# Stack unwinding The main usage of stack unwinding is: * To obtain a stack trace for debugger, crash reporter, profiler, garbage collector, etc. * With personality routines and language specific data area, to implement C++ exceptions (Itanium C++ ABI). See [C++ exception handling ABI](maskray-3.md) Stack unwinding tasks can be divided into two categories: * synchronous: triggered by the program itself, C++ throw, get its own stack trace, etc. This type of stack unwinding only occurs at the function call (in the function body, it will not appear in the prologue/epilogue) * asynchronous: triggered by a garbage collector, signals or an external program, this kind of stack unwinding can happen in function prologue/epilogue ## Frame pointer The most classic and simplest stack unwinding is based on the frame pointer: fix a register as the frame pointer (RBP on x86-64), put the frame pointer in the stack frame at the function prologue, and update the frame pointer to the address of the saved frame pointer. The frame pointer and its saved values in the stack form a singly linked list. After obtaining the initial frame pointer value (`__builtin_frame_address`), dereference the frame pointer continuously to get the frame pointer values of all stack frames. This method is not applicable to some instructions in the prologue/epilogue. ``` pushq %rbp movq %rsp, %rbp # after this, RBP references the current frame ... popq %rbp retq # RBP references the previous frame ``` ```c #include [[gnu::noinline]] void qux() { void **fp = __builtin_frame_address(0); for (;;) { printf("%p\n", fp); void **next_fp = *fp; if (next_fp <= fp) break; fp = next_fp; } } [[gnu::noinline]] void bar() { qux(); } [[gnu::noinline]] void foo() { bar(); } int main() { foo(); } ``` The frame pointer-based method is simple, but has several drawbacks. When the above code is compiled with `-O1` or above, foo and bar will have tail calls, and the program output will not include the stack frame of foo and bar (`-fomit-leaf-frame-pointer` does not hinder the tail call). In practice, it is not guaranteed that all libraries contain frame pointers. When unwinding a thread, it is necessary to check whether `next_fp` is like a stack address before dereferencing it to prevent segfaults. One way to check page accessibility is to parse `/proc/*/maps` to determine whether the address is readable (slow). There is a smart trick: ```c // Or use the write end of a pipe. int fd = open("/dev/random", O_WRONLY); if (write(fd, address, 1) < 0) // not readable ``` In addition, reserving a register for the frame pointer will increase text size and have negative performance impact (prologue, epilogue additional instruction overhead and register pressure caused by one fewer register), which may be quite significant on x86-32 which lack registers. On an architecture with relatively sufficient registers, e.g. x86-64, the performance loss can be more than 1%. ### Compiler behavior * -O0: Default `-fno-omit-frame-pointer`, all functions have frame pointer * -O1 or above: Preset `-fomit-frame-pointer`, set frame pointer only if necessary. Specify `-fno-omit-leaf-frame-pointer` to get a similar effect to -O0. You can additionally specify `-momti-leaf-frame-pointer` to remove the frame pointer of leaf functions ## libunwind C++ exception and stack unwinding of profiler/crash reporter usually use libunwind API and DWARF Call Frame Information. In the 1990s, Hewlett-Packard defined a set of libunwind API, which is divided into two categories: * `unw_*`: The entry points are `unw_init_local` (local unwinding, current process) and `unw_init_remote` (remote unwinding, other processes). Applications that usually use libunwind use this API. For example, Linux perf will call `unw_init_remote` * `_Unwind_*`: This part is standardized as Level 1: Base ABI of [Itanium C++ ABI: Exception