linux-kernel - Re: [PATCHv10 man-pages 5/5] execveat.2: initial man page for execveat(2)

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Date:	Mon, 12 Jan 2015 11:33:49 +0000
From:	David Drysdale <drysdale@...gle.com>
To:	Rich Felker <dalias@...ifal.cx>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>,
	Andy Lutomirski <luto@...capital.net>,
	Meredydd Luff <meredydd@...atehouse.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Miller <davem@...emloft.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Stephen Rothwell <sfr@...b.auug.org.au>,
	Oleg Nesterov <oleg@...hat.com>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Kees Cook <keescook@...omium.org>,
	Arnd Bergmann <arnd@...db.de>,
	Christoph Hellwig <hch@...radead.org>, X86 ML <x86@...nel.org>,
	linux-arch <linux-arch@...r.kernel.org>,
	Linux API <linux-api@...r.kernel.org>,
	sparclinux@...r.kernel.org
Subject: Re: [PATCHv10 man-pages 5/5] execveat.2: initial man page for execveat(2)

On Sat, Jan 10, 2015 at 1:33 AM, Rich Felker <dalias@...ifal.cx> wrote:
> On Fri, Jan 09, 2015 at 07:17:41PM -0600, Eric W. Biederman wrote:
>> Rich Felker <dalias@...ifal.cx> writes:
>>
>> > I'm not proposing code because I'm a libc developer not a kernel
>> > developer. I know what's needed for userspace to provide a conforming
>> > fexecve to applications, not how to implement that on the kernel side,
>> > although I'm trying to provide constructive ideas. The hostility is
>> > really not necessary.
>>
>> Conforming to what?
>>
>> The open group fexecve says nothing about requiring a file descriptor
>> passed to fexecve to have O_CLOEXEC.
>
> It doesn't require it but it allows it, and in multithreaded programs
> that might run child processes (or library code that might be used in
> such situations), O_CLOEXEC is mandatory everywhere to avoid fd leaks.

As a naive idea related to Andy's suggestion elsewhere, could you
just have an environment convention for fexecve-ing scripts?  That
would reduce FD leaks without any need for kernel involvement/changes.

For example, set _FEXECVED_VIA_FD=4 but don't set
O_CLOEXEC before fexecve, and the interpreter reads then
closes that FD.  Or just get the interpreter to spot scripts named
"/dev/fd/%d" and read-then-close the FD that way, cf. Eric's suggestion
at https://lkml.org/lkml/2014/10/22/652.

By the way, FreeBSD has a fexecve(2) syscall that behaves
in the same way as the current Linux code for an O_CLOEXEC
script -- the interpreter fails to open "/dev/fd/6" as it's gone.
Do you know if there are any other OSes that already do
something more sophisticated for this case?
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