linux-kernel - Re: Update of file offset on write() etc. is non-atomic with I/O

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20140304010517.GT18016@ZenIV.linux.org.uk>
Date:	Tue, 4 Mar 2014 01:05:17 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	George Spelvin <linux@...izon.com>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Update of file offset on write() etc. is non-atomic with I/O

On Mon, Mar 03, 2014 at 04:42:36PM -0800, Linus Torvalds wrote:
> On Mon, Mar 3, 2014 at 4:23 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
> >
> > I wonder if something like
> >
> > static inline struct fd fdget(int fd)
> > {
> >         unsigned long v = __fdget(fd);
> >         return (struct fd){(struct file *)(v & ~1), v & 1};
> > }
> 
> Make it use "&3" so that we have the two bits we need, and I'll
> happily take that approach, yes.

Hmm...  You want to have that second flag dealt with inside fs/file.c
as well?  OK, but that'll need one more thing - __attribute__((aligned(4)))
on struct file declaration.  We have at least one weird architecture that
is happy to align everything on 16bit boundaries.  Granted, it's half-dead,
but there might be something besides m68k with the same weirdness...

It's probably not worth replacing struct fd with typedef to unsigned long -
too easy to have it confused with a file descriptor itself and pass to
something that expects e.g. int.  In any case, since we leave fdget()
inlined, compiler will see the unsigned long it's been initialized with,
so if it decides that it's cheaper to replace f.file with v & ~3 through
the whole thing, keep v around and discard local struct fd completely,
it'll be able to do so just fine...

I'll play around with cross-builds a bit and see what falls out of all
that.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/