[PATCH v4] linux: Add openat2 (BZ 31664)

[Paul Eggert]

On 2025-08-25 15:28, Aleksa Sarai wrote:
> CHECK_FIELDS is an extension I proposed to most extensible-struct
> syscalls that would let userspace query which bit patterns are valid

I don't see how such a feature would work in general. If the struct has 
N bits, there are 2**N possible bit patterns and there's no way in 
general to find out which are valid without using O(2**N) time. One 
cannot simply ask "which bits can I turn on?" because that would imply 
that some forbidden bit patterns are allowed. For example, a valid call 
cannot set both O_DIRECTORY and O_CREAT, but a simple query saying "What 
bits can I set?" must yield both those bits.

Even if there's something along these lines that might work in the 
future, it's confusing that the read-only argument is 'struct open_how 
*' now.

The longstanding tradition in syscalls that get and set state is to have 
two arguments, one for getting and one for setting. (I am thinking of 
syscalls like sigaction and sigprocmask.) If openat2 is intended for 
both getting and setting it should have two arguments, not one.