1 //===-- Implementation of crt for x86_64 ----------------------------------===// 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 9 #include "config/linux/app.h" 10 #include "src/__support/OSUtil/syscall.h" 11 #include "src/string/memcpy.h" 12 13 #include <asm/prctl.h> 14 #include <linux/auxvec.h> 15 #include <linux/elf.h> 16 #include <stdint.h> 17 #include <sys/mman.h> 18 #include <sys/syscall.h> 19 20 extern "C" int main(int, char **, char **); 21 22 namespace __llvm_libc { 23 24 #ifdef SYS_mmap2 25 static constexpr long mmapSyscallNumber = SYS_mmap2; 26 #elif SYS_mmap 27 static constexpr long mmapSyscallNumber = SYS_mmap; 28 #else 29 #error "Target platform does not have SYS_mmap or SYS_mmap2 defined" 30 #endif 31 32 AppProperties app; 33 34 // TODO: The function is x86_64 specific. Move it to config/linux/app.h 35 // and generalize it. Also, dynamic loading is not handled currently. 36 void initTLS() { 37 if (app.tls.size == 0) 38 return; 39 40 // We will assume the alignment is always a power of two. 41 uintptr_t tlsSize = (app.tls.size + app.tls.align) & -app.tls.align; 42 43 // Per the x86_64 TLS ABI, the entry pointed to by the thread pointer is the 44 // address of the TLS block. So, we add more size to accomodate this address 45 // entry. 46 size_t tlsSizeWithAddr = tlsSize + sizeof(uintptr_t); 47 48 // We cannot call the mmap function here as the functions set errno on 49 // failure. Since errno is implemented via a thread local variable, we cannot 50 // use errno before TLS is setup. 51 long mmapRetVal = __llvm_libc::syscall( 52 mmapSyscallNumber, nullptr, tlsSizeWithAddr, PROT_READ | PROT_WRITE, 53 MAP_ANONYMOUS | MAP_PRIVATE, -1, 0); 54 // We cannot check the return value with MAP_FAILED as that is the return 55 // of the mmap function and not the mmap syscall. 56 if (mmapRetVal < 0 && static_cast<uintptr_t>(mmapRetVal) > -app.pageSize) 57 __llvm_libc::syscall(SYS_exit, 1); 58 uintptr_t *tlsAddr = reinterpret_cast<uintptr_t *>(mmapRetVal); 59 60 // x86_64 TLS faces down from the thread pointer with the first entry 61 // pointing to the address of the first real TLS byte. 62 uintptr_t endPtr = reinterpret_cast<uintptr_t>(tlsAddr) + tlsSize; 63 *reinterpret_cast<uintptr_t *>(endPtr) = endPtr; 64 65 __llvm_libc::memcpy(tlsAddr, reinterpret_cast<const void *>(app.tls.address), 66 app.tls.size); 67 if (__llvm_libc::syscall(SYS_arch_prctl, ARCH_SET_FS, endPtr) == -1) 68 __llvm_libc::syscall(SYS_exit, 1); 69 } 70 71 } // namespace __llvm_libc 72 73 using __llvm_libc::app; 74 75 struct Args { 76 // At the language level, argc is an int. But we use uint64_t as the x86_64 77 // ABI specifies it as an 8 byte value. 78 uint64_t argc; 79 80 // At the language level, argv is a char** value. However, we use uint64_t as 81 // the x86_64 ABI specifies the argv vector be an |argc| long array of 8-byte 82 // values. Even though a flexible length array would be more suitable here, we 83 // set the array length to 1 to avoid a compiler warning about it being a C99 84 // extension. Length of 1 is not really wrong as |argc| is guaranteed to be 85 // atleast 1, and there is an 8-byte null entry at the end of the argv array. 86 uint64_t argv[1]; 87 }; 88 89 // TODO: Would be nice to use the aux entry structure from elf.h when available. 90 struct AuxEntry { 91 uint64_t type; 92 uint64_t value; 93 }; 94 95 extern "C" void _start() { 96 uintptr_t *frame_ptr = 97 reinterpret_cast<uintptr_t *>(__builtin_frame_address(0)); 98 99 // This TU is compiled with -fno-omit-frame-pointer. Hence, the previous value 100 // of the base pointer is pushed on to the stack. So, we step over it (the 101 // "+ 1" below) to get to the args. 102 Args *args = reinterpret_cast<Args *>(frame_ptr + 1); 103 104 // After the argv array, is a 8-byte long NULL value before the array of env 105 // values. The end of the env values is marked by another 8-byte long NULL 106 // value. We step over it (the "+ 1" below) to get to the env values. 107 uint64_t *env_ptr = args->argv + args->argc + 1; 108 uint64_t *env_end_marker = env_ptr; 109 app.envPtr = env_ptr; 110 while (*env_end_marker) 111 ++env_end_marker; 112 113 // After the env array, is the aux-vector. The end of the aux-vector is 114 // denoted by an AT_NULL entry. 115 Elf64_Phdr *programHdrTable = nullptr; 116 uintptr_t programHdrCount; 117 for (AuxEntry *aux_entry = reinterpret_cast<AuxEntry *>(env_end_marker + 1); 118 aux_entry->type != AT_NULL; ++aux_entry) { 119 switch (aux_entry->type) { 120 case AT_PHDR: 121 programHdrTable = reinterpret_cast<Elf64_Phdr *>(aux_entry->value); 122 break; 123 case AT_PHNUM: 124 programHdrCount = aux_entry->value; 125 break; 126 case AT_PAGESZ: 127 app.pageSize = aux_entry->value; 128 break; 129 default: 130 break; // TODO: Read other useful entries from the aux vector. 131 } 132 } 133 134 for (uintptr_t i = 0; i < programHdrCount; ++i) { 135 Elf64_Phdr *phdr = programHdrTable + i; 136 if (phdr->p_type != PT_TLS) 137 continue; 138 // TODO: p_vaddr value has to be adjusted for static-pie executables. 139 app.tls.address = phdr->p_vaddr; 140 app.tls.size = phdr->p_memsz; 141 app.tls.align = phdr->p_align; 142 } 143 144 __llvm_libc::initTLS(); 145 146 __llvm_libc::syscall(SYS_exit, 147 main(args->argc, reinterpret_cast<char **>(args->argv), 148 reinterpret_cast<char **>(env_ptr))); 149 } 150