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

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Message-ID: <54B0D6E3.5030804@gmail.com>
Date:	Sat, 10 Jan 2015 08:38:11 +0100
From:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:	Rich Felker <dalias@...ifal.cx>
CC:	mtk.manpages@...il.com, David Drysdale <drysdale@...gle.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,
	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@...nel.org,
	linux-arch@...r.kernel.org, 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 05:13 PM, Rich Felker 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. It's AT_EXECFN,
> /proc/self/exe, and filenames shown elsewhere in /proc that may be
> derived in odd ways.
> 
> I would also move the text about O_CLOEXEC to a BUGS or NOTES section
> rather than the main description. The long-term intent should be that
> script execution this way should work. IIRC this was discussed earlier
> in the thread.

I agree, that something needs to be said. What I instead did was 
added "See BUGS" to the ENOEXEC error, and then this text:

   BUGS
       The  ENOENT  error  described above means that it is not possible
       possible to set the close-on-exec flag  on  the  file  descriptor
       given to a call of the form:

           execveat(fd, "", argv, envp, AT_EMPTY_PATH);

       However, the inability to set the close-on-exec flag means that a
       file descriptor referring to the  script  leaks  through  to  the
       script  itself.  As well as wasting a file descriptor, this leak‐
       age can lead to file-descriptor  exhaustion  in  scenarios  where
       scripts  recursively  employ  exceveat()  (or a future fexecve(3)
       implementation that might be based on execveat()).

Okay?

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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