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cpython/Python/jit_unwind.c
Diego Russo c7b7ca2cd5
GH-126910: Add gdb support for unwinding JIT frames (#146071) 
Co-authored-by: Pablo Galindo Salgado <pablogsal@gmail.com>
2026-05-02 13:42:03 +00:00

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/*
* Python JIT - DWARF .eh_frame builder
*
* This file contains the DWARF CFI generator used to build .eh_frame
* data for JIT code (perf jitdump and other unwinders).
*/
#include "Python.h"
#include "pycore_jit_unwind.h"
#include "pycore_lock.h"
#if defined(PY_HAVE_JIT_GDB_UNWIND)
# include "jit_unwind_info.h"
# if !JIT_UNWIND_INFO_SUPPORTED
# error "JIT unwind info was not generated for this target"
# endif
#endif
#if defined(PY_HAVE_PERF_TRAMPOLINE) || defined(PY_HAVE_JIT_GDB_UNWIND)
#if defined(PY_HAVE_JIT_GDB_UNWIND)
# include <elf.h>
#endif
#include <stdio.h>
#include <string.h>
// =============================================================================
// DWARF CONSTANTS
// =============================================================================
/*
* DWARF (Debug With Arbitrary Record Formats) constants
*
* DWARF is a debugging data format used to provide stack unwinding information.
* These constants define the various encoding types and opcodes used in
* DWARF Call Frame Information (CFI) records.
*/
/* DWARF Call Frame Information version */
#define DWRF_CIE_VERSION 1
/* DWARF CFA (Call Frame Address) opcodes */
enum {
DWRF_CFA_nop = 0x0, // No operation
DWRF_CFA_offset_extended = 0x5, // Extended offset instruction
DWRF_CFA_def_cfa = 0xc, // Define CFA rule
DWRF_CFA_def_cfa_register = 0xd, // Define CFA register
DWRF_CFA_def_cfa_offset = 0xe, // Define CFA offset
DWRF_CFA_offset_extended_sf = 0x11, // Extended signed offset
DWRF_CFA_advance_loc = 0x40, // Advance location counter
DWRF_CFA_offset = 0x80, // Simple offset instruction
DWRF_CFA_restore = 0xc0 // Restore register
};
/*
* Architecture-specific DWARF register numbers
*
* These constants define the register numbering scheme used by DWARF
* for each supported architecture. The numbers must match the ABI
* specification for proper stack unwinding.
*/
enum {
#ifdef __x86_64__
/* x86_64 register numbering (note: order is defined by x86_64 ABI) */
DWRF_REG_AX, // RAX
DWRF_REG_DX, // RDX
DWRF_REG_CX, // RCX
DWRF_REG_BX, // RBX
DWRF_REG_SI, // RSI
DWRF_REG_DI, // RDI
DWRF_REG_BP, // RBP
DWRF_REG_SP, // RSP
DWRF_REG_8, // R8
DWRF_REG_9, // R9
DWRF_REG_10, // R10
DWRF_REG_11, // R11
DWRF_REG_12, // R12
DWRF_REG_13, // R13
DWRF_REG_14, // R14
DWRF_REG_15, // R15
DWRF_REG_RA, // Return address (RIP)
#elif defined(__aarch64__) && defined(__AARCH64EL__) && !defined(__ILP32__)
/* AArch64 register numbering */
DWRF_REG_FP = 29, // Frame Pointer
DWRF_REG_RA = 30, // Link register (return address)
DWRF_REG_SP = 31, // Stack pointer
#else
# error "Unsupported target architecture"
#endif
};
// =============================================================================
// ELF OBJECT CONTEXT
// =============================================================================
/*
* Context for building ELF/DWARF structures
*
* This structure maintains state while constructing DWARF unwind information.
* It acts as a simple buffer manager with pointers to track current position
* and important landmarks within the buffer.
*/
typedef struct ELFObjectContext {
uint8_t* p; // Current write position in buffer
uint8_t* startp; // Start of buffer (for offset calculations)
uint8_t* fde_p; // Start of FDE data (for PC-relative calculations)
uintptr_t code_addr; // Address of the code section
size_t code_size; // Size of the code section
} ELFObjectContext;
// =============================================================================
// DWARF GENERATION UTILITIES
// =============================================================================
/*
* Append a null-terminated string to the ELF context buffer.
*
* Args:
* ctx: ELF object context
* str: String to append (must be null-terminated)
*
* Returns: Offset from start of buffer where string was written
*/
static uint32_t elfctx_append_string(ELFObjectContext* ctx, const char* str) {
uint8_t* p = ctx->p;
uint32_t ofs = (uint32_t)(p - ctx->startp);
/* Copy string including null terminator */
do {
*p++ = (uint8_t)*str;
} while (*str++);
ctx->p = p;
return ofs;
}
/*
* Append a SLEB128 (Signed Little Endian Base 128) value
*
* SLEB128 is a variable-length encoding used extensively in DWARF.
* It efficiently encodes small numbers in fewer bytes.
