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

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Message-ID: <54B0E282.7090203@gmail.com>
Date:	Sat, 10 Jan 2015 09:27:46 +0100
From:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:	David Drysdale <drysdale@...gle.com>,
	Rich Felker <dalias@...ifal.cx>
CC:	mtk.manpages@...il.com,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Andy Lutomirski <luto@...capital.net>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	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 01/09/2015 06:46 PM, David Drysdale wrote:
> On Fri, Jan 9, 2015 at 4:13 PM, Rich Felker <dalias@...ifal.cx> wrote:
>> On Fri, Jan 09, 2015 at 04:47:31PM +0100, Michael Kerrisk (man-pages) wrote:
>>> On 11/24/2014 12:53 PM, David Drysdale wrote:
>>>> Signed-off-by: David Drysdale <drysdale@...gle.com>
>>>> ---
>>>>  man2/execveat.2 | 153 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>>>>  1 file changed, 153 insertions(+)
>>>>  create mode 100644 man2/execveat.2
>>>
>>> David,
>>>
>>> Thanks for the very nicely prepared man page. I've done
>>> a few very light edits, and will release the version below
>>> with the next man-pages release.
>>>
>>> I have one question. In the message accompanying
>>> commit 51f39a1f0cea1cacf8c787f652f26dfee9611874 you wrote:
>>>
>>>   The filename fed to the executed program as argv[0] (or the name of the
>>>   script fed to a script interpreter) will be of the form "/dev/fd/<fd>"
>>>   (for an empty filename) or "/dev/fd/<fd>/<filename>", effectively
>>>   reflecting how the executable was found.  This does however mean that
>>>   execution of a script in a /proc-less environment won't work; also, script
>>>   execution via an O_CLOEXEC file descriptor fails (as the file will not be
>>>   accessible after exec).
>>>
>>> How does one produce this situation where the execed program sees
>>> argv[0] as a /dev/fd path? (i.e., what would the execveat()
>>> call look like?) I tried to produce this scenario, but could not.
>>
>> I think this is wrong. argv[0] is an arbitrary string provided by the
>> caller and would never be derived from the fd passed.
> 
> Yeah, I think I just wrote that wrong, it's only relevant for scripts.
> As Rich says, for normal binaries argv[0] is just the argv[0] that
> was passed into the execve[at] call.  For a script, the code in
> fs/binfmt_script.c will remove the original argv[0] and put the
> interpreter name and the script filename (e.g. "/bin/sh",
> "/dev/fd/6/script") in as 2 arguments in its place.

So, on reflection, I think it's worth saying something about this, and 
I added the following text to the man page:

   NOTES
       When asked to execute a script file, the argv[0] that  is  passed
       to  the  script  interpreter is a string of the form /dev/fd/N or
       /dev/fd/N/P, where N is the number of the file descriptor  passed
       via  the  dirfd argument.  A string of the first form occurs when
       AT_EMPTY_PATH is employed.  A string of the  second  form  occurs
       when the script is specified via both dirfd and pathname; in this
       case, P is the value given in pathname.

Thanks,

Michael


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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