Comment 3 for bug 1749393

There seem to be two parts to this. Firstly, with a big reserved-region, which is the default for 32-bit-guest-on-64-bit-host, this code in main.c:

        if (reserved_va) {
            mmap_next_start = reserved_va;
        }

says to start trying for the next mmap address at the top of the reserved section, which is typically right at the top of the guest's address space. This means that for a PIE executable we'll try to load it at a very high address, which then means there's no space above the data section for the brk segment.

Secondly, for the no-reserved-region case (-R 0, or 64-on-64), we still fail, but this time because we've chosen to mmap the dynamic interpreter at an address just above the executable. Again, no space to expand the data segment and brk fails.

Linux kernel commit a87938b2e246b81 message says something about there being a guaranteed 128MB "gap" between data segment and stack on x86-64 which we're obviously not honourin; presumbably there's similar requirements for other archs. (As an aside, is bash really happy with only having perhaps 128MB of allocatable memory? Otherwise it really ought to use mmap rather than brk for its allocator.)