*
* Args:
* ctx: ELF object context
* v: Signed value to encode
*/
static void elfctx_append_sleb128(ELFObjectContext* ctx, int32_t v) {
uint8_t* p = ctx->p;
/* Encode 7 bits at a time, with continuation bit in MSB */
for (; (uint32_t)(v + 0x40) >= 0x80; v >>= 7) {
*p++ = (uint8_t)((v & 0x7f) | 0x80); // Set continuation bit
}
*p++ = (uint8_t)(v & 0x7f); // Final byte without continuation bit
ctx->p = p;
}
/*
* Append a ULEB128 (Unsigned Little Endian Base 128) value
*
* Similar to SLEB128 but for unsigned values.
*
* Args:
* ctx: ELF object context
* v: Unsigned value to encode
*/
static void elfctx_append_uleb128(ELFObjectContext* ctx, uint32_t v) {
uint8_t* p = ctx->p;
/* Encode 7 bits at a time, with continuation bit in MSB */
for (; v >= 0x80; v >>= 7) {
*p++ = (char)((v & 0x7f) | 0x80); // Set continuation bit
}
*p++ = (char)v; // Final byte without continuation bit
ctx->p = p;
}
/*
* Macros for generating DWARF structures
*
* These macros provide a convenient way to write various data types
* to the DWARF buffer while automatically advancing the pointer.
*/
#define DWRF_U8(x) (*p++ = (x)) // Write unsigned 8-bit
#define DWRF_I8(x) (*(int8_t*)p = (x), p++) // Write signed 8-bit
#define DWRF_U16(x) (*(uint16_t*)p = (x), p += 2) // Write unsigned 16-bit
#define DWRF_U32(x) (*(uint32_t*)p = (x), p += 4) // Write unsigned 32-bit
#define DWRF_ADDR(x) (*(uintptr_t*)p = (x), p += sizeof(uintptr_t)) // Write address
#define DWRF_UV(x) (ctx->p = p, elfctx_append_uleb128(ctx, (x)), p = ctx->p) // Write ULEB128
#define DWRF_SV(x) (ctx->p = p, elfctx_append_sleb128(ctx, (x)), p = ctx->p) // Write SLEB128
#define DWRF_STR(str) (ctx->p = p, elfctx_append_string(ctx, (str)), p = ctx->p) // Write string
/* Align to specified boundary with NOP instructions */
#define DWRF_ALIGNNOP(s) \
while ((uintptr_t)p & ((s)-1)) { \
*p++ = DWRF_CFA_nop; \
}
/* Write a DWARF section with automatic size calculation */
#define DWRF_SECTION(name, stmt) \
{ \
uint32_t* szp_##name = (uint32_t*)p; \
p += 4; \
stmt; \
*szp_##name = (uint32_t)((p - (uint8_t*)szp_##name) - 4); \
}
// =============================================================================
// DWARF EH FRAME GENERATION
// =============================================================================
static void elf_init_ehframe_perf(ELFObjectContext* ctx);
#if defined(PY_HAVE_JIT_GDB_UNWIND)
static void elf_init_ehframe_gdb(ELFObjectContext* ctx);
#endif
static inline void elf_init_ehframe(ELFObjectContext* ctx, int absolute_addr) {
if (absolute_addr) {
#if defined(PY_HAVE_JIT_GDB_UNWIND)
elf_init_ehframe_gdb(ctx);
#else
Py_UNREACHABLE();
#endif
}
else {
elf_init_ehframe_perf(ctx);
}
}
size_t
_PyJitUnwind_EhFrameSize(int absolute_addr)
{
/* The .eh_frame we emit is small and bounded; keep a generous buffer. */
uint8_t scratch[512];
_Static_assert(sizeof(scratch) >= 256,
"scratch buffer may be too small for elf_init_ehframe");
ELFObjectContext ctx;
ctx.code_size = 1;
ctx.code_addr = 0;
ctx.startp = ctx.p = scratch;
ctx.fde_p = NULL;
/* Generate once into scratch to learn the required size. */
elf_init_ehframe(&ctx, absolute_addr);
ptrdiff_t size = ctx.p - ctx.startp;
assert(size <= (ptrdiff_t)sizeof(scratch));
return (size_t)size;
}
size_t
_PyJitUnwind_BuildEhFrame(uint8_t *buffer, size_t buffer_size,
const void *code_addr, size_t code_size,
int absolute_addr)
{
if (buffer == NULL || code_addr == NULL || code_size == 0) {
return 0;
}
/* Generate the frame twice: once to size-check, once to write. */
size_t required = _PyJitUnwind_EhFrameSize(absolute_addr);
if (required == 0 || required > buffer_size) {
return 0;
}
ELFObjectContext ctx;
ctx.code_size = code_size;
ctx.code_addr = (uintptr_t)code_addr;
ctx.startp = ctx.p = buffer;
ctx.fde_p = NULL;
elf_init_ehframe(&ctx, absolute_addr);
size_t written = (size_t)(ctx.p - ctx.startp);
/* The frame size is independent of code_addr/code_size (fixed-width fields). */
assert(written == required);
return written;
}
/*
* Generate a minimal .eh_frame for a single JIT code region.
*
* The .eh_frame section contains Call Frame Information (CFI) that describes
* how to unwind the stack at any point in the code. This is essential for
* unwinding through JIT-generated code.
*
* The generated data contains:
* 1. A CIE (Common Information Entry) describing the calling convention.
* 2. An FDE (Frame Description Entry) describing how to unwind the JIT frame.
