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 "include/sys/mman.h" 11 #include "include/sys/syscall.h" 12 #include "src/__support/OSUtil/syscall.h" 13 #include "src/string/memcpy.h" 14 15 #include <asm/prctl.h> 16 #include <linux/auxvec.h> 17 #include <linux/elf.h> 18 #include <stdint.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 // TODO: Declare var an extern var in config/linux/app.h so that other 33 // libc functions can make use of the application wide information. For 34 // example, mmap can pick up the page size from here. 35 AppProperties app; 36 37 // TODO: The function is x86_64 specific. Move it to config/linux/app.h 38 // and generalize it. Also, dynamic loading is not handled currently. 39 void initTLS() { 40 if (app.tls.size == 0) 41 return; 42 43 // We will assume the alignment is always a power of two. 44 uintptr_t tlsSize = (app.tls.size + app.tls.align) & -app.tls.align; 45 46 // Per the x86_64 TLS ABI, the entry pointed to by the thread pointer is the 47 // address of the TLS block. So, we add more size to accomodate this address 48 // entry. 49 size_t tlsSizeWithAddr = tlsSize + sizeof(uintptr_t); 50 51 // We cannot call the mmap function here as the functions set errno on 52 // failure. Since errno is implemented via a thread local variable, we cannot 53 // use errno before TLS is setup. 54 long mmapRetVal = __llvm_libc::syscall( 55 mmapSyscallNumber, nullptr, tlsSizeWithAddr, PROT_READ | PROT_WRITE, 56 MAP_ANONYMOUS | MAP_PRIVATE, -1, 0); 57 // We cannot check the return value with MAP_FAILED as that is the return 58 // of the mmap function and not the mmap syscall. 59 if (mmapRetVal < 0 && static_cast<uintptr_t>(mmapRetVal) > -app.pageSize) 60 __llvm_libc::syscall(SYS_exit, 1); 61 uintptr_t *tlsAddr = reinterpret_cast<uintptr_t *>(mmapRetVal); 62 63 // x86_64 TLS faces down from the thread pointer with the first entry 64 // pointing to the address of the first real TLS byte. 65 uintptr_t endPtr = reinterpret_cast<uintptr_t>(tlsAddr) + tlsSize; 66 *reinterpret_cast<uintptr_t *>(endPtr) = endPtr; 67 68 __llvm_libc::memcpy(tlsAddr, reinterpret_cast<const void *>(app.tls.address), 69 app.tls.size); 70 if (__llvm_libc::syscall(SYS_arch_prctl, ARCH_SET_FS, endPtr) == -1) 71 __llvm_libc::syscall(SYS_exit, 1); 72 } 73 74 } // namespace __llvm_libc 75 76 using __llvm_libc::app; 77 78 struct Args { 79 // At the language level, argc is an int. But we use uint64_t as the x86_64 80 // ABI specifies it as an 8 byte value. 81 uint64_t argc; 82 83 // At the language level, argv is a char** value. However, we use uint64_t as 84 // the x86_64 ABI specifies the argv vector be an |argc| long array of 8-byte 85 // values. Even though a flexible length array would be more suitable here, we 86 // set the array length to 1 to avoid a compiler warning about it being a C99 87 // extension. Length of 1 is not really wrong as |argc| is guaranteed to be 88 // atleast 1, and there is an 8-byte null entry at the end of the argv array. 89 uint64_t argv[1]; 90 }; 91 92 // TODO: Would be nice to use the aux entry structure from elf.h when available. 93 struct AuxEntry { 94 uint64_t type; 95 uint64_t value; 96 }; 97 98 extern "C" void _start() { 99 uintptr_t *frame_ptr = 100 reinterpret_cast<uintptr_t *>(__builtin_frame_address(0)); 101 102 // This TU is compiled with -fno-omit-frame-pointer. Hence, the previous value 103 // of the base pointer is pushed on to the stack. So, we step over it (the 104 // "+ 1" below) to get to the args. 105 Args *args = reinterpret_cast<Args *>(frame_ptr + 1); 106 107 // After the argv array, is a 8-byte long NULL value before the array of env 108 // values. The end of the env values is marked by another 8-byte long NULL 109 // value. We step over it (the "+ 1" below) to get to the env values. 110 uint64_t *env_ptr = args->argv + args->argc + 1; 111 uint64_t *env_end_marker = env_ptr; 112 while (*env_end_marker) 113 ++env_end_marker; 114 115 // After the env array, is the aux-vector. The end of the aux-vector is 116 // denoted by an AT_NULL entry. 117 Elf64_Phdr *programHdrTable = nullptr; 118 uintptr_t programHdrCount; 119 for (AuxEntry *aux_entry = reinterpret_cast<AuxEntry *>(env_end_marker + 1); 120 aux_entry->type != AT_NULL; ++aux_entry) { 121 switch (aux_entry->type) { 122 case AT_PHDR: 123 programHdrTable = reinterpret_cast<Elf64_Phdr *>(aux_entry->value); 124 break; 125 case AT_PHNUM: 126 programHdrCount = aux_entry->value; 127 break; 128 case AT_PAGESZ: 129 app.pageSize = aux_entry->value; 130 break; 131 default: 132 break; // TODO: Read other useful entries from the aux vector. 133 } 134 } 135 136 for (uintptr_t i = 0; i < programHdrCount; ++i) { 137 Elf64_Phdr *phdr = programHdrTable + i; 138 if (phdr->p_type != PT_TLS) 139 continue; 140 // TODO: p_vaddr value has to be adjusted for static-pie executables. 141 app.tls.address = phdr->p_vaddr; 142 app.tls.size = phdr->p_memsz; 143 app.tls.align = phdr->p_align; 144 } 145 146 __llvm_libc::initTLS(); 147 148 __llvm_libc::syscall(SYS_exit, 149 main(args->argc, reinterpret_cast<char **>(args->argv), 150 reinterpret_cast<char **>(env_ptr))); 151 } 152