1 //===------------------ mach-o/compact_unwind_encoding.h ------------------===// 2 // 3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. 4 // See https://llvm.org/LICENSE.txt for license information. 5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception 6 // 7 // 8 // Darwin's alternative to DWARF based unwind encodings. 9 // 10 //===----------------------------------------------------------------------===// 11 12 13 #ifndef __COMPACT_UNWIND_ENCODING__ 14 #define __COMPACT_UNWIND_ENCODING__ 15 16 #include <stdint.h> 17 18 // 19 // Compilers can emit standard DWARF FDEs in the __TEXT,__eh_frame section 20 // of object files. Or compilers can emit compact unwind information in 21 // the __LD,__compact_unwind section. 22 // 23 // When the linker creates a final linked image, it will create a 24 // __TEXT,__unwind_info section. This section is a small and fast way for the 25 // runtime to access unwind info for any given function. If the compiler 26 // emitted compact unwind info for the function, that compact unwind info will 27 // be encoded in the __TEXT,__unwind_info section. If the compiler emitted 28 // DWARF unwind info, the __TEXT,__unwind_info section will contain the offset 29 // of the FDE in the __TEXT,__eh_frame section in the final linked image. 30 // 31 // Note: Previously, the linker would transform some DWARF unwind infos into 32 // compact unwind info. But that is fragile and no longer done. 33 34 35 // 36 // The compact unwind endoding is a 32-bit value which encoded in an 37 // architecture specific way, which registers to restore from where, and how 38 // to unwind out of the function. 39 // 40 typedef uint32_t compact_unwind_encoding_t; 41 42 43 // architecture independent bits 44 enum { 45 UNWIND_IS_NOT_FUNCTION_START = 0x80000000, 46 UNWIND_HAS_LSDA = 0x40000000, 47 UNWIND_PERSONALITY_MASK = 0x30000000, 48 }; 49 50 51 52 53 // 54 // x86 55 // 56 // 1-bit: start 57 // 1-bit: has lsda 58 // 2-bit: personality index 59 // 60 // 4-bits: 0=old, 1=ebp based, 2=stack-imm, 3=stack-ind, 4=DWARF 61 // ebp based: 62 // 15-bits (5*3-bits per reg) register permutation 63 // 8-bits for stack offset 64 // frameless: 65 // 8-bits stack size 66 // 3-bits stack adjust 67 // 3-bits register count 68 // 10-bits register permutation 69 // 70 enum { 71 UNWIND_X86_MODE_MASK = 0x0F000000, 72 UNWIND_X86_MODE_EBP_FRAME = 0x01000000, 73 UNWIND_X86_MODE_STACK_IMMD = 0x02000000, 74 UNWIND_X86_MODE_STACK_IND = 0x03000000, 75 UNWIND_X86_MODE_DWARF = 0x04000000, 76 77 UNWIND_X86_EBP_FRAME_REGISTERS = 0x00007FFF, 78 UNWIND_X86_EBP_FRAME_OFFSET = 0x00FF0000, 79 80 UNWIND_X86_FRAMELESS_STACK_SIZE = 0x00FF0000, 81 UNWIND_X86_FRAMELESS_STACK_ADJUST = 0x0000E000, 82 UNWIND_X86_FRAMELESS_STACK_REG_COUNT = 0x00001C00, 83 UNWIND_X86_FRAMELESS_STACK_REG_PERMUTATION = 0x000003FF, 84 85 UNWIND_X86_DWARF_SECTION_OFFSET = 0x00FFFFFF, 86 }; 87 88 enum { 89 UNWIND_X86_REG_NONE = 0, 90 UNWIND_X86_REG_EBX = 1, 91 UNWIND_X86_REG_ECX = 2, 92 UNWIND_X86_REG_EDX = 3, 93 UNWIND_X86_REG_EDI = 4, 94 UNWIND_X86_REG_ESI = 5, 95 UNWIND_X86_REG_EBP = 6, 96 }; 97 98 // 99 // For x86 there are four modes for the compact unwind encoding: 100 // UNWIND_X86_MODE_EBP_FRAME: 101 // EBP based frame where EBP is push on stack immediately after return address, 102 // then ESP is moved to EBP. Thus, to unwind ESP is restored with the current 103 // EPB value, then EBP is restored by popping off the stack, and the return 104 // is done by popping the stack once more into the pc. 105 // All non-volatile registers that need to be restored must have been saved 106 // in a small range in the stack that starts EBP-4 to EBP-1020. The offset/4 107 // is encoded in the UNWIND_X86_EBP_FRAME_OFFSET bits. The registers saved 108 // are encoded in the UNWIND_X86_EBP_FRAME_REGISTERS bits as five 3-bit entries. 