Handling](https://itanium-cxx-abi.github.io/cxx-abi/abi-eh.html). The Level 2 C++ ABI calls these `_Unwind_*` APIs. Among them, `_Unwind_Resume` is the only API that is directly called by C++ compiled code. `_Unwind_Backtrace` is used by a few applications to obtain stack traces. Other functions are called by libsupc++/libc++abi `__cxa_*` functions and `__gxx_personality_v0`. Hewlett-Packard has open sourced https://www.nongnu.org/libunwind/ (in addition to many projects called "libunwind"). The common implementations of this API on Linux are: * libgcc/unwind-\* (`libgcc_s.so.1` or `libgcc_eh.a`): Implemented `_Unwind_*` and introduced some extensions: `_Unwind_Resume_or_Rethrow`, `_Unwind_FindEnclosingFunction`, `__register_frame` etc. * llvm-project/libunwind (`libunwind.so` or `libunwind.a`) is a simplified implementation of HP API, which provides part of `unw_*`, but does not implement `unw_init_remote`. Part of the code is taken from ld64. If you use Clang, you can use `--rtlib=compiler-rt --unwindlib=libunwind` to choose * glibc's internal implementation of `_Unwind_Find_FDE`, usually not exported, and related to `__register_frame_info` ## DWARF Call Frame Information The unwind instructions required by different areas of the program are described by DWARF Call Frame Information (CFI) and stored by `.eh_frame` on the ELF platform. Compiler/assembler/linker/libunwind provides corresponding support. `.eh_frame` is composed of Common Information Entry (CIE) and Frame Description Entry (FDE). CIE has these fields: * `length` * `CIE_id`: Constant 0. This field is used to distinguish CIE and FDE. In FDE, this field is non-zero, representing `CIE_pointer` * `version`: Constant 1 * `augmentation_string`: A string describing the CIE/FDE parameter list. The `P` character indicates the personality routine pointer; the `L` character indicates that the augmentation data of the FDE stores the language-specific data area (LSDA) * `address_size`: Generally 4 or 8 * `segment_selector_size`: For x86 * `code_alignment_factor`: Assuming that the instruction length is a multiple of 2 or 4 (for RISC), it affects the multiplier of parameters such as `DW_CFA_advance_loc` * `data_alignment_factor`: The multiplier that affects parameters such as `DW_CFA_offset` `DW_CFA_val_offset` * `return_address_register` * `augmentation_data_length` * `augmentation_data`: personality * `initial_instructions`: bytecode for unwinding, a common prefix used by all FDEs using this CIE * padding Each FDE has an associated CIE. FDE has these fields: * `length`: The length of FDE itself. If it is `0xffffffff`, the next 8 bytes (`extended_length`) record the actual length. Unless specially constructed, `extended_length` is not used * `CIE_pointer`: Subtract CIE_pointer from the current position to get the associated CIE * `initial_location`: The address of the first location described by the FDE. There is a relocation referring to the section symbol in .o * `address_range`: initial_location and address_range describe an address range * `instructions`: bytecode for unwinding, essentially (address,opcode) pairs * `augmentation_data_length` * `augmentation_data`: If the associated CIE augmentation contains `L` characters, language-specific data area will be recorded here * padding A CIE may optionally refer to a personality routine in the text section. A FDE may optionally refer to its associated LSDA in `.gcc_except_table`. The personality routine and LSDA are used in Level 2: C++ ABI of Itanium C++ ABI. `.eh_frame` is based on `.debug_frame` introduced in DWARF v2. They have some differences, though: * `.eh_frame` has the flag of `SHF_ALLOC` (indicating that a section should be part of the mirror image in memory) but `.debug_frame` does not, so the latter has very few usage scenarios. * `debug_frame` supports DWARF64 format (supports 64-bit