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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.
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