109 // Each entry contains which register to restore. 110 // UNWIND_X86_MODE_STACK_IMMD: 111 // A "frameless" (EBP not used as frame pointer) function with a small 112 // constant stack size. To return, a constant (encoded in the compact 113 // unwind encoding) is added to the ESP. Then the return is done by 114 // popping the stack into the pc. 115 // All non-volatile registers that need to be restored must have been saved 116 // on the stack immediately after the return address. The stack_size/4 is 117 // encoded in the UNWIND_X86_FRAMELESS_STACK_SIZE (max stack size is 1024). 118 // The number of registers saved is encoded in UNWIND_X86_FRAMELESS_STACK_REG_COUNT. 119 // UNWIND_X86_FRAMELESS_STACK_REG_PERMUTATION constains which registers were 120 // saved and their order. 121 // UNWIND_X86_MODE_STACK_IND: 122 // A "frameless" (EBP not used as frame pointer) function large constant 123 // stack size. This case is like the previous, except the stack size is too 124 // large to encode in the compact unwind encoding. Instead it requires that 125 // the function contains "subl $nnnnnnnn,ESP" in its prolog. The compact 126 // encoding contains the offset to the nnnnnnnn value in the function in 127 // UNWIND_X86_FRAMELESS_STACK_SIZE. 128 // UNWIND_X86_MODE_DWARF: 129 // No compact unwind encoding is available. Instead the low 24-bits of the 130 // compact encoding is the offset of the DWARF FDE in the __eh_frame section. 131 // This mode is never used in object files. It is only generated by the 132 // linker in final linked images which have only DWARF unwind info for a 133 // function. 134 // 135 // The permutation encoding is a Lehmer code sequence encoded into a 136 // single variable-base number so we can encode the ordering of up to 137 // six registers in a 10-bit space. 138 // 139 // The following is the algorithm used to create the permutation encoding used 140 // with frameless stacks. It is passed the number of registers to be saved and 141 // an array of the register numbers saved. 142 // 143 //uint32_t permute_encode(uint32_t registerCount, const uint32_t registers[6]) 144 //{ 145 // uint32_t renumregs[6]; 146 // for (int i=6-registerCount; i < 6; ++i) { 147 // int countless = 0; 148 // for (int j=6-registerCount; j < i; ++j) { 149 // if ( registers[j] < registers[i] ) 150 // ++countless; 151 // } 152 // renumregs[i] = registers[i] - countless -1; 153 // } 154 // uint32_t permutationEncoding = 0; 155 // switch ( registerCount ) { 156 // case 6: 157 // permutationEncoding |= (120*renumregs[0] + 24*renumregs[1] 158 // + 6*renumregs[2] + 2*renumregs[3] 159 // + renumregs[4]); 160 // break; 161 // case 5: 162 // permutationEncoding |= (120*renumregs[1] + 24*renumregs[2] 163 // + 6*renumregs[3] + 2*renumregs[4] 164 // + renumregs[5]); 165 // break; 166 // case 4: 167 // permutationEncoding |= (60*renumregs[2] + 12*renumregs[3] 168 // + 3*renumregs[4] + renumregs[5]); 169 // break; 170 // case 3: 171 // permutationEncoding |= (20*renumregs[3] + 4*renumregs[4] 172 // + renumregs[5]); 173 // break; 174 // case 2: 175 // permutationEncoding |= (5*renumregs[4] + renumregs[5]); 176 // break; 177 // case 1: 178 // permutationEncoding |= (renumregs[5]); 179 // break; 180 // } 181 // return permutationEncoding; 182 //} 183 // 184 185 186 187 188 // 189 // x86_64 190 // 191 // 1-bit: start 192 // 1-bit: has lsda 193 // 2-bit: personality index 194 // 195 // 4-bits: 0=old, 1=rbp based, 2=stack-imm, 3=stack-ind, 4=DWARF 196 // rbp based: 197 // 15-bits (5*3-bits per reg) register permutation 198 // 8-bits for stack offset 199 // frameless: 