offsets but the volume will be slightly larger) but `.eh_frame` does not support (in fact, it can be expanded, but lacks demand) * There is no augmentation_data_length and augmentation_data in the CIE of `.debug_frame` * The version field in CIE is different * The meaning of CIE_pointer in FDE is different. `.debug_frame` indicates a section offset (absolute) and `.eh_frame` indicates a relative offset. This change made by `.eh_frame` is great. If the length of `.eh_frame` exceeds 32-bit, `.debug_frame` has to be converted to DWARF64 to represent `CIE_pointer`, and relative offset does not need to worry about this issue (if the distance between FDE and CIE exceeds 32-bit, add a CIE OK) For the following function: ```c void f() { __builtin_unwind_init(); } ``` The compiler produces `.cfi_*` (CFI directives) to annotate the assembly, `.cfi_startproc` and `.cfi_endproc` annotate the FDE area, and other CFI directives describe CFI instructions. A call frame is indicated by an address on the stack. This address is called Canonical Frame Address (CFA), and is usually the stack pointer value of the call site. The following example demonstrates the usage of CFI instructions: ``` f: # At the function entry, CFA = rsp+8 .cfi_startproc # %bb.0: pushq %rbp # Redefine CFA = rsp+16 .cfi_def_cfa_offset 16 # rbp is saved at the address CFA-16 .cfi_offset %rbp, -16 movq %rsp, %rbp # CFA = rbp+16. CFA does not needed to be redefined when rsp changes .cfi_def_cfa_register %rbp pushq %r15 pushq %r14 pushq %r13 pushq %r12 pushq %rbx # rbx is saved at the address CFA-56 .cfi_offset %rbx, -56 .cfi_offset %r12, -48 .cfi_offset %r13, -40 .cfi_offset %r14, -32 .cfi_offset %r15, -24 popq %rbx popq %r12 popq %r13 popq %r14 popq %r15 popq %rbp # CFA = rsp+8 .cfi_def_cfa %rsp, 8 retq .Lfunc_end0: .size f, .Lfunc_end0-f .cfi_endproc ``` The assembler parses CFI directives and generates `.eh_frame` (this mechanism was introduced by Alan Modra in 2003). Linker collects `.eh_frame` input sections in .o/.a files to generate output `.eh_frame`. In 2006, GNU as introduced `.cfi_personality` and `.cfi_lsda`. ### `.eh_frame_hdr` and `PT_EH_FRAME` To locate the FDE where a pc is located, you need to scan `.eh_frame` from the beginning to find the appropriate FDE (whether the pc falls in the interval indicated by initial_location and address_range). The time spent is proportional to the number of scanned CIE and FDE records. https://sourceware.org/pipermail/binutils/2001-December/015674.html introduced `.eh_frame_hdr`, a binary search index table describing (`initial_location`, FDE address) pairs. The linker collects all `.eh_frame` input sections. With `--eh-frame-hdr`, `ld` generates `.eh_frame_hdr` and creates a program header `PT_EH_FRAME` to describe `.eh_frame_hdr`. An unwinder can parse the program headers and look for `PT_EH_FRAME` to locate `.eh_frame_hdr`. Please check out the example below. ### `__register_frame_info` Before `.eh_frame_hdr` and `PT_EH_FRAME` were invented, there was a static constructor `frame_dummy` in crtbegin (`crtstuff.c`): calling `__register_frame_info` to register the executable file `.eh_frame`. Now `__register_frame_info` is only used by programs linked with `-static`. Correspondingly, if you specify `-Wl,--no-eh-frame-hdr` when linking, you cannot unwind (if you use a C++ exception, the program will call `std::terminate`). ### libunwind example ```c #include #include void backtrace() { unw_context_t context; unw_cursor_t cursor; // Store register values into context. unw_getcontext(&context); // Locate the PT_GNU_EH_FRAME which contains PC. unw_init_local(&cursor, &context); size_t rip, rsp; do { unw_get_reg(&cursor, UNW_X86_64_RIP, &rip); unw_get_reg(&cursor, UNW_X86_64_RSP, &rsp); printf("rip: %zx rsp: %zx\n", rip, rsp); } while (unw_step(&cursor) > 0); } void bar() {backtrace();} void foo() {bar();} int