*
* Two flavors are emitted, dispatched on the absolute_addr flag:
*
* - absolute_addr == 0 (elf_init_ehframe_perf): PC-relative FDE address
* encoding for perf's synthesized DSO layout. The CIE describes the
* trampoline's entry state and the FDE walks through the prologue and
* epilogue with advance_loc instructions. This matches the pre-existing
* perf_jit_trampoline behavior byte-for-byte.
*
* - absolute_addr == 1 (elf_init_ehframe_gdb): absolute FDE address
* encoding for the GDB JIT in-memory ELF. The CIE describes the
* steady-state frame layout (CFA = %rbp+16 / x29+16, with saved fp and
* return-address column at fixed offsets) and the FDE emits no further
* CFI. The same rule applies at every PC in the registered region,
* which is correct for executor stencils (they pin the frame pointer
* across the region). This is the GDB-side fix; see elf_init_ehframe_gdb
* for details.
*/
static void elf_init_ehframe_perf(ELFObjectContext* ctx) {
int fde_ptr_enc = DWRF_EH_PE_pcrel | DWRF_EH_PE_sdata4;
uint8_t* p = ctx->p;
uint8_t* framep = p; // Remember start of frame data
/*
* DWARF Unwind Table for Trampoline Function
*
* This section defines DWARF Call Frame Information (CFI) using encoded macros
* like `DWRF_U8`, `DWRF_UV`, and `DWRF_SECTION` to describe how the trampoline function
* preserves and restores registers. This is used by profiling tools (e.g., `perf`)
* and debuggers for stack unwinding in JIT-compiled code.
*
* -------------------------------------------------
* TO REGENERATE THIS TABLE FROM GCC OBJECTS:
* -------------------------------------------------
*
* 1. Create a trampoline source file (e.g., `trampoline.c`):
*
* #include <Python.h>
* typedef PyObject* (*py_evaluator)(void*, void*, int);
* PyObject* trampoline(void *ts, void *f, int throwflag, py_evaluator evaluator) {
* return evaluator(ts, f, throwflag);
* }
*
* 2. Compile to an object file with frame pointer preservation:
*
* gcc trampoline.c -I. -I./Include -O2 -fno-omit-frame-pointer -mno-omit-leaf-frame-pointer -c
*
* 3. Extract DWARF unwind info from the object file:
*
* readelf -w trampoline.o
*
* Example output from `.eh_frame`:
*
* 00000000 CIE
* Version: 1
* Augmentation: "zR"
* Code alignment factor: 4
* Data alignment factor: -8
* Return address column: 30
* DW_CFA_def_cfa: r31 (sp) ofs 0
*
* 00000014 FDE cie=00000000 pc=0..14
* DW_CFA_advance_loc: 4
* DW_CFA_def_cfa_offset: 16
* DW_CFA_offset: r29 at cfa-16
* DW_CFA_offset: r30 at cfa-8
* DW_CFA_advance_loc: 12
* DW_CFA_restore: r30
* DW_CFA_restore: r29
* DW_CFA_def_cfa_offset: 0
*
* -- These values can be verified by comparing with `readelf -w` or `llvm-dwarfdump --eh-frame`.
*
* ----------------------------------
* HOW TO TRANSLATE TO DWRF_* MACROS:
* ----------------------------------
*
* After compiling your trampoline with:
*
* gcc trampoline.c -I. -I./Include -O2 -fno-omit-frame-pointer -mno-omit-leaf-frame-pointer -c
*
* run:
*
* readelf -w trampoline.o
*
* to inspect the generated `.eh_frame` data. You will see two main components:
*
* 1. A CIE (Common Information Entry): shared configuration used by all FDEs.
* 2. An FDE (Frame Description Entry): function-specific unwind instructions.
*
* ---------------------
* Translating the CIE:
* ---------------------
* From `readelf -w`, you might see:
*
* 00000000 0000000000000010 00000000 CIE
* Version: 1
* Augmentation: "zR"
* Code alignment factor: 4
* Data alignment factor: -8
* Return address column: 30
* Augmentation data: 1b
* DW_CFA_def_cfa: r31 (sp) ofs 0
*
* Map this to:
*
* DWRF_SECTION(CIE,
* DWRF_U32(0); // CIE ID (always 0 for CIEs)
* DWRF_U8(DWRF_CIE_VERSION); // Version: 1
* DWRF_STR("zR"); // Augmentation string "zR"
* DWRF_UV(4); // Code alignment factor = 4
* DWRF_SV(-8); // Data alignment factor = -8
* DWRF_U8(DWRF_REG_RA); // Return address register (e.g., x30 = 30)
* DWRF_UV(1); // Augmentation data length = 1
* DWRF_U8(DWRF_EH_PE_pcrel | DWRF_EH_PE_sdata4); // Encoding for FDE pointers
*
* DWRF_U8(DWRF_CFA_def_cfa); // DW_CFA_def_cfa
* DWRF_UV(DWRF_REG_SP); // Register: SP (r31)
* DWRF_UV(0); // Offset = 0
*
* DWRF_ALIGNNOP(sizeof(uintptr_t)); // Align to pointer size boundary
* )
*
* Notes:
* - Use `DWRF_UV` for unsigned LEB128, `DWRF_SV` for signed LEB128.
* - `DWRF_REG_RA` and `DWRF_REG_SP` are architecture-defined constants.
