1 //! TLS utilities.
2 //!
3 //! # Safety
4 //!
5 //! This file contains code that reads the raw phdr array pointed to by the
6 //! kernel-provided AUXV values.
7 #![allow(unsafe_code)]
8
9 use crate::backend::c;
10 use crate::backend::param::auxv::exe_phdrs;
11 use core::arch::global_asm;
12 use core::ptr::{null, NonNull};
13 use linux_raw_sys::elf::*;
14
15 /// For use with [`set_thread_area`].
16 ///
17 /// [`set_thread_area`]: crate::runtime::set_thread_area
18 #[cfg(target_arch = "x86")]
19 pub type UserDesc = linux_raw_sys::general::user_desc;
20
startup_tls_info() -> StartupTlsInfo21 pub(crate) fn startup_tls_info() -> StartupTlsInfo {
22 let mut base = null();
23 let mut tls_phdr = null();
24 let mut stack_size = 0;
25
26 let (first_phdr, phent, phnum) = exe_phdrs();
27 let mut current_phdr = first_phdr.cast::<Elf_Phdr>();
28
29 // The dynamic address of the dynamic section, which we can compare with
30 // the `PT_DYNAMIC` header's static address, if present.
31 let dynamic_addr: *const c::c_void = unsafe { &_DYNAMIC };
32
33 // SAFETY: We assume the phdr array pointer and length the kernel provided
34 // to the process describe a valid phdr array.
35 unsafe {
36 let phdrs_end = current_phdr.cast::<u8>().add(phnum * phent).cast();
37 while current_phdr != phdrs_end {
38 let phdr = &*current_phdr;
39 current_phdr = current_phdr.cast::<u8>().add(phent).cast();
40
41 match phdr.p_type {
42 // Compute the offset from the static virtual addresses
43 // in the `p_vaddr` fields to the dynamic addresses. We don't
44 // always get a `PT_PHDR` or `PT_DYNAMIC` header, so use
45 // whichever one we get.
46 PT_PHDR => base = first_phdr.cast::<u8>().wrapping_sub(phdr.p_vaddr),
47 PT_DYNAMIC => base = dynamic_addr.cast::<u8>().wrapping_sub(phdr.p_vaddr),
48
49 PT_TLS => tls_phdr = phdr,
50 PT_GNU_STACK => stack_size = phdr.p_memsz,
51 _ => {}
52 }
53 }
54
55 if tls_phdr.is_null() {
56 StartupTlsInfo {
57 addr: NonNull::dangling().as_ptr(),
58 mem_size: 0,
59 file_size: 0,
60 align: 1,
61 stack_size: 0,
62 }
63 } else {
64 StartupTlsInfo {
65 addr: base.cast::<u8>().wrapping_add((*tls_phdr).p_vaddr).cast(),
66 mem_size: (*tls_phdr).p_memsz,
67 file_size: (*tls_phdr).p_filesz,
68 align: (*tls_phdr).p_align,
69 stack_size,
70 }
71 }
72 }
73 }
74
75 extern "C" {
76 /// Declare the `_DYNAMIC` symbol so that we can compare its address with
77 /// the static address in the `PT_DYNAMIC` header to learn our offset. Use
78 /// a weak symbol because `_DYNAMIC` is not always present.
79 static _DYNAMIC: c::c_void;
80 }
81 // Rust has `extern_weak` but it isn't stable, so use a `global_asm`.
82 global_asm!(".weak _DYNAMIC");
83
84 /// The values returned from [`startup_tls_info`].
85 ///
86 /// [`startup_tls_info`]: crate::runtime::startup_tls_info
87 pub struct StartupTlsInfo {
88 /// The base address of the TLS segment.
89 pub addr: *const c::c_void,
90 /// The size of the memory region.
91 pub mem_size: usize,
92 /// The size beyond which all memory is zero-initialized.
93 pub file_size: usize,
94 /// The required alignment for the TLS segment.
95 pub align: usize,
96 /// The requested minimum size for stacks.
97 pub stack_size: usize,
98 }
99