On Fri, Jan 9, 2015 at 4:13 PM, Rich Felker <dalias@xxxxxxxxxx> 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. 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. [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). -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
