Document kernel settings to run SheepShaver in Linux somewhere obvious

[rakslice]

To run SheepShaver in Linux, one that was made with the default configure addressing setting, requires changing the kernel setting vm.mmap_min_addr to 0.

Otherwise you get an error like:
$ SheepShaver
Cannot map Low Memory Globals: Operation not permitted.

It would be nice if this was documented somewhere obvious like the readme.

You can set the setting by echoing 0 to /proc/sys/vm/mmap_min_addr as root, or doing

sudo sysctl -w vm.mmap_min_addr=0

and you can make it happen automatically at startup by putting

vm.mmap_min_addr = 0

into /etc/sysctl.d/whatever.conf

Edit:
The kernel null deref protection afforded by mmap minimum address enforcement has defense in depth value. If we can make some working build other than a real addressing build be the default configure option, great.

This issue is just a practical matter: while real addressing is still the default configure option, without instructions, users encounter the error message and show up in the issue tracker for support.

[ianfixes]

Thanks for contributing this. Is this something that works in CI but fails locally? To say it another way, does the CI config need to be updated in some way to expose this issue?

[rakslice]

This is a setting that has to be set on the local machine where the the software is used, it has nothing to do with building.

[rakslice]

I've edited the issue a bit to try to clarify that.

[ianfixes]

If I'm understanding this correctly, you're talking about the proper way to run SheepShaver after it is already built

[rakslice]

Yes, that's correct.

[SegHaxx]

I highly recommend not messing with vm.mmap_min_addr or encouraging others to do so. It is the way it is for good reason. Compile with --enable-addressing=direct instead.

Linux, FreeBSD and probably every other unix OS are moving towards making real addressing impossible going forwards, as it is a security risk.

We should really change the default SheepShaver build to use direct addressing.

[rakslice]

Note that if you change the default addressing mode, for arches and compiler versions where the default is to build the JIT using pregenerated stuff, you'll want to reroll the pregenerated stuff to match the new addressing mode (which to some degree is an archaeology project for the taking AFAICT - see cebix/macemu#222) or at least make JIT not the default in the meantime.

[SegHaxx]

It'd be better to comment on my PR if you want me to change it. :)

[rakslice]

Oh, there was no PR on the issue when I opened the page. :D I'm surprised you got that far without running into a problem?