On Wed, Nov 6, 2024 at 11:53 AM Noah Goldstein <goldstein....@gmail.com> wrote:
>
> On Wed, Nov 6, 2024 at 11:26 AM Richard Henderson
> <richard.hender...@linaro.org> wrote:
> >
> > On 11/6/24 17:03, Noah Goldstein wrote:
> > > On Wed, Nov 6, 2024 at 3:38 AM Richard Henderson
> > > <richard.hender...@linaro.org> wrote:
> > >>
> > >> On 11/5/24 23:54, Noah Goldstein wrote:
> > >>>>> You still need to handle is_proc_myself, for the guest binary.
> > >>>
> > >>> Would this by handled by basically do:
> > >>>
> > >>> ```
> > >>> if (is_proc_myself(p, "exe")) {
> > >>> exe = exec_path;
> > >>> if (through_qemu)
> > >>> argp[argp_offset] = exec_path;
> > >>> }
> > >>> ```
> > >>> Or am I missing something?
> > >>
> > >> Something like that, yes.
> > >>
> > >>>>> I wonder if those two cases are related. Do we need to also add an
> > >>>>> argument so that we
> > >>>>> can pass the executable to the next qemu via file descriptor? I.e.
> > >>>>> execvat becomes
> > >>>>>
> > >>>>> f = openat()
> > >>>>> execv(qemu, "-execfd", f)
> > >>>>>
> > >>>>> and is_proc_myself uses execfd, which we already have open.
> > >>>
> > >>> How does passing a fd from one process to another work?
> > >> As long as the fd is not marked O_CLOEXEC, it stays open in the new
> > >> process. Providing
> > >> the number via command-line, or whatever, is sufficient for the new
> > >> process to know what
> > >> is going on.
> > >
> > > Err I guess I was thinking its a bit weird having an option that is
> > > only really applicable
> > > if qemu is a child process. I.e the `-execfd` argument is not usable
> > > from commandline.
> >
> > qemu-foo -execfd 3 3< /some/file
> >
> > Or perhaps opened via other tooling.
> >
> > >> I now realize this is necessary for the AT_EMPTY_PATH flag, where we
> > >> only have the file
> > >> descriptor.
> > >
> > > We could also do something along the lines of:
> > >
> > > ```
> > > fd = openat(dirfd, exe);
> > > char new_exe[PATH_MAX];
> > > char fd_path[PATH_MAX];
> > > sprintf(fd_path, "/proc/self/fd/%d", fd);
> > > readlink(fd_path, new_exe, PATH_MAX);
> >
> > Reading the link doesn't always work.
> > Reading or passing the link means AT_SYMLINK_NOFOLLOW isn't honored.
>
> Okay, fair enough, I will get started on adding `-execfd`
Question about impl regarding handling of `-execfd` with/without a program name.
1) `-execfd` + program name ie: `qemu -execfd <some_fd> ls -a`.
2) `-execfd` without program name i.e: `qemu -execfd <some_fd> -a`.
Do you want to allow both of these? If you want to allow (1), what should
we use for `argv[0]`/`exec_path`. The program pass ("ls") or
`readlink(<some_fd>)`?