*
* ---------------------
* Translating the FDE:
* ---------------------
* From `readelf -w`:
*
* 00000014 0000000000000020 00000018 FDE cie=00000000 pc=0000000000000000..0000000000000014
* DW_CFA_advance_loc: 4
* DW_CFA_def_cfa_offset: 16
* DW_CFA_offset: r29 at cfa-16
* DW_CFA_offset: r30 at cfa-8
* DW_CFA_advance_loc: 12
* DW_CFA_restore: r30
* DW_CFA_restore: r29
* DW_CFA_def_cfa_offset: 0
*
* Map the FDE header and instructions to:
*
* DWRF_SECTION(FDE,
* DWRF_U32((uint32_t)(p - framep)); // Offset to CIE (relative from here)
* DWRF_U32(pc_relative_offset); // PC-relative location of the code (calculated dynamically)
* DWRF_U32(ctx->code_size); // Code range covered by this FDE
* DWRF_U8(0); // Augmentation data length (none)
*
* DWRF_U8(DWRF_CFA_advance_loc | 1); // Advance location by 1 unit (1 * 4 = 4 bytes)
* DWRF_U8(DWRF_CFA_def_cfa_offset); // CFA = SP + 16
* DWRF_UV(16);
*
* DWRF_U8(DWRF_CFA_offset | DWRF_REG_FP); // Save x29 (frame pointer)
* DWRF_UV(2); // At offset 2 * 8 = 16 bytes
*
* DWRF_U8(DWRF_CFA_offset | DWRF_REG_RA); // Save x30 (return address)
* DWRF_UV(1); // At offset 1 * 8 = 8 bytes
*
* DWRF_U8(DWRF_CFA_advance_loc | 3); // Advance location by 3 units (3 * 4 = 12 bytes)
*
* DWRF_U8(DWRF_CFA_offset | DWRF_REG_RA); // Restore x30
* DWRF_U8(DWRF_CFA_offset | DWRF_REG_FP); // Restore x29
*
* DWRF_U8(DWRF_CFA_def_cfa_offset); // CFA = SP
* DWRF_UV(0);
* )
*
* To regenerate:
* 1. Get the `code alignment factor`, `data alignment factor`, and `RA column` from the CIE.
* 2. Note the range of the function from the FDE's `pc=...` line and map it to the JIT code as
* the code is in a different address space every time.
* 3. For each `DW_CFA_*` entry, use the corresponding `DWRF_*` macro:
* - `DW_CFA_def_cfa_offset` → DWRF_U8(DWRF_CFA_def_cfa_offset), DWRF_UV(value)
* - `DW_CFA_offset: rX` → DWRF_U8(DWRF_CFA_offset | reg), DWRF_UV(offset)
* - `DW_CFA_restore: rX` → DWRF_U8(DWRF_CFA_offset | reg) // restore is same as reusing offset
* - `DW_CFA_advance_loc: N` → DWRF_U8(DWRF_CFA_advance_loc | (N / code_alignment_factor))
* 4. Use `DWRF_REG_FP`, `DWRF_REG_RA`, etc., for register numbers.
* 5. Use `sizeof(uintptr_t)` (typically 8) for pointer size calculations and alignment.
*/
/*
* Emit DWARF EH CIE (Common Information Entry)
*
* The CIE describes the calling conventions and basic unwinding rules
* that apply to all functions in this compilation unit.
*/
DWRF_SECTION(CIE,
DWRF_U32(0); // CIE ID (0 indicates this is a CIE)
DWRF_U8(DWRF_CIE_VERSION); // CIE version (1)
DWRF_STR("zR"); // Augmentation string ("zR" = has LSDA)
#ifdef __x86_64__
DWRF_UV(1); // Code alignment factor (x86_64: 1 byte)
#elif defined(__aarch64__) && defined(__AARCH64EL__) && !defined(__ILP32__)
DWRF_UV(4); // Code alignment factor (AArch64: 4 bytes per instruction)
#endif
DWRF_SV(-(int64_t)sizeof(uintptr_t)); // Data alignment factor (negative)
DWRF_U8(DWRF_REG_RA); // Return address register number
DWRF_UV(1); // Augmentation data length
DWRF_U8(fde_ptr_enc); // FDE pointer encoding
/* Initial CFI instructions - describe default calling convention */
#ifdef __x86_64__
/* x86_64 initial CFI state */
DWRF_U8(DWRF_CFA_def_cfa); // Define CFA (Call Frame Address)
DWRF_UV(DWRF_REG_SP); // CFA = SP register
DWRF_UV(sizeof(uintptr_t)); // CFA = SP + pointer_size
DWRF_U8(DWRF_CFA_offset|DWRF_REG_RA); // Return address is saved
DWRF_UV(1); // At offset 1 from CFA
#elif defined(__aarch64__) && defined(__AARCH64EL__) && !defined(__ILP32__)
/* AArch64 initial CFI state */
DWRF_U8(DWRF_CFA_def_cfa); // Define CFA (Call Frame Address)
DWRF_UV(DWRF_REG_SP); // CFA = SP register
DWRF_UV(0); // CFA = SP + 0 (AArch64 starts with offset 0)
// No initial register saves in AArch64 CIE
#endif
DWRF_ALIGNNOP(sizeof(uintptr_t)); // Align to pointer boundary
)
/*
* Emit DWARF EH FDE (Frame Description Entry)
*
* The FDE describes unwinding information specific to this function.
