feat(env): implement tls_info in terms of linker symbols

Author: mkroening

Cargo.lock

@@ -295,6 +295,7 @@ build-time = "0.1.3"
 crossbeam-utils = { version = "0.8", default-features = false }
 delegate = "0.13"
 document-features = { version = "0.2", optional = true }
+elf = { version = "0.8", default-features = false }
 embedded-io = { version = "0.7", features = ["alloc"] }
 endian-num = { version = "0.2", optional = true, features = ["linux-types"] }
 enum_dispatch = "0.3"

Cargo.toml

@@ -1,5 +1,9 @@
 use core::ops::Range;
+use core::{ptr, slice};
 
+use elf::abi;
+use elf::file::Elf64_Ehdr;
+use elf::segment::Elf64_Phdr;
 use hermit_entry::boot_info::TlsInfo;
 
 pub fn executable_ptr_range() -> Range<*mut ()> {
@@ -57,5 +61,109 @@ fn executable_end() -> *mut () {
 }
 
 pub fn tls_info() -> Option<TlsInfo> {
-	super::boot_info().load_info.tls_info
+	let ehdr = ehdr();
+	ehdr.sanity_check_ident();
+
+	let phdrs = unsafe { ehdr.phdrs() };
+	let tls_phdr = phdrs.iter().find(|phdr| phdr.p_type == abi::PT_TLS)?;
+	let executable_start = executable_ptr_range().start.expose_provenance() as u64;
+
+	let tls_info = TlsInfo {
+		start: executable_start + tls_phdr.p_vaddr,
+		filesz: tls_phdr.p_filesz,
+		memsz: tls_phdr.p_memsz,
+		align: tls_phdr.p_align,
+	};
+
+	assert!(tls_info_eq(
+		&tls_info,
+		&super::boot_info().load_info.tls_info.unwrap()
+	));
+
+	Some(tls_info)
+}
+
+fn ehdr() -> &'static Elf64_Ehdr {
+	unsafe extern "C" {
+		/// ELF file header.
+		///
+		/// Apart from changelogs, it is not documented, but defined by:
+		///
+		/// - ld: [binutils-gdb@`62655c7`]
+		/// - gold: [binutils-gdb@`eabc84f`]
+		/// - lld: [llvm/llvm-project@`4f7a5c3`]
+		/// - mold: [rui314/mold@`ccc7f83`]
+		/// - Wild: [wild-linker/wild@`fb7da78`]
+		///
+		/// [binutils-gdb@`62655c7`]: https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=62655c7b8bfc33e6c12694f439ff8f7e8da3005a
+		/// [binutils-gdb@`eabc84f`]: https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=eabc84f4848311a68f65df04e428c8b53a92f1c0
+		/// [llvm/llvm-project@`4f7a5c3`]: https://github.com/llvm/llvm-project/commit/4f7a5c3bb429c945ff4e456478b5532f828d1143
+		/// [rui314/mold@`ccc7f83`]: https://github.com/rui314/mold/commit/ccc7f83cde954048424fa488464be41f210e60d1
+		/// [wild-linker/wild@`fb7da78`]: https://github.com/wild-linker/wild/commit/fb7da7841ad9e64e2cd1128a62d23d488dae921d
+		static __ehdr_start: Elf64_Ehdr;
+	}
+
+	unsafe { &__ehdr_start }
+}
+
+#[expect(non_camel_case_types)]
+trait Elf64_EhdrExt {
+	fn sanity_check_ident(&self);
+
+	unsafe fn phdrs(&self) -> &[Elf64_Phdr];
+}
+
+impl Elf64_EhdrExt for Elf64_Ehdr {
+	fn sanity_check_ident(&self) {
+		let ident = &self.e_ident;
+
+		let magic = &ident[..abi::EI_CLASS];
+		assert_eq!(magic, abi::ELFMAGIC);
+
+		let version = ident[abi::EI_VERSION];
+		assert_eq!(version, abi::EV_CURRENT);
+
+		let class = ident[abi::EI_CLASS];
+		assert_eq!(class, abi::ELFCLASS64);
+
+		/// 2's complement values, with native endianness.
+		pub const ELFDATA2NATIVE: u8 = if cfg!(target_endian = "little") {
+			abi::ELFDATA2LSB
+		} else if cfg!(target_endian = "big") {
+			abi::ELFDATA2MSB
+		} else {
+			unreachable!()
+		};
+
+		let data = ident[abi::EI_DATA];
+		assert_eq!(data, ELFDATA2NATIVE);
+
+		/// Stand-alone (embedded) ABI
+		pub const ELFOSABI_STANDALONE: u8 = 255;
+
+		let osabi = ident[abi::EI_OSABI];
+		// For some reason `x86_64-unknown-none` uses `ELFOSABI_GNU`.
+		// We need to allow this for `no_std` applications such as our integration tests.
+		assert!(osabi == ELFOSABI_STANDALONE || osabi == abi::ELFOSABI_GNU);
+
+		let abiversion = ident[abi::EI_ABIVERSION];
+		assert_eq!(abiversion, 0);
+	}
+
+	unsafe fn phdrs(&self) -> &[Elf64_Phdr] {
+		let ptr = unsafe {
+			ptr::from_ref(self)
+				.byte_add(self.e_phoff as usize)
+				.cast::<Elf64_Phdr>()
+		};
+		let len = self.e_phnum as usize;
+		unsafe { slice::from_raw_parts(ptr, len) }
+	}
+}
+
+fn tls_info_eq(this: &TlsInfo, other: &TlsInfo) -> bool {
+	this.start == other.start
+		&& this.filesz == other.filesz
+		&& this.memsz == other.memsz
+		&& this.align == other.align
 }