[hotline1337]

Summary

• Replaced the single contiguous ELF staging buffer with one buffer per non-empty PT_LOAD segment in common/lib/elf.c.
• Validated usable memory, 4 GiB limits, and entry point placement per segment in the Multiboot 1 and 2 loaders.
• Preserved the full image span for Multiboot 2 relocation bounds while checking each segment independently.
• Added tests for widely separated and overlapping ELF segments.

Background & Problem

Kernels with disjoint ELF segments, such as seL4 with bootstrap headers at 1 MiB and kernel code at 16 MiB, can fail to boot under UEFI via Multiboot 1 or 2 (#704):

1. Synthetic single range: Limine represented the whole [min_paddr, max_paddr) span as one elsewhere_range, including gaps between loadable segments.
2. Reserved memory in gaps: check_usable_memory() rejected the entire range if firmware reserved memory in a gap, even when every segment's target was usable.
3. Staging buffer size: Sparse ELF images required a buffer covering the full span, increasing memory use with the size of the gaps.

Changes

1. common/lib/elf.c, common/lib/elf.h: Return a staging range for each non-empty PT_LOAD segment. Sort ranges by target address, reject overlapping loadable targets, and check for overlaps between a target and another segment's staging buffer.
2. common/protos/multiboot1.c: Check each kernel range against the 4 GiB limit and usable memory; validate the entry point and reserve each target range.
3. common/protos/multiboot2.c: Apply the relocation slide to all kernel ranges. Keep the full image span for relocation bounds, but check usable and loader memory per segment. Validate the 4 GiB limit and entry point, and reserve each target range.
4. checks/cases/multiboot_elf.py: Add Multiboot 1 and 2 boot cases with a distant header segment, plus hostile ELF cases with overlapping segments.

Verification & Testing

1. Build:
   • Compiled the BIOS and x86-64 UEFI targets.
2. Multiboot ELF checks:
   • Ran checks/run-checks -k 'multiboot_elf.*' on BIOS and x86-64 UEFI: 8/8 passed.
   • Covered disjoint-segment boots and hostile ELF inputs for Multiboot 1 and 2.

AI assistance disclosure

The fix was developed by a human, and verified with material AI help (gemini-4-carbon). The commit carries the corresponding Assisted-by: trailer.

Fixes #704