200 // 8-bits stack size 201 // 3-bits stack adjust 202 // 3-bits register count 203 // 10-bits register permutation 204 // 205 enum { 206 UNWIND_X86_64_MODE_MASK = 0x0F000000, 207 UNWIND_X86_64_MODE_RBP_FRAME = 0x01000000, 208 UNWIND_X86_64_MODE_STACK_IMMD = 0x02000000, 209 UNWIND_X86_64_MODE_STACK_IND = 0x03000000, 210 UNWIND_X86_64_MODE_DWARF = 0x04000000, 211 212 UNWIND_X86_64_RBP_FRAME_REGISTERS = 0x00007FFF, 213 UNWIND_X86_64_RBP_FRAME_OFFSET = 0x00FF0000, 214 215 UNWIND_X86_64_FRAMELESS_STACK_SIZE = 0x00FF0000, 216 UNWIND_X86_64_FRAMELESS_STACK_ADJUST = 0x0000E000, 217 UNWIND_X86_64_FRAMELESS_STACK_REG_COUNT = 0x00001C00, 218 UNWIND_X86_64_FRAMELESS_STACK_REG_PERMUTATION = 0x000003FF, 219 220 UNWIND_X86_64_DWARF_SECTION_OFFSET = 0x00FFFFFF, 221 }; 222 223 enum { 224 UNWIND_X86_64_REG_NONE = 0, 225 UNWIND_X86_64_REG_RBX = 1, 226 UNWIND_X86_64_REG_R12 = 2, 227 UNWIND_X86_64_REG_R13 = 3, 228 UNWIND_X86_64_REG_R14 = 4, 229 UNWIND_X86_64_REG_R15 = 5, 230 UNWIND_X86_64_REG_RBP = 6, 231 }; 232 // 233 // For x86_64 there are four modes for the compact unwind encoding: 234 // UNWIND_X86_64_MODE_RBP_FRAME: 235 // RBP based frame where RBP is push on stack immediately after return address, 236 // then RSP is moved to RBP. Thus, to unwind RSP is restored with the current 237 // EPB value, then RBP is restored by popping off the stack, and the return 238 // is done by popping the stack once more into the pc. 239 // All non-volatile registers that need to be restored must have been saved 240 // in a small range in the stack that starts RBP-8 to RBP-2040. The offset/8 241 // is encoded in the UNWIND_X86_64_RBP_FRAME_OFFSET bits. The registers saved 242 // are encoded in the UNWIND_X86_64_RBP_FRAME_REGISTERS bits as five 3-bit entries. 243 // Each entry contains which register to restore. 244 // UNWIND_X86_64_MODE_STACK_IMMD: 245 // A "frameless" (RBP not used as frame pointer) function with a small 246 // constant stack size. To return, a constant (encoded in the compact 247 // unwind encoding) is added to the RSP. Then the return is done by 248 // popping the stack into the pc. 249 // All non-volatile registers that need to be restored must have been saved 250 // on the stack immediately after the return address. The stack_size/8 is 251 // encoded in the UNWIND_X86_64_FRAMELESS_STACK_SIZE (max stack size is 2048). 252 // The number of registers saved is encoded in UNWIND_X86_64_FRAMELESS_STACK_REG_COUNT. 253 // UNWIND_X86_64_FRAMELESS_STACK_REG_PERMUTATION constains which registers were 254 // saved and their order. 255 // UNWIND_X86_64_MODE_STACK_IND: 256 // A "frameless" (RBP not used as frame pointer) function large constant 257 // stack size. This case is like the previous, except the stack size is too 258 // large to encode in the compact unwind encoding. Instead it requires that 259 // the function contains "subq $nnnnnnnn,RSP" in its prolog. The compact 260 // encoding contains the offset to the nnnnnnnn value in the function in 261 // UNWIND_X86_64_FRAMELESS_STACK_SIZE. 262 // UNWIND_X86_64_MODE_DWARF: 263 // No compact unwind encoding is available. Instead the low 24-bits of the 264 // compact encoding is the offset of the DWARF FDE in the __eh_frame section. 265 // This mode is never used in object files. It is only generated by the 266 // linker in final linked images which have only DWARF unwind info for a 267 // function. 