Re: [PATCH v9] pidfd: add ioctl to retrieve pid info

[Jonathan Corbet <corbet@lwn.net>]

Aleksa Sarai <cyphar@cyphar.com> writes:

>> In fairness, this is how statx works and statx does this to not require
>> syscall retries to figure out what flags the current kernel supports and
>> instead defers that to stx_mask.
>> 
>> However, I think verifying the value is slightly less fragile -- as long
>> as we get a cheap way for userspace to check what flags are supported
>> (such as CHECK_FIELDS[1]). It would kind of suck if userspace would have
>> to do 50 syscalls to figure out what request_mask values are valid.
>
> Unfortunately, we probably need to find a different way to do
> CHECK_FIELDS for extensible-struct ioctls because CHECK_FIELDS uses the
> top bit in a u64 but we can't set a size that large with ioctl
> numbers...

Add a separate PIDFD_GET_VALID_REQUEST_MASK ioctl()?

But then I'm bad at designing interfaces...

jon