* It references the CIE and provides function-specific CFI instructions.
*
* The PC-relative offset is calculated after the entire EH frame is built
* to ensure accurate positioning relative to the synthesized DSO layout.
*/
DWRF_SECTION(FDE,
DWRF_U32((uint32_t)(p - framep)); // Offset to CIE (backwards reference)
/*
* In perf jitdump mode the FDE PC field is encoded PC-relative and
* points back to code_start. Record where that field lives so we can
* patch in the final offset after the rest of the synthetic DSO
* layout is known.
*/
ctx->fde_p = p; // Remember where PC offset field is located for later calculation
DWRF_U32(0); // Placeholder for PC-relative offset (calculated below)
DWRF_U32(ctx->code_size); // Address range covered by this FDE (code length)
DWRF_U8(0); // Augmentation data length (none)
/*
* Architecture-specific CFI instructions
*
* These instructions describe how registers are saved and restored
* during function calls. Each architecture has different calling
* conventions and register usage patterns.
*/
#ifdef __x86_64__
/* x86_64 calling convention unwinding rules */
# if defined(__CET__) && (__CET__ & 1)
DWRF_U8(DWRF_CFA_advance_loc | 4); // Advance past endbr64 (4 bytes)
# endif
DWRF_U8(DWRF_CFA_advance_loc | 1); // Advance past push %rbp (1 byte)
DWRF_U8(DWRF_CFA_def_cfa_offset); // def_cfa_offset 16
DWRF_UV(16); // New offset: SP + 16
DWRF_U8(DWRF_CFA_offset | DWRF_REG_BP); // offset r6 at cfa-16
DWRF_UV(2); // Offset factor: 2 * 8 = 16 bytes
DWRF_U8(DWRF_CFA_advance_loc | 3); // Advance past mov %rsp,%rbp (3 bytes)
DWRF_U8(DWRF_CFA_def_cfa_register); // def_cfa_register r6
DWRF_UV(DWRF_REG_BP); // Use base pointer register
DWRF_U8(DWRF_CFA_advance_loc | 3); // Advance past call *%rcx (2 bytes) + pop %rbp (1 byte) = 3
DWRF_U8(DWRF_CFA_def_cfa); // def_cfa r7 ofs 8
DWRF_UV(DWRF_REG_SP); // Use stack pointer register
DWRF_UV(8); // New offset: SP + 8
#elif defined(__aarch64__) && defined(__AARCH64EL__) && !defined(__ILP32__)
/* AArch64 calling convention unwinding rules */
DWRF_U8(DWRF_CFA_advance_loc | 1); // Advance by 1 instruction (4 bytes)
DWRF_U8(DWRF_CFA_def_cfa_offset); // CFA = SP + 16
DWRF_UV(16); // Stack pointer moved by 16 bytes
DWRF_U8(DWRF_CFA_offset | DWRF_REG_FP); // x29 (frame pointer) saved
DWRF_UV(2); // At CFA-16 (2 * 8 = 16 bytes from CFA)
DWRF_U8(DWRF_CFA_offset | DWRF_REG_RA); // x30 (link register) saved
DWRF_UV(1); // At CFA-8 (1 * 8 = 8 bytes from CFA)
DWRF_U8(DWRF_CFA_advance_loc | 3); // Advance by 3 instructions (12 bytes)
DWRF_U8(DWRF_CFA_def_cfa_register); // CFA = FP (x29) + 16
DWRF_UV(DWRF_REG_FP);
DWRF_U8(DWRF_CFA_restore | DWRF_REG_RA); // Restore x30 - NO DWRF_UV() after this!
DWRF_U8(DWRF_CFA_restore | DWRF_REG_FP); // Restore x29 - NO DWRF_UV() after this!
DWRF_U8(DWRF_CFA_def_cfa); // CFA = SP + 0 (stack restored)
DWRF_UV(DWRF_REG_SP);
DWRF_UV(0);
#else
# error "Unsupported target architecture"
#endif
DWRF_ALIGNNOP(sizeof(uintptr_t)); // Align to pointer boundary
)
ctx->p = p; // Update context pointer to end of generated data
/* Calculate and update the PC-relative offset in the FDE
*
* When perf processes the jitdump, it creates a synthesized DSO with this layout:
*
* Synthesized DSO Memory Layout:
* ┌─────────────────────────────────────────────────────────────┐ < code_start
* │ Code Section │
* │ (round_up(code_size, 8) bytes) │
* ├─────────────────────────────────────────────────────────────┤ < start of EH frame data
* │ EH Frame Data │
* │ ┌─────────────────────────────────────────────────────┐ │
* │ │ CIE data │ │
* │ └─────────────────────────────────────────────────────┘ │
* │ ┌─────────────────────────────────────────────────────┐ │
* │ │ FDE Header: │ │
* │ │ - CIE offset (4 bytes) │ │
* │ │ - PC offset (4 bytes) <─ fde_offset_in_frame ─────┼────┼─> points to code_start
* │ │ - address range (4 bytes) │ │ (this specific field)
* │ │ CFI Instructions... │ │
* │ └─────────────────────────────────────────────────────┘ │
* ├─────────────────────────────────────────────────────────────┤ < reference_point
* │ EhFrameHeader │
* │ (navigation metadata) │
* └─────────────────────────────────────────────────────────────┘
*
* The PC offset field in the FDE must contain the distance from itself to code_start:
*
* distance = code_start - fde_pc_field
*
* Where:
* fde_pc_field_location = reference_point - eh_frame_size + fde_offset_in_frame
* code_start_location = reference_point - eh_frame_size - round_up(code_size, 8)
*
* Therefore:
* distance = code_start_location - fde_pc_field_location
* = (ref - eh_frame_size - rounded_code_size) - (ref - eh_frame_size + fde_offset_in_frame)
* = -rounded_code_size - fde_offset_in_frame
* = -(round_up(code_size, 8) + fde_offset_in_frame)
*
* Note: fde_offset_in_frame is the offset from EH frame start to the PC offset field.
