Re: RFC: Adding arch-specific user ABI documentation in linux-man

On Wed, May 06, 2020 at 12:47:17PM +0200, Michael Kerrisk (man-pages) wrote:

Hello Dave, et al.

On 5/4/20 5:32 PM, Dave Martin wrote:

quoted

Hi all,

I considering trying to plug some gaps in the arch-specific ABI
documentation in the linux man-pages, specifically for arm64 (and
possibly arm, where compat means we have some overlap).

Sounds good to me.

quoted

For arm64, there are now significant new extensions (Pointer
authentication, SVE, MTE etc.)  Currently there is some user-facing
documentation mixed in with the kernel-facing documentation in the
kernel tree, but this situation isn't ideal.

Do you have an opinion on where in the man-pages documentation should be
added, and how to structure it?


Affected areas include:

 * exec interface

Not sure what the details are here, so I have no opinion yet.
But probably, as additions to execve(2).

Having looked at that page again, probably yes.

This would include things like how arch-specific registers get reset,
and how various thread properties are reset/inherited.

Possibly, much of this would be documented by cross-referencing other
pages.

quoted

 * aux vector, hwcaps

==> getauxval(3)

Ah, yes.  I keep forgetting that because it's provided by libc.

There are a few things missing, but we can add them in there.

quoted

 * arch-specific signals
 * signal frame

Not sure what the details are here, so I have no opinion yet.

Stuff like architecture-specific si_codes and how they are used.

Signal handlers that poke about in the signal frame need to know how it
is structured, and for arm64 this is now quite complex.  I want to
document some example code for parsing it somewhere, and I feel that
will be too much noise for signal(7).

I guess I can try to write something and than we can figure out where to
put it.

quoted

 * mmap/mprotect extensions

See below.

quoted

 * prctl calls

As additions in prctl(2) would be fine, I think.

quoted

 * ptrace quirks and extensions

See below.

quoted

 * coredump contents

Not sure what the details are here, so I have no opinion yet.
Possibly as additions to core(5).

Things like arch-specific regsets.  Some of that can probabably live
in the ptrace page, with cross-references from core(5).

quoted

Not everything has an obvious home in an existing page, 

Yes.

quoted

and adding
specifics for every architecture could make some existing manpages very
unwieldy.

Still, I think it's worth adding details, especially for widely
used architectures.

Agreed.  If there's a lot of explanation required for somethings which
feels out of scope for the core page, then I might still be tempted to
push some of that out into a more arch-specific page and cross-reference
it.  We can play that by ear, though.

quoted

I think for some arch features, we really need some "overview" pages
too: just documenting the low-level details is of limited value
without some guide as to how to use them together.


Does the following sketch look reasonable?

 * man7/arm64.7: new page: overview of arm64-specific ABI extensions

I'm a little unclear on what would land in here, but sounds 
reasonable in principle.

I probably won't attempt to add such a page until I find a use for it.

I mainly intended this as an overview of what areas are impacted by the
architecture and which other pages to go look at.

quoted

 * man7/sve.7 (or man7/arm64-sve.7 or man7/sve.7arm64): new page:
   overview of arm64 SVE ABI

Sounds reasonable to me.

quoted

 * man2/arm64-ptrace.2 (or man2/ptrace.2arm64): new page:
   arm64 ptrace extensions

I think maybe better is: ptrace-arm64.2

Agreed.  That's probably easier to for people to place in their mental
map.

I'm agnostic about whether there should be a new page, or whether 
these should be added to ptrace(2). But, we could start with the
idea of a new page.

quoted

 * man2/mmap.2: extend with arm64-specific flags (only two flags, so we
   add them to the existing man page rather than creating a new one).

Sounds good to me

quoted

etc.


Ideally, I'd like to adopt a pattern that other arches can follow.

Well, if they do follow. Arch-specific documentation is woefully
thin at the moment. I'm not going to worry too much about the right
pattern (don't let perfect get in the way of good, yadda, yadda),
until I get so much arch-specific documentation that some refactoring
may be required. (I'm not going to hold my breath waiting for that
to happen ;-).)

Sure.  I don't want to do something obviously broken, but it sounds like
you're reasonably happy with my approach.  In any case I'll start with the
simple stuff.

Where it's easy to do so, I may try to trawl up some undocumented non-
ARM stuff, but I'm not the expert there...

Cheers
---Dave

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