May 29, 2020 at 11:02:16PM -0700, Eric Biggers wrote:
> > > > +	if (len <= DNAME_INLINE_LEN - 1) {
> > > > +		unsigned int i;
> > > > +
> > > > +		for (i = 0; i < len; i++)
> > > > +			strbuf[i] = READ_ONCE(str[i]);
> > > > +		strbuf[len] = 0;
> > > 
> > > This READ_ONCE is going to force the compiler to use byte accesses.
> > > What's wrong with using a plain memcpy()?
> > > 
> > 
> > It's undefined behavior when the source can be concurrently modified.
> > 
> > Compilers can assume that it's not, and remove the memcpy() (instead just using
> > the source data directly) if they can prove that the destination array is never
> > modified again before it goes out of scope.
> > 
> > Do you have any suggestions that don't involve undefined behavior?
> 
> Even memcpy(strbuf, (volatile void *)str, len)?  It's been a while since I've
> looked at these parts of C99...

That doesn't make sense.  memcpy() takes a non-volatile pointer, so the pointer
just gets implicitly cast back to (void *), and you get a compiler warning.

- Eric