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Date:   Mon, 1 Jun 2020 00:05:26 -0700
From:   Eric Biggers <ebiggers@kernel.org>
To:     Matthew Wilcox <willy@infradead.org>
Cc:     linux-ext4@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-f2fs-devel@lists.sourceforge.net, stable@vger.kernel.org,
        Al Viro <viro@zeniv.linux.org.uk>,
        Daniel Rosenberg <drosen@google.com>,
        Gabriel Krisman Bertazi <krisman@collabora.co.uk>
Subject: Re: [PATCH] ext4: avoid utf8_strncasecmp() with unstable name
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On Sat, May 30, 2020 at 01:41:32PM -0700, Matthew Wilcox 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?
> 
> 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.

The compiler can still reason about it if it's in the same file, if it's an
inline function, or if link-time-optimization is enabled.  (LTO isn't yet
supported by the mainline kernel, but people have been working on it.)

Also, as I mentioned to Al, it's necessary to cast away 'volatile' to call
memcpy().  So the 'volatile' serves no purpose.

How about using barrier(), which expands to  asm("" : : : "memory") to tell the
compiler that memory was clobbered?

        if (len <= DNAME_INLINE_LEN - 1) {
                memcpy(strbuf, str, len);
                strbuf[len] = 0;
                /* prevent compiler from optimizing out the temporary buffer */
                barrier();
        }

I think it's still technically undefined to call memcpy() on concurrently
modified memory at all, but I think the above would be okay in practice...

Using 'noinline' could be another option.

- Eric