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:

quoted

On 11/24/2014 12:53 PM, David Drysdale wrote:

quoted

Signed-off-by: David Drysdale <redacted>
---
 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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