268 // 269 270 271 // ARM64 272 // 273 // 1-bit: start 274 // 1-bit: has lsda 275 // 2-bit: personality index 276 // 277 // 4-bits: 4=frame-based, 3=DWARF, 2=frameless 278 // frameless: 279 // 12-bits of stack size 280 // frame-based: 281 // 4-bits D reg pairs saved 282 // 5-bits X reg pairs saved 283 // DWARF: 284 // 24-bits offset of DWARF FDE in __eh_frame section 285 // 286 enum { 287 UNWIND_ARM64_MODE_MASK = 0x0F000000, 288 UNWIND_ARM64_MODE_FRAMELESS = 0x02000000, 289 UNWIND_ARM64_MODE_DWARF = 0x03000000, 290 UNWIND_ARM64_MODE_FRAME = 0x04000000, 291 292 UNWIND_ARM64_FRAME_X19_X20_PAIR = 0x00000001, 293 UNWIND_ARM64_FRAME_X21_X22_PAIR = 0x00000002, 294 UNWIND_ARM64_FRAME_X23_X24_PAIR = 0x00000004, 295 UNWIND_ARM64_FRAME_X25_X26_PAIR = 0x00000008, 296 UNWIND_ARM64_FRAME_X27_X28_PAIR = 0x00000010, 297 UNWIND_ARM64_FRAME_D8_D9_PAIR = 0x00000100, 298 UNWIND_ARM64_FRAME_D10_D11_PAIR = 0x00000200, 299 UNWIND_ARM64_FRAME_D12_D13_PAIR = 0x00000400, 300 UNWIND_ARM64_FRAME_D14_D15_PAIR = 0x00000800, 301 302 UNWIND_ARM64_FRAMELESS_STACK_SIZE_MASK = 0x00FFF000, 303 UNWIND_ARM64_DWARF_SECTION_OFFSET = 0x00FFFFFF, 304 }; 305 // For arm64 there are three modes for the compact unwind encoding: 306 // UNWIND_ARM64_MODE_FRAME: 307 // This is a standard arm64 prolog where FP/LR are immediately pushed on the 308 // stack, then SP is copied to FP. If there are any non-volatile registers 309 // saved, then are copied into the stack frame in pairs in a contiguous 310 // range right below the saved FP/LR pair. Any subset of the five X pairs 311 // and four D pairs can be saved, but the memory layout must be in register 312 // number order. 313 // UNWIND_ARM64_MODE_FRAMELESS: 314 // A "frameless" leaf function, where FP/LR are not saved. The return address 315 // remains in LR throughout the function. If any non-volatile registers 316 // are saved, they must be pushed onto the stack before any stack space is 317 // allocated for local variables. The stack sized (including any saved 318 // non-volatile registers) divided by 16 is encoded in the bits 319 // UNWIND_ARM64_FRAMELESS_STACK_SIZE_MASK. 320 // UNWIND_ARM64_MODE_DWARF: 321 // No compact unwind encoding is available. Instead the low 24-bits of the 322 // compact encoding is the offset of the DWARF FDE in the __eh_frame section. 323 // This mode is never used in object files. It is only generated by the 324 // linker in final linked images which have only DWARF unwind info for a 325 // function. 326 // 327 328 329 330 331 332 //////////////////////////////////////////////////////////////////////////////// 333 // 334 // Relocatable Object Files: __LD,__compact_unwind 335 // 336 //////////////////////////////////////////////////////////////////////////////// 337 338 // 339 // A compiler can generated compact unwind information for a function by adding 340 // a "row" to the __LD,__compact_unwind section. This section has the 341 // S_ATTR_DEBUG bit set, so the section will be ignored by older linkers. 342 // It is removed by the new linker, so never ends up in final executables. 343 // This section is a table, initially with one row per function (that needs 344 // unwind info). The table columns and some conceptual entries are: 345 // 346 // range-start pointer to start of function/range 347 // range-length 348 // compact-unwind-encoding 32-bit encoding 349 // personality-function or zero if no personality function 350 // lsda or zero if no LSDA data 351 // 352 // The length and encoding fields are 32-bits. The other are all pointer sized. 353 // 354 // In x86_64 assembly, these entry would look like: 355 // 356 // .section __LD,__compact_unwind,regular,debug 357 // 358 // #compact unwind for _foo 359 // .quad _foo 360 // .set L1,LfooEnd-_foo 361 // .long L1 362 // .long 0x01010001 363 // .quad 0 364 // .quad 0 365 // 366 // #compact unwind for _bar 367 // .quad _bar 368 // .set L2,LbarEnd-_bar 369 // .long L2 370 // .long 0x01020011 371 // .quad __gxx_personality 372 // .quad except_tab1 373 // 374 // 375 // Notes: There is no need for any labels in the the __compact_unwind section. 