main() {foo();} ``` If you use llvm-project/libunwind： ```sh $CC a.c -Ipath/to/include -Lpath/to/lib -lunwind ``` If you use nongnu.org/libunwind, there are two options: (a) Add `#define UNW_LOCAL_ONLY` before `#include ` (b) Link one more library, on x86-64 it is `-l:libunwind-x86_64.so`. If you use Clang, you can also use `clang --rtlib=compiler-rt --unwindlib=libunwind -I path/to/include a.c`, in addition to providing `unw_*`, it can ensure that `libgcc_s.so` is not linked * `unw_getcontext`: Get register value (including PC) * `unw_init_local` * Use `dl_iterate_phdr` to traverse executable files and shared objects, and find the `PT_LOAD` program header that contains the PC * Find the `PT_EH_FRAME`(`.eh_frame_hdr`) of the module where you are, and save it in cursor * `unw_step` * Binary search for the `.eh_frame_hdr` item corresponding to the PC, record the FDE found and the CIE it points to * Execute `initial_instructions` in CIE * Execute the instructions (bytecode) in FDE. An automaton maintains the current location and CFA. Among the instructions, `DW_CFA_advance_loc` advances the location; `DW_CFA_def_cfa_*` updates CFA; `DW_CFA_offset` indicates that the value of a register is stored at CFA+offset * The automaton stops when the current location is greater than or equal to PC. In other words, the executed instruction is a prefix of FDE instructions An unwinder locates the applicable FDE according to the program counter, and executes all the CFI instructions before the program counter. There are several important * `DW_CFA_def_cfa_*` * `DW_CFA_offset` * `DW_CFA_advance_loc` A `-DCMAKE_BUILD_TYPE=Release -DLLVM_TARGETS_TO_BUILD=X86` clang, `.text` 51.7MiB, `.eh_frame` 4.2MiB, `.eh_frame_hdr` 646, 2 CIE, 82745 FDE. ### Remarks CFI instructions are suitable for the compiler to generate code, but cumbersome to write in hand-written assembly. In 2015, Alex Dowad contributed an awk script to musl libc to parse the assembly and automatically generate CFI directives. In fact, generating precise CFI instructions is challenging for ompilers as well. For a function that does not use a frame pointer, adjusting SP requires outputting a CFI directive to redefine CFA. GCC does not parse inline assembly, so adjusting SP in inline assembly often results in imprecise CFI. ```c void foo() { asm("subq $128, %rsp\n" // Cannot unwind if -fomit-leaf-frame-pointer "nop\n" "addq $128, %rsp\n"); } int main() { foo(); } ``` The CFIInstrInserter pass in LLVM can insert `.cfi_def_cfa_*` `.cfi_offset` `.cfi_restore` to adjust the CFA and callee-saved registers. The DWARF scheme also has very low information density. The various compact unwind schemes have made improvement on this aspect. To list a few issues: * CIE `address_size`: nobody uses different values for an architecture. Even if they do (ILP32 ABIs in AArch64 and x86-64), the information is already available elsewhere. * CIE `segment_selector_size`: It is nice that they cared x86, but x86 itself does not need it anymore :/ * CIE `code_alignment_factor` and `data_alignment_factor`: A RISC architecture with such preference can hard code the values. * CIE `return_address_register`: I do not know when an architecture wants to use a different register for the return address. * `length`: The DWARF's 8-byte form is definitely overengineered... For standard form prologue/epilogue, the field should not be needed. * `initial_location` and `address_range`: if a binary search index table is always needed, why do we need the length field? * `instructions`: bytecode is flexible but commonly a function prologue/epilogue is of a standard form and the few callee-saved registers can be encoded in a more compact way. * `augmentation_data`: While this provide flexibility, in practice very rarely a function needs anything more than a personality and a LSDA pointer. Callee-saved