Re: [PATCH] kernel/fork: stop playing lockless games for exe_file replacement

Oleg Nesterov
2023-08-14T15:59:10Z

On 08/14, David Hildenbrand wrote:
>
> >OK, I seem to understand... without mmap_read_lock() it is possible that
> >
> > - dup_mm_exe_file() sees mm->exe_file = old_exe_file
> >
> > - replace_mm_exe_file() does allow_write_access(old_exe_file)
> >
> > - another process does get_write_access(old_exe_file)
> >
> > - dup_mm_exe_file()->deny_write_access() fails
> >
> >Right?
>
> From what I recall, yes.

Thanks! but then... David, this all is subjective, feel free to ignore, but
the current code doesn't look good to me, I mean the purpose of mmap_read_lock()
is very unclear. To me something like

if (old_exe_file) {
/*
* Ensure that if we race with dup_mm_exe_file() and it sees
* mm->exe_file == old_exe_file deny_write_access(old_exe_file)
* can't fail after we do allow_write_access() and another task
* does get_write_access(old_exe_file).
*/
mmap_read_lock(mm);
mmap_read_unlock(mm);

allow_write_access(old_exe_file);
fput(old_exe_file);
}

looks more understandable...

But this patch from Mateusz looks even better to me. So, FWIW,

Acked-by: Oleg Nesterov <oleg@redhat.com>