376 // The use of the .set directive is to force the evaluation of the 377 // range-length at assembly time, instead of generating relocations. 378 // 379 // To support future compiler optimizations where which non-volatile registers 380 // are saved changes within a function (e.g. delay saving non-volatiles until 381 // necessary), there can by multiple lines in the __compact_unwind table for one 382 // function, each with a different (non-overlapping) range and each with 383 // different compact unwind encodings that correspond to the non-volatiles 384 // saved at that range of the function. 385 // 386 // If a particular function is so wacky that there is no compact unwind way 387 // to encode it, then the compiler can emit traditional DWARF unwind info. 388 // The runtime will use which ever is available. 389 // 390 // Runtime support for compact unwind encodings are only available on 10.6 391 // and later. So, the compiler should not generate it when targeting pre-10.6. 392 393 394 395 396 //////////////////////////////////////////////////////////////////////////////// 397 // 398 // Final Linked Images: __TEXT,__unwind_info 399 // 400 //////////////////////////////////////////////////////////////////////////////// 401 402 // 403 // The __TEXT,__unwind_info section is laid out for an efficient two level lookup. 404 // The header of the section contains a coarse index that maps function address 405 // to the page (4096 byte block) containing the unwind info for that function. 406 // 407 408 #define UNWIND_SECTION_VERSION 1 409 struct unwind_info_section_header 410 { 411 uint32_t version; // UNWIND_SECTION_VERSION 412 uint32_t commonEncodingsArraySectionOffset; 413 uint32_t commonEncodingsArrayCount; 414 uint32_t personalityArraySectionOffset; 415 uint32_t personalityArrayCount; 416 uint32_t indexSectionOffset; 417 uint32_t indexCount; 418 // compact_unwind_encoding_t[] 419 // uint32_t personalities[] 420 // unwind_info_section_header_index_entry[] 421 // unwind_info_section_header_lsda_index_entry[] 422 }; 423 424 struct unwind_info_section_header_index_entry 425 { 426 uint32_t functionOffset; 427 uint32_t secondLevelPagesSectionOffset; // section offset to start of regular or compress page 428 uint32_t lsdaIndexArraySectionOffset; // section offset to start of lsda_index array for this range 429 }; 430 431 struct unwind_info_section_header_lsda_index_entry 432 { 433 uint32_t functionOffset; 434 uint32_t lsdaOffset; 435 }; 436 437 // 438 // There are two kinds of second level index pages: regular and compressed. 439 // A compressed page can hold up to 1021 entries, but it cannot be used 440 // if too many different encoding types are used. The regular page holds 441 // 511 entries. 442 // 443 444 struct unwind_info_regular_second_level_entry 445 { 446 uint32_t functionOffset; 447 compact_unwind_encoding_t encoding; 448 }; 449 450 #define UNWIND_SECOND_LEVEL_REGULAR 2 451 struct unwind_info_regular_second_level_page_header 452 { 453 uint32_t kind; // UNWIND_SECOND_LEVEL_REGULAR 454 uint16_t entryPageOffset; 455 uint16_t entryCount; 456 // entry array 457 }; 458 459 #define UNWIND_SECOND_LEVEL_COMPRESSED 3 460 struct unwind_info_compressed_second_level_page_header 461 { 462 uint32_t kind; // UNWIND_SECOND_LEVEL_COMPRESSED 463 uint16_t entryPageOffset; 464 uint16_t entryCount; 465 uint16_t encodingsPageOffset; 466 uint16_t encodingsCount; 467 // 32-bit entry array 468 // encodings array 469 }; 470 471 #define UNWIND_INFO_COMPRESSED_ENTRY_FUNC_OFFSET(entry) (entry & 0x00FFFFFF) 472 #define UNWIND_INFO_COMPRESSED_ENTRY_ENCODING_INDEX(entry) ((entry >> 24) & 0xFF) 473 474 475 476 #endif 477 478
