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Message-ID: <20200601064514.GC11054@sol.localdomain> 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
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