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; 42 if (tlsSize != app.tls.size) 43 tlsSize += app.tls.align; 44 45 // Per the x86_64 TLS ABI, the entry pointed to by the thread pointer is the 46 // address of the TLS block. So, we add more size to accomodate this address 47 // entry. 48 size_t tlsSizeWithAddr = tlsSize + sizeof(uintptr_t); 49 50 // We cannot call the mmap function here as the functions set errno on 51 // failure. Since errno is implemented via a thread local variable, we cannot 52 // use errno before TLS is setup. 53 long mmapRetVal = __llvm_libc::syscall( 54 mmapSyscallNumber, nullptr, tlsSizeWithAddr, PROT_READ | PROT_WRITE, 55 MAP_ANONYMOUS | MAP_PRIVATE, -1, 0); 56 // We cannot check the return value with MAP_FAILED as that is the return 57 // of the mmap function and not the mmap syscall. 58 if (mmapRetVal < 0 && static_cast<uintptr_t>(mmapRetVal) > -app.pageSize) 59 __llvm_libc::syscall(SYS_exit, 1); 60 uintptr_t *tlsAddr = reinterpret_cast<uintptr_t *>(mmapRetVal); 61 62 // x86_64 TLS faces down from the thread pointer with the first entry 63 // pointing to the address of the first real TLS byte. 64 uintptr_t endPtr = reinterpret_cast<uintptr_t>(tlsAddr) + tlsSize; 65 *reinterpret_cast<uintptr_t *>(endPtr) = endPtr; 66 67 __llvm_libc::memcpy(tlsAddr, reinterpret_cast<const void *>(app.tls.address), 68 app.tls.size); 69 if (__llvm_libc::syscall(SYS_arch_prctl, ARCH_SET_FS, endPtr) == -1) 70 __llvm_libc::syscall(SYS_exit, 1); 71 } 72 73 } // namespace __llvm_libc 74 75 using __llvm_libc::app; 76 77 // TODO: Would be nice to use the aux entry structure from elf.h when available. 78 struct AuxEntry { 79 uint64_t type; 80 uint64_t value; 81 }; 82 83 extern "C" void _start() { 84 // This TU is compiled with -fno-omit-frame-pointer. Hence, the previous value 85 // of the base pointer is pushed on to the stack. So, we step over it (the 86 // "+ 1" below) to get to the args. 87 app.args = reinterpret_cast<__llvm_libc::Args *>( 88 reinterpret_cast<uintptr_t *>(__builtin_frame_address(0)) + 1); 89 90 // The x86_64 ABI requires that the stack pointer is aligned to a 16-byte 91 // boundary. We align it here but we cannot use any local variables created 92 // before the following alignment. Best would be to not create any local 93 // variables before the alignment. Also, note that we are aligning the stack 94 // downwards as the x86_64 stack grows downwards. This ensures that we don't 95 // tread on argc, argv etc. 96 // NOTE: Compiler attributes for alignment do not help here as the stack 97 // pointer on entry to this _start function is controlled by the OS. In fact, 98 // compilers can generate code assuming the alignment as required by the ABI. 99 // If the stack pointers as setup by the OS are already aligned, then the 100 // following code is a NOP. 101 __asm__ __volatile__("andq $0xfffffffffffffff0, %%rsp\n\t" ::: "%rsp"); 102 __asm__ __volatile__("andq $0xfffffffffffffff0, %%rbp\n\t" ::: "%rbp"); 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 = app.args->argv + app.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, main(app.args->argc, 147 reinterpret_cast<char **>(app.args->argv), 148 reinterpret_cast<char **>(env_ptr))); 149 } 150