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
Date: Sat, 30 May 2020 10:35:47 -0700 [thread overview]
Message-ID: <20200530173547.GA12299@sol.localdomain> (raw)
In-Reply-To: <20200530171814.GD19604@bombadil.infradead.org>
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?
- Eric
WARNING: multiple messages have this Message-ID (diff)
From: Eric Biggers <ebiggers@kernel.org>
To: Matthew Wilcox <willy@infradead.org>
Cc: Daniel Rosenberg <drosen@google.com>,
stable@vger.kernel.org, linux-f2fs-devel@lists.sourceforge.net,
Al Viro <viro@zeniv.linux.org.uk>,
linux-fsdevel@vger.kernel.org, linux-ext4@vger.kernel.org,
Gabriel Krisman Bertazi <krisman@collabora.co.uk>
Subject: Re: [f2fs-dev] [PATCH] ext4: avoid utf8_strncasecmp() with unstable name
Date: Sat, 30 May 2020 10:35:47 -0700 [thread overview]
Message-ID: <20200530173547.GA12299@sol.localdomain> (raw)
In-Reply-To: <20200530171814.GD19604@bombadil.infradead.org>
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?
- Eric
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Linux-f2fs-devel mailing list
Linux-f2fs-devel@lists.sourceforge.net
https://lists.sourceforge.net/lists/listinfo/linux-f2fs-devel
next prev parent reply other threads:[~2020-05-30 17:35 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-30 6:02 [PATCH] ext4: avoid utf8_strncasecmp() with unstable name Eric Biggers
2020-05-30 6:02 ` [f2fs-dev] " Eric Biggers
2020-05-30 6:17 ` Gabriel Krisman Bertazi
2020-05-30 6:17 ` [f2fs-dev] " Gabriel Krisman Bertazi
2020-05-30 6:44 ` Eric Biggers
2020-05-30 6:44 ` [f2fs-dev] " Eric Biggers
2020-05-30 6:52 ` Gabriel Krisman Bertazi
2020-05-30 17:18 ` Matthew Wilcox
2020-05-30 17:18 ` [f2fs-dev] " Matthew Wilcox
2020-05-30 17:35 ` Eric Biggers [this message]
2020-05-30 17:35 ` Eric Biggers
2020-05-30 17:59 ` Al Viro
2020-05-30 17:59 ` [f2fs-dev] " Al Viro
2020-06-01 6:45 ` Eric Biggers
2020-06-01 6:45 ` [f2fs-dev] " Eric Biggers
2020-05-30 20:41 ` Matthew Wilcox
2020-05-30 20:41 ` [f2fs-dev] " Matthew Wilcox
2020-06-01 7:05 ` Eric Biggers
2020-06-01 7:05 ` [f2fs-dev] " Eric Biggers
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