Re: [PATCH RFC v3 00/10] extensible syscalls: CHECK_FIELDS to allow for easier feature detection

* Aleksa Sarai:

This is something that I've been thinking about for a while. We had a
discussion at LPC 2020 about this[1] but the proposals suggested there
never materialised.

In short, it is quite difficult for userspace to detect the feature
capability of syscalls at runtime. This is something a lot of programs
want to do, but they are forced to create elaborate scenarios to try to
figure out if a feature is supported without causing damage to the
system. For the vast majority of cases, each individual feature also
needs to be tested individually (because syscall results are
all-or-nothing), so testing even a single syscall's feature set can
easily inflate the startup time of programs.

This patchset implements the fairly minimal design I proposed in this
talk[2] and in some old LKML threads (though I can't find the exact
references ATM). The general flow looks like:

By the way, I have recently tried to document things from a glibc
perspective (which is a bit broader because we also have purely
userspace types):

  [PATCH RFC] manual: Document how types change
  <https://inbox.sourceware.org/libc-alpha/8734m4n1ij.fsf@oldenburg3.str.redhat.com/>

(This patch has not yet been reviewed.)