*
*/
int32_t rounded_code_size =
(int32_t)_Py_SIZE_ROUND_UP(ctx->code_size, 8);
int32_t fde_offset_in_frame = (int32_t)(ctx->fde_p - framep);
*(int32_t *)ctx->fde_p = -(rounded_code_size + fde_offset_in_frame);
}
/*
* Build .eh_frame data for the GDB JIT interface.
*
* The executor runs inside the frame established by _PyJIT_Entry, but the
* synthetic executor FDE collapses that state into a single logical JIT frame
* that unwinds directly into _PyEval_*. Executor stencils never touch the
* frame pointer - enforced by Tools/jit/_optimizers.py _validate() and
* -mframe-pointer=reserved - so the steady-state rule is valid at every PC
* and the FDE body is empty. Tools/jit/_targets.py derives the initial CFI
* rules from the row active at the executor call in the compiled shim object.
*/
#if defined(PY_HAVE_JIT_GDB_UNWIND)
static void elf_init_ehframe_gdb(ELFObjectContext* ctx) {
int fde_ptr_enc = DWRF_EH_PE_absptr;
uint8_t* p = ctx->p;
uint8_t* framep = p;
DWRF_SECTION(CIE,
DWRF_U32(0); // CIE ID
DWRF_U8(DWRF_CIE_VERSION);
DWRF_STR("zR"); // aug data length + FDE ptr encoding follow
DWRF_UV(JIT_UNWIND_CODE_ALIGNMENT_FACTOR);
DWRF_SV(JIT_UNWIND_DATA_ALIGNMENT_FACTOR);
DWRF_U8(JIT_UNWIND_RA_REG);
DWRF_UV(1); // Augmentation data length
DWRF_U8(fde_ptr_enc); // FDE pointer encoding
/* Executor steady-state rule (our invariant, not the compiler's). */
DWRF_U8(DWRF_CFA_def_cfa);
DWRF_UV(JIT_UNWIND_CFA_REG);
DWRF_UV(JIT_UNWIND_CFA_OFFSET);
DWRF_U8(DWRF_CFA_offset | JIT_UNWIND_FP_REG);
DWRF_UV(JIT_UNWIND_FP_OFFSET);
DWRF_U8(DWRF_CFA_offset | JIT_UNWIND_RA_REG);
DWRF_UV(JIT_UNWIND_RA_OFFSET);
DWRF_ALIGNNOP(sizeof(uintptr_t));
)
DWRF_SECTION(FDE,
DWRF_U32((uint32_t)(p - framep)); // Offset to CIE (backwards reference)
DWRF_ADDR(ctx->code_addr); // Absolute code start
DWRF_ADDR((uintptr_t)ctx->code_size); // Code range covered
DWRF_U8(0); // Augmentation data length (none)
DWRF_ALIGNNOP(sizeof(uintptr_t));
)
ctx->p = p;
}
#endif
#if defined(PY_HAVE_JIT_GDB_UNWIND)
enum {
JIT_NOACTION = 0,
JIT_REGISTER_FN = 1,
JIT_UNREGISTER_FN = 2,
};
struct jit_code_entry {
struct jit_code_entry *next;
struct jit_code_entry *prev;
const char *symfile_addr;
uint64_t symfile_size;
const void *code_addr;
};
struct jit_descriptor {
uint32_t version;
uint32_t action_flag;
struct jit_code_entry *relevant_entry;
struct jit_code_entry *first_entry;
};
PyMutex _Py_jit_debug_mutex = {0};
Py_EXPORTED_SYMBOL volatile struct jit_descriptor __jit_debug_descriptor = {
1, JIT_NOACTION, NULL, NULL
};
Py_EXPORTED_SYMBOL void __attribute__((noinline))
__jit_debug_register_code(void)
{
/* Keep this call visible to debuggers and not optimized away. */
(void)__jit_debug_descriptor.action_flag;
#if defined(__GNUC__) || defined(__clang__)
__asm__ __volatile__("" ::: "memory");
#endif
}
static uint16_t
gdb_jit_machine_id(void)
{
/* Map the current target to ELF e_machine; return 0 to skip registration. */
#if defined(__x86_64__) || defined(_M_X64)
return EM_X86_64;
#elif defined(__aarch64__) && !defined(__ILP32__)
return EM_AARCH64;
#else
return 0;
#endif
}
static struct jit_code_entry *
gdb_jit_register_code(
const void *code_addr,
size_t code_size,
const char *symname,
const uint8_t *eh_frame,
size_t eh_frame_size
)
{
/*
* Build a minimal in-memory ELF for GDB's JIT interface and link it into
* __jit_debug_descriptor so debuggers can resolve JIT code.
