Returning error if offset doesn't fit into libc::off_t.

[lvella]

Slightly improves the situation of issue #85.

[RazrFalcon]

I guess the map_offset should be updated to #[should_panic] on 32bit Linux.

[adamreichold]

I just noticed that libc actually defines mmap64 taking off64_t. I think we should just use that function and fix this for good.

[RazrFalcon]

@adamreichold Looks like it's not available for musl? And requires _FILE_OFFSET_BITS flag check?

[adamreichold]

Looks like it's not available for musl?

The libc sources say

// * musl has 64-bit versions only so aliases the LFS64 symbols to the standard ones

And requires _FILE_OFFSET_BITS flag check?

I though this macro only controls whether these 64-bit wide interface replace the default interfaces or stay add-ons? But they are always available as add-ons (for glibc)?

In any case, I'd say it would be preferable to use #[cfg(..)] to make this work on those platforms where it can and fallback to try_into otherwise.

[RazrFalcon]

The libc sources say

I haven't been able to understand what they mean. But I would assume that mmap64 isn't available in musl and we have to use mmap, which should already support 64bit offset?

I will try adding musl target to CI.

[adamreichold]

I haven't been able to understand what they mean. But I would assume that mmap64 isn't available in musl and we have to use mmap, which should already support 64bit offset?

Yes, my understanding is that Musl v1.2+ on 32-bit platforms has mem::size_of::<off_t>() == 8 and hence this problem does not affect it and we can just continue to call mmap. It is only Glibc on 32-bit platforms which is problematic as per the OP.

[adamreichold]

Relevant part of Musl FAQ: https://wiki.musl-libc.org/faq.html#Q:-Do-I-need-to-define-%3Ccode%3E_LARGEFILE64_SOURCE%3C/code%3E-to-get-64bit-%3Ccode%3Eoff_t%3C/code%3E?

[lvella]

The logic could be: use mmap if off_t is 64 bits, use mmap64 if available and off_t is less than 64 bits, otherwise use mmap with the overflow runtime check of this PR.

Problem is, I have no idea how to do that. I tried detecting if mmap64 is available in a build script using crate autocfg, but I could not make it use libc in the test built files.

[RazrFalcon]

@lvella Too complicated. Let's try using mmap64 for now and see how it will go.