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

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Message-ID: <87y12x7wzt.fsf@trenco.lwn.net>
Date: Wed, 09 Oct 2024 16:05:42 -0600
From: Jonathan Corbet <corbet@....net>
To: Aleksa Sarai <cyphar@...har.com>
Cc: luca.boccassi@...il.com, linux-fsdevel@...r.kernel.org,
 christian@...uner.io, linux-kernel@...r.kernel.org, oleg@...hat.com
Subject: Re: [PATCH v9] pidfd: add ioctl to retrieve pid info

Aleksa Sarai <cyphar@...har.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