*/
if (code_addr == NULL || code_size == 0 || symname == NULL) {
return NULL;
}
const uint16_t machine = gdb_jit_machine_id();
if (machine == 0) {
return NULL;
}
enum {
SH_NULL = 0,
SH_TEXT,
SH_EH_FRAME,
SH_SHSTRTAB,
SH_STRTAB,
SH_SYMTAB,
SH_NUM,
};
static const char shstrtab[] =
"\0.text\0.eh_frame\0.shstrtab\0.strtab\0.symtab";
_Static_assert(sizeof(shstrtab) ==
1 + sizeof(".text") + sizeof(".eh_frame") +
sizeof(".shstrtab") + sizeof(".strtab") + sizeof(".symtab"),
"shstrtab size mismatch");
const size_t shstrtab_size = sizeof(shstrtab);
const size_t sh_text = 1;
const size_t sh_eh_frame = sh_text + sizeof(".text");
const size_t sh_shstrtab = sh_eh_frame + sizeof(".eh_frame");
const size_t sh_strtab = sh_shstrtab + sizeof(".shstrtab");
const size_t sh_symtab = sh_strtab + sizeof(".strtab");
const size_t text_size = code_size;
const size_t text_padded = _Py_SIZE_ROUND_UP(text_size, 8);
const size_t strtab_size = 1 + strlen(symname) + 1;
const size_t symtab_size = 3 * sizeof(Elf64_Sym);
size_t offset = sizeof(Elf64_Ehdr);
offset = _Py_SIZE_ROUND_UP(offset, 16);
const size_t text_off = offset;
const size_t eh_off = text_off + text_padded;
offset = eh_off + eh_frame_size;
const size_t shstr_off = offset;
offset += shstrtab_size;
const size_t str_off = offset;
offset += strtab_size;
/* Elf64_Sym requires 8-byte alignment for st_value/st_size. */
offset = _Py_SIZE_ROUND_UP(offset, 8);
const size_t sym_off = offset;
offset += symtab_size;
offset = _Py_SIZE_ROUND_UP(offset, sizeof(Elf64_Shdr));
const size_t sh_off = offset;
const size_t shnum = SH_NUM;
const size_t total_size = sh_off + shnum * sizeof(Elf64_Shdr);
uint8_t *buf = (uint8_t *)PyMem_RawMalloc(total_size);
if (buf == NULL) {
return NULL;
}
memset(buf, 0, total_size);
Elf64_Ehdr *ehdr = (Elf64_Ehdr *)buf;
memcpy(ehdr->e_ident, ELFMAG, SELFMAG);
ehdr->e_ident[EI_CLASS] = ELFCLASS64;
ehdr->e_ident[EI_DATA] = ELFDATA2LSB;
ehdr->e_ident[EI_VERSION] = EV_CURRENT;
ehdr->e_ident[EI_OSABI] = ELFOSABI_NONE;
ehdr->e_type = ET_DYN;
ehdr->e_machine = machine;
ehdr->e_version = EV_CURRENT;
ehdr->e_entry = 0;
ehdr->e_phoff = 0;
ehdr->e_shoff = sh_off;
ehdr->e_ehsize = sizeof(Elf64_Ehdr);
ehdr->e_shentsize = sizeof(Elf64_Shdr);
ehdr->e_shnum = shnum;
ehdr->e_shstrndx = SH_SHSTRTAB;
memcpy(buf + text_off, code_addr, text_size);
memcpy(buf + eh_off, eh_frame, eh_frame_size);
char *shstr = (char *)(buf + shstr_off);
memcpy(shstr, shstrtab, shstrtab_size);
char *strtab = (char *)(buf + str_off);
strtab[0] = '\0';
memcpy(strtab + 1, symname, strlen(symname));
strtab[strtab_size - 1] = '\0';
Elf64_Sym *syms = (Elf64_Sym *)(buf + sym_off);
memset(syms, 0, symtab_size);
/* Section symbol for .text (local) */
syms[1].st_info = ELF64_ST_INFO(STB_LOCAL, STT_SECTION);
syms[1].st_shndx = 1;
/* Function symbol */
syms[2].st_name = 1;
syms[2].st_info = ELF64_ST_INFO(STB_GLOBAL, STT_FUNC);
syms[2].st_other = STV_DEFAULT;
syms[2].st_shndx = 1;
/* For ET_DYN/ET_EXEC, st_value is the absolute virtual address. */
syms[2].st_value = (Elf64_Addr)(uintptr_t)code_addr;
syms[2].st_size = code_size;
Elf64_Shdr *shdrs = (Elf64_Shdr *)(buf + sh_off);
memset(shdrs, 0, shnum * sizeof(Elf64_Shdr));
shdrs[SH_TEXT].sh_name = sh_text;
shdrs[SH_TEXT].sh_type = SHT_PROGBITS;
shdrs[SH_TEXT].sh_flags = SHF_ALLOC | SHF_EXECINSTR;
shdrs[SH_TEXT].sh_addr = (Elf64_Addr)(uintptr_t)code_addr;
shdrs[SH_TEXT].sh_offset = text_off;
shdrs[SH_TEXT].sh_size = text_size;
shdrs[SH_TEXT].sh_addralign = 16;
shdrs[SH_EH_FRAME].sh_name = sh_eh_frame;
shdrs[SH_EH_FRAME].sh_type = SHT_PROGBITS;