registers other than FP are oftentimes unneeded but there is no compiler option to drop them. ## `SHT_X86_64_UNWIND` `.eh_frame` has special processing in linker/dynamic loader, so conventionally it should use a separate section type, but `SHT_PROGBITS` was used in the design. In the x86-64 psABI, the type of `.eh_frame` is `SHT_X86_64_UNWIND` (influenced by Solaris). * In GNU as, `.section .eh_frame,"a",@unwind` will generate `SHT_X86_64_UNWIND`, and `.cfi_*` will generate `SHT_PROGBITS`. * Since Clang 3.8, `.cfi_*` generates `SHT_X86_64_UNWIND` `.section .eh_frame,"a",@unwind` is rare (glibc's x86 port, libffi, LuaJIT and other packages), so checking the type of `.eh_frame` is a good way to distinguish Clang/GCC object file :) For LLD 11.0.0, I contributed https://reviews.llvm.org/D85785 to allow mixed types for `.eh_frame` in a relocatable link ;-) Suggestion to future architectures: When defining processor-specific section types, please do not use 0x70000001 (`SHT_ARM_EXIDX=SHT_IA_64_UNWIND=SHT_PARISC_UNWIND=SHT_X86_64_UNWIND=SHT_LOPROC+1`) for purposes other than unwinding :) `SHT_CSKY_ATTRIBUTES=0x70000001` :) ### Linker perspective Usually in the case of COMDAT group and `-ffunction-sections`, `.data`/`.rodata` needs to be split like `.text`, but `.eh_frame` is monolithic. Like many other metadata sections, the main problem with the monolithic section is that garbage collection is challenging in the linker. Unlike some other metadata sections, simply abandoning garbage collecting is not a choice: `.eh_frame_hdr` is a binary search index table and duplicate/unused entries can confuse the customers. When a linker processes `.eh_frame`, it needs to conceptually split `.eh_frame` into CIE/FDE. During `--gc-sections`, the conceptual reference relationship is reversed considering the actual relocation: a FDE has a relocation referencing the text section; during GC, if the pointed text section is discarded, the FDE that references it should also be discarded. LLD has some special handling for `.eh_frame`: * `-M` requires special code * `--gc-sections` occurs before `.eh_frame` deduplication/GC. The personality in a CIE is a valid reference. However, `initial_location` in FDE should be ignored. Moreover, a LSDA reference in a FDE in a section group should be ignored. * In a relocatable link, a relocation from `.eh_frame` to a `STT_SECTION` symbol in a discarded section (due to COMDAT group rule) should be allowed (normally such a `STB_LOCAL` relocation from outside of the group is disallowed). ## Compact unwind descriptors On macOS, Apple designed the compact unwind descriptors mechanism to accelerate unwinding. In theory, this technique can be used to save some space in `__eh_frame`, but it has not been implemented. The main idea is: * The FDE of most functions has a fixed mode (specify CFA at the prologue, store callee-saved registers), and the FDE instructions can be compressed to 32-bit. * Personality/lsda described by CIE/FDE augmentation data is very common and can be extracted as a fixed field. Only 64-bit will be discussed below. A descriptor occupies 32 bytes ``` .quad _foo .set L1, Lfoo_end-_foo .long L1 .long compact_unwind_description .quad personality .quad lsda_address ``` If you study `.eh_frame_hdr` (binary search index table) and `.ARM.exidx`, you can know that the length field is redundant. The Compact unwind descriptor is encoded as: ```c uint32_t : 24; // vary with different modes uint32_t mode : 4; uint32_t flags : 4; ``` Five modes are defined: * 0: reserved * 1: FP-based frame: RBP is frame pointer, frame size is variable * 2: SP-based frame: frame pointer is not used, frame size is fixed during compilation * 3: large SP-based frame: frame pointer is not used, the frame size is fixed at compile time but the value is large and cannot be represented by mode 2 * 4: DWARF CFI