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

["Michael Kerrisk (man-pages)" <mtk.manpages-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

On 01/09/2015 06:46 PM, David Drysdale wrote:
> On Fri, Jan 9, 2015 at 4:13 PM, Rich Felker <dalias-/miJ2pyFWUyWIDz0JBNUog@public.gmane.org> 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-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>
>>>> ---
>>>>  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.

Yep, got it now.

> [As an aside, IIRC the filename does get put into the new
> process's memory, up above the environment strings -- but
> that copy isn't visible via argv nor envp.]
> 
>> 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 may be misremembering, but I thought we hoped to be able to fix
> execveat of a script without /proc in future, but didn't expect to fix
> execveat of a script via an O_CLOEXEC fd (because in the latter
> case the fd gets closed before the script interpreter runs, so even
> if the interpreter (or a special filesystem) does clever things for names
> starting with "/dev/fd/..." the file descriptor is already gone).

See my other replies (and of course, Rich's). It does seem there is 
a real problem to be solved here.

Thanks,

Michael


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