1//===-- Implementation of tls 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 "src/__support/OSUtil/syscall.h"
10#include "src/__support/macros/config.h"
11#include "src/string/memory_utils/inline_memcpy.h"
12#include "startup/linux/do_start.h"
13
14#include <asm/prctl.h>
15#include <sys/mman.h>
16#include <sys/syscall.h>
17
18namespace LIBC_NAMESPACE_DECL {
19
20#ifdef SYS_mmap2
21static constexpr long MMAP_SYSCALL_NUMBER = SYS_mmap2;
22#elif SYS_mmap
23static constexpr long MMAP_SYSCALL_NUMBER = SYS_mmap;
24#else
25#error "mmap and mmap2 syscalls not available."
26#endif
27
28// TODO: Also generalize this routine and handle dynamic loading properly.
29void init_tls(TLSDescriptor &tls_descriptor) {
30 if (app.tls.size == 0) {
31 tls_descriptor.size = 0;
32 tls_descriptor.tp = 0;
33 return;
34 }
35
36 // We will assume the alignment is always a power of two.
37 uintptr_t tls_size = app.tls.size & -app.tls.align;
38 if (tls_size != app.tls.size)
39 tls_size += app.tls.align;
40
41 // Per the x86_64 TLS ABI, the entry pointed to by the thread pointer is the
42 // address of the TLS block. So, we add more size to accomodate this address
43 // entry.
44 // We also need to include space for the stack canary. The canary is at
45 // offset 0x28 (40) and is of size uintptr_t.
46 uintptr_t tls_size_with_addr = tls_size + sizeof(uintptr_t) + 40;
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 mmap_retval = syscall_impl<long>(
52 MMAP_SYSCALL_NUMBER, nullptr, tls_size_with_addr, 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 (mmap_retval < 0 && static_cast<uintptr_t>(mmap_retval) > -app.page_size)
57 syscall_impl<long>(SYS_exit, 1);
58 uintptr_t *tls_addr = reinterpret_cast<uintptr_t *>(mmap_retval);
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 end_ptr = reinterpret_cast<uintptr_t>(tls_addr) + tls_size;
63 *reinterpret_cast<uintptr_t *>(end_ptr) = end_ptr;
64
65 inline_memcpy(reinterpret_cast<char *>(tls_addr),
66 reinterpret_cast<const char *>(app.tls.address),
67 app.tls.init_size);
68 uintptr_t *stack_guard_addr = reinterpret_cast<uintptr_t *>(end_ptr + 40);
69 // Setting the stack guard to a random value.
70 // We cannot call the get_random function here as the function sets errno on
71 // failure. Since errno is implemented via a thread local variable, we cannot
72 // use errno before TLS is setup.
73 long stack_guard_retval =
74 syscall_impl(SYS_getrandom, reinterpret_cast<long>(stack_guard_addr),
75 sizeof(uint64_t), 0);
76 if (stack_guard_retval < 0)
77 syscall_impl(SYS_exit, 1);
78
79 tls_descriptor = {tls_size_with_addr, reinterpret_cast<uintptr_t>(tls_addr),
80 end_ptr};
81 return;
82}
83
84void cleanup_tls(uintptr_t addr, uintptr_t size) {
85 if (size == 0)
86 return;
87 syscall_impl<long>(SYS_munmap, addr, size);
88}
89
90// Sets the thread pointer to |val|. Returns true on success, false on failure.
91bool set_thread_ptr(uintptr_t val) {
92 return syscall_impl(SYS_arch_prctl, ARCH_SET_FS, val) != -1;
93}
94} // namespace LIBC_NAMESPACE_DECL
95

source code of libc/startup/linux/x86_64/tls.cpp