Cheri ELF
Reference
gABI
Morello AArch64 ABI
ELF for dynamic linking
As in gABI(retrieve date 20221117)
Note Section
| Type |
Name |
| 0x0 |
[NT_CHERI_GLOBALS_ABI] |
| 0x1 |
[NT_CHERI_TLS_ABI] |
| 0x80000000 - 0xffffffff |
– (Reserved for processor-specific use) |
NT_CHERI_GLOBALS_ABI, this note describes the ABI variant in use for accessing globals. Capabilities for globals can be obtained in the following different ways.
- 0x0: CHERI_GLOBALS_ABI_PCREL. By indexing a table relative to the program counter.
- 0x1: CHERI_GLOBALS_ABI_PLT_FPTR. By indexing a table pointed to by a reserved register or equivalent that is defined on entry to functions.
- Function pointers are sealed entry capabilities to trampolines that install the correct value for the target.
- 0x2: CHERI_GLOBALS_ABI_FDESC. By indexing a table pointed to by a reserved register or equivalent that is defined on entry to functions.
- Function pointers are capabilities to function descriptors, the format of which is processor-specific.
NT_CHERI_TLS_ABI, this note describes the ABI variant in use for accessing thread-locals.
- 0x0: CHERI_TLS_ABI_TRAD. Capabilities for thread-locals are obtained using traditional TLS Variant I or II with pointers implemented using capabilities.
ARM Morello
RISC-V
MIPS
([CheriABI TR932] ch4.6) The dynamic linker (RTLD) is extended to initialize external symbol references using new dynamic relocations that initialize and bound the capability.
- a new ELF relocation
R_MIPS_CHERI_CAPABILITY for external symbol references;
- for local (non-preemptible) symbols, use a special section (__cap_relocs)
- GOT (Global offset table) in binary contains global variables and functions. CheriABI changes this table to contain tightly bounded capabilities:
- parse ELF symbol table and use the
st_size field in the Elf_Sym structure as the size of global variables.
st_size is emitted by the compiler and will correspond to the size of the C/C++ declaration; and for functions it is the size of all instructions in that function.- However, “we only use exact bounds for data symbols and bound function symbols to the containing shared object’s code segment”. This wide bound for code will :
- preserve the ability of code to use branches in place of jumps between functions.
- facilitate the existing practice of referencing global variables using program-counter-relative addressing.
- Same logic for
dlsym() API.
readelf -a .exe
Examples: hello.elf kernel.elf