On Thu, Mar 19, 2020 at 4:25 PM Peter Maydell [off-list ref] wrote:

On Thu, 19 Mar 2020 at 15:13, Linus Walleij [off-list ref] wrote:

quoted

On Tue, Mar 17, 2020 at 12:58 PM Peter Maydell [off-list ref] wrote:

quoted

What in particular does this personality setting affect?
My copy of the personality(2) manpage just says:

       PER_LINUX32 (since Linux 2.2)
              [To be documented.]

which isn't very informative.

It's not a POSIX thing (not part of the Single Unix Specification)
so as with most Linux things it has some fuzzy semantics
defined by the community...

I usually just go to the source.

If we're going to decide that this is the way to say
"give me 32-bit semantics" we need to actually document
that and define in at least broad terms what we mean
by it, so that when new things are added that might or
might not check against the setting there is a reference
defining whether they should or not, and so that
userspace knows what it's opting into by setting the flag.
The kernel loves undocumented APIs but userspace
consumers of them are not so enamoured :-)

OK I guess we can at least take this opportunity to add
some kerneldoc to the include file.

As a concrete example, should "give me 32-bit semantics
via PER_LINUX32" mean "mmap should always return addresses
within 4GB" ? That would seem like it would make sense --

Incidentally that thing in particular has its own personality
flag (personalities are additive, it's a bit schizophrenic)
so PER_LINUX_32BIT is defined as:
PER_LINUX_32BIT =       0x0000 | ADDR_LIMIT_32BIT,
and that is specifically for limiting the address space to
32bit.

There is also PER_LINUX32_3GB for a 3GB lowmem
limit.

Since the personality is kind of additive, if
we want a flag *specifically* for indicating that we want
32bit hashes from the file system, there are bits left so we
can provide that.

Is this what we want to do? I just think we shouldn't
decide on that lightly as we will be using up personality
bug bits, but sometimes you have to use them.

PER_LINUX32 as it stands means 32bit personality
but very specifically does not include memory range
limitations since that has its own flags.

Yours,
Linus Walleij