On 14.08.23 17:20, Oleg Nesterov wrote:
On 08/14, Oleg Nesterov wrote:
On 08/13, Mateusz Guzik wrote:
fe69d560b5bd ("kernel/fork: always deny write access to current MM
exe_file") added another lock trip to synchronize the state of exe_file
against fork, further defeating the point of xchg.
As such I think the atomic here only adds complexity for no benefit.
Just write-lock around the replacement.
Well, I tend to agree but can't really comment because I forgot everything
about these code paths.
But I have to admit that I don't understand the code in replace_mm_exe_file()
without this patch...
old_exe_file = xchg(&mm->exe_file, new_exe_file);
if (old_exe_file) {
/*
* Don't race with dup_mmap() getting the file and disallowing
* write access while someone might open the file writable.
*/
mmap_read_lock(mm);
allow_write_access(old_exe_file);
fput(old_exe_file);
mmap_read_unlock(mm);
}
Can someone please explain me which exactly race this mmap_read_lock() tries
to avoid and how ?
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.
--
Cheers,
David / dhildenb
