Fix Base64#encode, exclude last 3 bytes from bswap

[kostya]

There is no specs, because I dont know how to really test guarded memory access, but test for various lengths percent 3 (from 0 to 7), already is: https://github.com/crystal-lang/crystal/blob/master/spec/std/base64_spec.cr#L7

[kostya]

Also possible remove bswap at all, and use this code in main loop also, but this seems 7% slower.

[didactic-drunk]

There is no specs, because I dont know how to really test guarded memory access, but test for various lengths percent 3 (from 0 to 7), already is: https://github.com/crystal-lang/crystal/blob/master/spec/std/base64_spec.cr#L7

If Mmap was part of std you could allocate your own guard page. The libc functions are already mapped. It just needs a small wrapper class. Is this something that would be accepted as a PR? @asterite

[asterite]

I think this is fine, thanks!

@didactic-drunk we already use mmap for fibers, so that's already a thing, though you have to use LibC.mmap I guess. Is that to allocate memory for the string that, if outside of range, will error?

[asterite]

Thank you!

[didactic-drunk]

@asterite Today's purpose is allocatong a guard page after the Slice and probably the Slice (Does an unallocated memory range exist after a random String? Probably not)

A general purpose Mmap class would at minimum hold the raw pointer and contain methods for changing protection levels, remapping, resizing, syncing and to_slice. I could use it to make Sodium like allocated memory regions with guard pages on each side without a Sodium dependency similar to memguard LockedBuffer. I have a much longer feature request on secret handling that requires this. The post is still WIP and may finish today.

This shard uses mmap to map files and would also benefit from a general purpose Mmap class.

[didactic-drunk]

Never mind. I'm withdrawing my mmap request. I'll make a shard and if necessary you can import it later.

[asterite]

Oh, I think I understand now what you mean by protrcting the heap. But we use boehm GC for allocating memory, I'm not sure how we can also use mmap. Plus string literals use ROM data. So I'm not sure it can help in this case. But I know nothing about mmap.

[sdogruyol]

Thank you @kostya 🙏

[didactic-drunk]

@asterite I'm not suggesting protecting the heap, but mmap block of memory and mmap a PROT_NONE guard page after it. Then provide the first mmap block as a Slice to Base64.

Pseudo code

size = 48
writable_page = mmap(starting_address, 4096, PROT_READ | PROT_WRITE)
guard_page = mmap(starting_address + 4096, 4096, PROT_NONE)
writable_addr = writable_page + (4096 - size)
slice = Slice.new writable_addr, size
Base64.encode slice

[straight-shoota]

Thanks, @kostya