On 01/09/2015 06:46 PM, David Drysdale wrote:

On Fri, Jan 9, 2015 at 4:13 PM, Rich Felker [off-list ref] wrote:

quoted

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.

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.]

quoted

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/