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@xxxxxxxxxx>
>>> ---
>>> 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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