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

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Date:   Sat, 30 May 2020 13:41:32 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Eric Biggers <ebiggers@...nel.org>
Cc:     linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net, stable@...r.kernel.org,
        Al Viro <viro@...iv.linux.org.uk>,
        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 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?

void *memcpy_unsafe(void *dst, volatile void *src, __kernel_size_t);

It can just call memcpy() of course, but the compiler can't reason about
this function because it's not a stdlib function.