escape ### FP-based frame (`UNWIND_MODE_BP_FRAME`) The compact unwind encoding is: ```c uint32_t regs : 15; uint32_t : 1; // 0 uint32_t stack_adjust : 8; uint32_t mode : 4; uint32_t flags : 4; ``` The callee-saved registers on x86-64 are: RBX, R12, R13, R14, R15, RBP. 3 bits can encode a register, 15 bits are enough to represent 5 registers except RBP (whether to save and where). `stack_adjust` records the extra stack space outside the save register. ### SP-based frame (`UNWIND_MODE_STACK_IMMD`) The compact unwind encoding is: ```c uint32_t reg_permutation : 10; uint32_t cnt : 3; uint32_t : 3; uint32_t size : 8; uint32_t mode : 4; uint32_t flags : 4; ``` `cnt` represents the number of saved registers (maximum 6). `reg_permutation` indicates the sequence number of the saved register. `size*8` represents the stack frame size. ### Large SP-based frame (`UNWIND_MODE_STACK_IND`) Compact unwind descriptor编码为： ```c uint32_t reg_permutation : 10; uint32_t cnt : 3; uint32_t adj : 3; uint32_t size_offset : 8; uint32_t mode : 4; uint32_t flags : 4; ``` Similar to SP-based frame. In particular: the stack frame size is read from the text section. The RSP adjustment is usually represented by `subq imm, %rsp`, and `size_offset` is used to represent the distance from the instruction to the beginning of the function. The actual stack size also includes `adj*8`. ### DWARF CFI escape If for various reasons, the compact unwind descriptor cannot be expressed, it must fall back to DWARF CFI. In the LLVM implementation, each function is represented by only a compact unwind descriptor. If asynchronous stack unwinding occurs in epilogue, existing implementations cannot distinguish it from stack unwinding in function body. Canonical Frame Address will be calculated incorrectly, and the caller-saved register will be read incorrectly. If it happens in prologue, and the prologue has other instructions outside the push register and `subq imm, $rsp`, an error will occur. In addition, if shrink wrapping is enabled for a function, prologue may not be at the beginning of the function. The asynchronous stack unwinding from the beginning to the prologue also fails. It seems that most people don't care about this issue. It may be because the profiler loses a few percentage points of the profile. In fact, if you use multiple descriptors to describe each area of a function, you can still unwind accurately. OpenVMS proposed [\[RFC\] Improving compact x86-64 compact unwind descriptors](http://lists.llvm.org/pipermail/llvm-dev/2018-January/120741.html) in 2018, but unfortunately there is no relevant implementation. ### ARM exception handling Divided into `.ARM.exidx` and `.ARM.extab` `.ARM.exidx` is a binary search index table, composed of 2-word pairs. The first word is 31-bit PC-relative offset to the start of the region. The second word uses the program description more clearly: ```c if (indexData == EXIDX_CANTUNWIND) return false; // like an absent .eh_frame entry. In the case of C++ exceptions, std::terminate if (indexData & 0x80000000) { extabAddr = &indexData; extabData = indexData; // inline } else { extabAddr = &indexData + signExtendPrel31(indexData); extabData = read32(&indexData + signExtendPrel31(indexData)); // stored in .ARM.extab } ``` `tableData & 0x80000000` means a compact model entry, otherwise means a generic model entry. `.ARM.exidx` is equivalent to enhanced `.eh_frame_hdr`, compact model is equivalent to inlining the personality and lsda in `.eh_frame`. Consider the following three situations: * If the C++ exception will not be triggered and the function that may trigger the exception will not be called: no personality is needed, only one `EXIDX_CANTUNWIND` entry is needed, no `.ARM.extab` * If a C++ exception is triggered but no landing pad is required: personality is `__aeabi_unwind_cpp_pr0`, only a compact model