shdrs[SH_EH_FRAME].sh_flags = SHF_ALLOC;
shdrs[SH_EH_FRAME].sh_addr =
(Elf64_Addr)((uintptr_t)code_addr + text_padded);
shdrs[SH_EH_FRAME].sh_offset = eh_off;
shdrs[SH_EH_FRAME].sh_size = eh_frame_size;
shdrs[SH_EH_FRAME].sh_addralign = 8;
shdrs[SH_SHSTRTAB].sh_name = sh_shstrtab;
shdrs[SH_SHSTRTAB].sh_type = SHT_STRTAB;
shdrs[SH_SHSTRTAB].sh_offset = shstr_off;
shdrs[SH_SHSTRTAB].sh_size = shstrtab_size;
shdrs[SH_SHSTRTAB].sh_addralign = 1;
shdrs[SH_STRTAB].sh_name = sh_strtab;
shdrs[SH_STRTAB].sh_type = SHT_STRTAB;
shdrs[SH_STRTAB].sh_offset = str_off;
shdrs[SH_STRTAB].sh_size = strtab_size;
shdrs[SH_STRTAB].sh_addralign = 1;
shdrs[SH_SYMTAB].sh_name = sh_symtab;
shdrs[SH_SYMTAB].sh_type = SHT_SYMTAB;
shdrs[SH_SYMTAB].sh_offset = sym_off;
shdrs[SH_SYMTAB].sh_size = symtab_size;
shdrs[SH_SYMTAB].sh_link = SH_STRTAB;
shdrs[SH_SYMTAB].sh_info = 2;
shdrs[SH_SYMTAB].sh_addralign = 8;
shdrs[SH_SYMTAB].sh_entsize = sizeof(Elf64_Sym);
struct jit_code_entry *entry = PyMem_RawMalloc(sizeof(*entry));
if (entry == NULL) {
PyMem_RawFree(buf);
return NULL;
}
entry->symfile_addr = (const char *)buf;
entry->symfile_size = total_size;
entry->code_addr = code_addr;
PyMutex_Lock(&_Py_jit_debug_mutex);
entry->prev = NULL;
entry->next = __jit_debug_descriptor.first_entry;
if (entry->next != NULL) {
entry->next->prev = entry;
}
__jit_debug_descriptor.first_entry = entry;
__jit_debug_descriptor.relevant_entry = entry;
__jit_debug_descriptor.action_flag = JIT_REGISTER_FN;
__jit_debug_register_code();
__jit_debug_descriptor.action_flag = JIT_NOACTION;
__jit_debug_descriptor.relevant_entry = NULL;
PyMutex_Unlock(&_Py_jit_debug_mutex);
return entry;
}
#endif // defined(PY_HAVE_JIT_GDB_UNWIND)
void *
_PyJitUnwind_GdbRegisterCode(const void *code_addr,
size_t code_size,
const char *entry,
const char *filename)
{
#if defined(PY_HAVE_JIT_GDB_UNWIND)
/* GDB expects a stable symbol name and absolute addresses in .eh_frame. */
if (entry == NULL) {
entry = "";
}
if (filename == NULL) {
filename = "";
}
size_t name_size = snprintf(NULL, 0, "py::%s:%s", entry, filename) + 1;
char *name = (char *)PyMem_RawMalloc(name_size);
if (name == NULL) {
return NULL;
}
snprintf(name, name_size, "py::%s:%s", entry, filename);
uint8_t buffer[1024];
size_t eh_frame_size = _PyJitUnwind_BuildEhFrame(
buffer, sizeof(buffer), code_addr, code_size, 1);
if (eh_frame_size == 0) {
PyMem_RawFree(name);
return NULL;
}
void *handle = gdb_jit_register_code(code_addr, code_size, name,
buffer, eh_frame_size);
PyMem_RawFree(name);
return handle;
#else
(void)code_addr;
(void)code_size;
(void)entry;
(void)filename;
return NULL;
#endif
}
void
_PyJitUnwind_GdbUnregisterCode(void *handle)
{
#if defined(PY_HAVE_JIT_GDB_UNWIND)
struct jit_code_entry *entry = (struct jit_code_entry *)handle;
if (entry == NULL) {
return;
}
PyMutex_Lock(&_Py_jit_debug_mutex);
if (entry->prev != NULL) {
entry->prev->next = entry->next;
}
else {
__jit_debug_descriptor.first_entry = entry->next;
}
if (entry->next != NULL) {
entry->next->prev = entry->prev;
}
__jit_debug_descriptor.relevant_entry = entry;
__jit_debug_descriptor.action_flag = JIT_UNREGISTER_FN;
__jit_debug_register_code();
__jit_debug_descriptor.action_flag = JIT_NOACTION;
__jit_debug_descriptor.relevant_entry = NULL;
PyMutex_Unlock(&_Py_jit_debug_mutex);
PyMem_RawFree((void *)entry->symfile_addr);
PyMem_RawFree(entry);
#else
(void)handle;
#endif
}
#endif // defined(PY_HAVE_PERF_TRAMPOLINE) || defined(PY_HAVE_JIT_GDB_UNWIND)
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