ELF64 Loader

Document Revision: 26h1.0
Source: arch/s390x/init/zxfl/common/elfload.c

1. Overview

zxfl_load_elf64 loads the kernel ELF64 image from DASD into physical memory. It processes only PT_LOAD program headers; all other segment types are ignored.

2. Load Sequence

zxfl_load_elf64(schid, dataset_name, load_base_out)
  │
  ├─ Read ELF header (first 64 bytes)
  ├─ Validate: magic 0x7F 'E' 'L' 'F', EI_CLASS=2 (64-bit),
  │            EI_DATA=2 (big-endian), e_machine=0x16 (s390)
  ├─ Read program header table (e_phoff, e_phnum entries)
  ├─ For each PT_LOAD segment:
  │    ├─ Compute physical load address:
  │    │    pa = p_paddr − CONFIG_KERNEL_VIRT_OFFSET
  │    ├─ Read p_filesz bytes from file offset p_offset → pa
  │    └─ Zero-fill [pa + p_filesz, pa + p_memsz)
  └─ Return load_min (lowest p_paddr seen, stripped of HHDM offset)

3. Address Computation

The kernel is linked with virtual addresses in the HHDM range (p_vaddr ≥ 0xFFFF800000000000). The physical load address is derived by subtracting CONFIG_KERNEL_VIRT_OFFSET:

$$pa = p_paddr - \texttt{CONFIG_KERNEL_VIRT_OFFSET}$$

The loader does not use p_vaddr directly; it uses p_paddr to avoid ambiguity when the linker script sets AT() addresses.

4. Constraints

  • The kernel ELF must be ET_EXEC (executable, not shared object).
  • e_machine must be 0x16 (EM_S390). Any other value causes an immediate panic.
  • All PT_LOAD segments must have p_paddr ≥ CONFIG_KERNEL_VIRT_OFFSET. A segment below the HHDM offset is rejected.
  • The kernel entry point (e_entry) must be ≥ 0xFFFF800000040000 (HHDM + 256 KB). The loader enforces this before the final jump.
  • The total loaded image (all PT_LOAD segments) must fit within the memory probed by the write-pattern test.

5. BSS Zeroing

Segments where p_memsz > p_filesz have a BSS tail. The loader zeros this region with memset immediately after reading the file data. This ensures the kernel's BSS is clean before any ZXVL verification.