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SubjectRe: execve(NULL, argv, envp) for nommu?
On 09/12, Rob Landley wrote:
>
> On 09/11/2017 10:15 AM, Oleg Nesterov wrote:
> > On 09/08, Rob Landley wrote:
> >>
> >> So is exec(NULL, argv, envp) a reasonable thing to want?
> >
> > I think that something like prctl(PR_OPEN_EXE_FILE) which does
> >
> > dentry_open(current->mm->exe_file->path, O_PATH)
> >
> > and returns fd make more sense.
> >
> > Then you can do execveat(fd, "", ..., AT_EMPTY_PATH).
> I'm all for it? That sounds like a cosmetic difference, a more verbose
> way of achieving the same outcome.

Simpler to implement. Something like the (untested) patch below. Not sure
it is correct, not sure it is good idea, etc.

> (Of course now you've got a filehandle you can read xattrs and such
> through from otherwise jailed contexts letting you do things you
> couldn't necessarily do before,

I can be easily wrong, this is not my area, but afaics no. Note that
you get the FMODE_PATH file (see O_PATH), you can do almost nothing
with it.

So. IIUC with this patch you can do

fd = prctl(PR_OPEN_EXE_FILE);

execveat(fd, "", NULL, NULL, AT_EMPTY_PATH);

and execveat should succeed even if the binary was unlinked/renamed in
between.

otoh it should fail if, say, you do "chmod a-x exename" in between.

However. This won't work after chroot() so I am not sure this solves your
problems.

> but I assume you know the security
> implications of that more than I do.

Unlikely ;)


> > But to be honest, I can't understand the problem, because I know nothing
> > about nommu.
> >
> > You need to unblock parent sleeping in vfork(), and you can't do another
> > fork (I don't undestand why).
>
> A nommu system doesn't have a memory management unit, so all addresses
> are physical addresses. This means two processes can't see different
> things at the same address: either they see the same thing or one of
> them can't see that address (due to a range register making it).

Yes, yes, I understand, and thanks for your detailed explanation...

> > Perhaps the child can create another thread? The main thread can exit
> > after that and unblock the parent. Or perhaps even something like
> > clone(CLONE_VM | CLONE_PARENT), I dunno...
>
> Launching a new thread doesn't unblock the parent.

Well, this doesn't really matter, but see above, the main thread can exit
after that. This should unblock the parent.

> And even without that, we're still in the "vfork but add concurrency"
> territory. Your threads don't have their own independent mappings,

Of course!

Just I misinterpreted your initial email as if this is fine for your
use-case, and all you need is unblock the parent and nothing else.

Oleg.
---


--- x/kernel/sys.c
+++ x/kernel/sys.c
@@ -2183,6 +2183,40 @@ static int propagate_has_child_subreaper(struct task_struct *p, void *data)
return 1;
}

+static int open_mm_exe_file(void)
+{
+ struct file *exe_file, *file;
+ struct path *path;
+ int fd = -ENOENT;
+
+ exe_file = get_mm_exe_file(current->mm);
+ if (!exe_file)
+ goto out;
+
+ path = &exe_file->f_path;
+ if (!path->dentry)
+ goto put_exe_file;
+
+ fd = get_unused_fd_flags(O_CLOEXEC); // flags?
+ if (fd < 0)
+ goto put_exe_file;
+
+ file = dentry_open(path, O_PATH, current_cred());
+ if (IS_ERR(file)) {
+ put_unused_fd(fd);
+ fd = PTR_ERR(file);
+ goto put_exe_file;
+ }
+
+ path_get(path);
+ fd_install(fd, file);
+
+put_exe_file:
+ fput(exe_file);
+out:
+ return fd;
+}
+
SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
unsigned long, arg4, unsigned long, arg5)
{
@@ -2196,6 +2230,9 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,

error = 0;
switch (option) {
+ case PR_OPEN_EXE_FILE:
+ error = open_mm_exe_file();
+ break;
case PR_SET_PDEATHSIG:
if (!valid_signal(arg2)) {
error = -EINVAL;
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