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

[Matthew Wilcox <willy@infradead.org>]

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.