linux-kernel - Re: [PATCH] Introduce O

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Message-Id: <20070531162021.4cd4e91a.akpm@linux-foundation.org>
Date:	Thu, 31 May 2007 16:20:21 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Ulrich Drepper <drepper@...hat.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu,
	torvalds@...ux-foundation.org, Chris Zankel <chris@...kel.net>
Subject: Re: [PATCH] Introduce O_CLOEXEC (take >2)

On Thu, 31 May 2007 14:09:15 -0400
Ulrich Drepper <drepper@...hat.com> wrote:

> I've brought this topic up before but didn't provide a patch.  Well, here
> we go again, this time with a patch.  I even throw in a test program.
> 
> The problem is as follows: in multi-threaded code (or more correctly: all
> code using clone() with CLONE_FILES) we have a race when exec'ing.
> 
>    thread #1                       thread #2
> 
>    fd=open()
> 
>                                    fork + exec
> 
>   fcntl(fd,F_SETFD,FD_CLOEXEC)
> 
> In some applications this can happen frequently.  Take a web browser.  One
> thread opens a file and another thread starts, say, an external PDF viewer.
> The result can even be a security issue if that open file descriptor refers
> to a sensitive file and the external program can somehow be tricked into
> using that descriptor.
> 
> Just adding O_CLOEXEC support to open() doesn't solve the whole set of
> problems.  There are other ways to create file descriptors (socket,
> epoll_create, Unix domain socket transfer, etc).  These can and should
> be addressed separately though.  open() is such an easy case that it makes
> not much sense putting the fix off.
> 
> ...
>
> diff --git a/include/asm-generic/fcntl.h b/include/asm-generic/fcntl.h
> index c154b9d..b847741 100644
> --- a/include/asm-generic/fcntl.h
> +++ b/include/asm-generic/fcntl.h
> @@ -48,6 +48,9 @@
>  #ifndef O_NOATIME
>  #define O_NOATIME	01000000
>  #endif
> +#ifndef O_CLOEXEC
> +#define O_CLOEXEC	02000000	/* set close_on_exec */
> +#endif
>  #ifndef O_NDELAY
>  #define O_NDELAY	O_NONBLOCK
>  #endif

This will break xtensa, because that architecture (and only that
architecture) doesn't include asm-generic/fcntl.h from asm/fcntl.h.

But let's leave this patch as-is: it's xtensa which needs fixing.

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