entry is needed, no `.ARM.extab` * If there is a catch: `__gxx_personality_v0` is required, `.ARM.extab` is required `.ARM.extab` is equivalent to the combined `.eh_frame` and `.gcc_except_table`. ### Generic model ```c uint32_t personality; // bit 31 is 0 uint32_t : 24; uint32_t num : 8; uint32_t opcodes[]; // opcodes, variable length uint8_t lsda[]; // variable length ``` In construction. ## Windows ARM64 exception handling See https://docs.microsoft.com/en-us/cpp/build/arm64-exception-handling, this is my favorite coding scheme. Support the unwinding of mid-prolog and mid-epilog. Support function fragments (used to represent unconventional stack frames such as shrink wrapping). Saved in two sections `.pdata` and `.xdata`. ```c uint32_t function_start_rva; uint32_t Flag : 2; uint32_t Data : 30; ``` For canonical form functions, Packed Unwind Data is used, and no `.xdata` record is required; for descriptors that cannot be represented by Packed Unwind Data, it is stored in `.xdata`. ### Packed Unwind Data ```c uint32_t FunctionStartRVA; uint32_t Flag : 2; uint32_t FunctionLength : 11; uint32_t RegF : 3; uint32_t RegI : 4; uint32_t H : 1; uint32_t CR : 2; uint32_t FrameSize : 9; ``` ## MIPS compact exception tables In construction. ## Linux kernel ORC unwind tables For x86-64, the Linux kernel uses its own unwind tables: ORC. You can find its documentation on https://www.kernel.org/doc/html/latest/x86/orc-unwinder.html and there is an lwn.net introduction [The ORCs are coming](https://lwn.net/Articles/728339/). `objtool orc generate a.o` parses `.eh_frame` and generates `.orc_unwind` and `.orc_unwind_ip`. For an object file assembled from: ``` .globl foo .type foo, @function foo: ret ``` At two addresses the unwind information changes: the start of foo and the end of foo, so 2 ORC entries will be produced. If the DWARF CFA changes (e.g. due to push/pop) in the middle of the function, there may be more entries. `.orc_unwind_ip` contains two entries, representing the PC-relative addresses. ``` Relocation section '.rela.orc_unwind_ip' at offset 0x2028 contains 2 entries: Offset Info Type Symbol's Value Symbol's Name + Addend 0000000000000000 0000000500000002 R_X86_64_PC32 0000000000000000 .text + 0 0000000000000004 0000000500000002 R_X86_64_PC32 0000000000000000 .text + 1 ``` `.orc_unwind` contains two entries of type `orc_entry`. The entries encode how IP/SP/BP of the previous frame are stored. ```c struct orc_entry { s16 sp_offset; // sp_offset and sp_reg encode where SP of the previous frame is stored s16 bp_offset; // bp_offset and bp_reg encode where BP of the previous frame is stored unsigned sp_reg:4; unsigned bp_reg:4; unsigned type:2; // how IP of the previous frame is stored unsigned end:1; } __attribute__((__packed__)); ``` You may find similarities in this scheme and `UNWIND_MODE_BP_FRAME` and `UNWIND_MODE_STACK_IMMD` in Apples's compact unwind descriptors. The ORC scheme uses 16-bit integers so assumably `UNWIND_MODE_STACK_IND` will not be needed. During unwinding, most callee-saved registers other than BP are unneeded, so ORC does not bother recording them. The linker will resolve relocations in `.orc_unwind_ip` and create `__start_orc_unwind_ip`/`__stop_orc_unwind_ip`/`__start_orc_unwind`/ `__stop_orc_unwind` delimiter the section contents. Then, a host utility scripts/sorttable sorts the contents of `.orc_unwind_ip` and `.orc_unwind`. To unwind a stack frame, `unwind_next_frame` * performs a binary search into the `.orc_unwind_ip` table to figure out the relevant ORC entry * retrieves the previous SP with the current SP, `orc->sp_reg` and `orc->sp_offset`. * retrieves the previous IP with `orc->type` and other values. * retrieves the previous BP with the currrent BP, the previous SP, `orc->bp_reg` and `orc->bp_offset`. `bp->reg` can be `ORC_REG_UNDEFINED`/`ORC_REG_PREV_SP`/`ORC_REG_BP`.