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// Copyright 2021 The Fuchsia Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#ifndef SRC_LIB_ELFLDLTL_INCLUDE_LIB_ELFLDLTL_SELF_H_
#define SRC_LIB_ELFLDLTL_INCLUDE_LIB_ELFLDLTL_SELF_H_
#include <lib/stdcompat/span.h>
#include <climits>
#include <cstdint>
#include <variant>
#include "layout.h"
namespace elfldltl {
// The elfldltl::Self calls provide introspection for a program to inspect its
// own ELF headers. Note these always refer to the containing ELF module's
// static link image; i.e. calls made inside a shared library (or a static
// library linked into it) refer to the shared library's runtime image, while
// calls inside the main executable (or a static library linked into it) refer
// to the main executable's runtime image. The elfldltl::Self class itself is
// always an empty object, just used for scoping and template parameterization.
// All methods are static.
class SelfBase {
public:
// Compute the load bias: the difference between the runtime load address of
// the first PT_LOAD segment and its page-truncated p_vaddr. (That aligned
// p_vaddr is usually zero, but not always.)
static uintptr_t LoadBias() {
#ifndef __PIC__
// When compiled for fixed-address use, _DYNAMIC et al might not be
// defined. But it's a compile-time assumption throughout the generated
// code that the load bias is zero, so we can assume it here too.
return 0;
#endif
// The linker stores the link-time address of _DYNAMIC in GOT[0] and there
// is no relocation to fix it up. So we can compare that to our runtime
// PC-relative address for _DYNAMIC to see the difference in runtime and
// link-time addresses, i.e. the load bias. Unlike just taking the address
// of __ehdr_start or the like and calling that the "load address", this
// still works both if there is no ready symbol for the beginning of the
// load image (__ehdr_start is canonical in that it's supported by the
// linker, but only in certain layouts, and there's no such canonical
// symbol for all layouts) and if the link-time base address is nonzero.
#if defined(__aarch64__) && defined(__clang__)
// TODO(https://bugs.llvm.org/show_bug.cgi?id=49672): lld fails to observe
// the _GLOBAL_OFFSET_TABLE_[0] protocol on aarch64 but it always defines
// __dso_handle at the "base address" even when there are no ELF headers
// and we never link at nonzero on aarch64 so this suffices in practice.
return reinterpret_cast<uintptr_t>(kBase);
#endif
return reinterpret_cast<uintptr_t>(kDynamic) - kGot[0];
}
protected:
// These are defined implicitly by the linker. _DYNAMIC should be defined in
// any -pie or -shared link, while __ehdr_start is defined only in standard
// layouts (i.e. not in non-ELF raw kernel images via custom linker scripts).
[[gnu::visibility("hidden")]] static std::byte kImage[] __asm__("__ehdr_start");
[[gnu::visibility("hidden")]] static std::byte kImageEnd[] __asm__("_end");
[[gnu::visibility("hidden")]] static const std::byte kDynamic[] __asm__("_DYNAMIC");
[[gnu::visibility("hidden")]] static const uintptr_t kGot[] __asm__("_GLOBAL_OFFSET_TABLE_");
#if defined(__aarch64__) && defined(__clang__)
// TODO(https://bugs.llvm.org/show_bug.cgi?id=49672): see above
[[gnu::visibility("hidden")]] static const std::byte kBase[] __asm__("__dso_handle");
#endif
};
template <ElfClass Class = ElfClass::kNative>
class Self : public SelfBase {
public:
using Elf = ::elfldltl::Elf<Class>;
// Access the calling ELF module's own ELF file header. Using this in a
// program with a nonstandard layout without visible ELF headers will cause
// a link-time failure.
static const typename Elf::Ehdr& Ehdr() {
return *reinterpret_cast<const typename Elf::Ehdr*>(kImage);
}
// Dynamically check if the calling ELF module's file header matches this
// instantiation's ElfClass. See Ehdr() above about link-time constraints.
static bool Match() { return Ehdr().elfclass == Class; }
// Dynamically check if the calling ELF module's file header passes basic
// format checks for this instantiation's ElfClass and native byte order.
// See Ehdr() above about link-time constraints.
static bool Valid() { return Ehdr().Valid(); }
// Examine the calling ELF module's file header to find its own program
// headers. See Ehdr() above about link-time constraints.
static cpp20::span<const typename Elf::Phdr> Phdrs() {
const auto phoff = Ehdr().phoff;
size_t phnum;
if (Ehdr().phnum == Elf::Ehdr::kPnXnum) [[unlikely]] {
// This is the marker that the count might exceed 16 bits. In that case,
// it's instead stored in the special stub section header at index 0.
// This is the only time the section header table is used at runtime,
// and there are still no actual sections (index 0 is always a stub).
const auto shoff = Ehdr().shoff;
auto shdr0 = reinterpret_cast<const typename Elf::Shdr*>(kImage + shoff);
phnum = shdr0->info;
} else {
phnum = Ehdr().phnum;
}
return {reinterpret_cast<const typename Elf::Phdr*>(kImage + phoff), phnum};
}
// Get the calling ELF module's own dynamic section. This works in any
// program linked to have a dynamic section, even if the ELF headers are not
// preserved at runtime. Note that the returned span's size is only an
// extreme upper bound on the actual dynamic section that can be accessed.
// It must always be examined linearly from the front and not examined past
// the elfldltl::ElfDynTag::kNull terminator entry.
static cpp20::span<const typename Elf::Dyn> Dynamic() {
// This is pedantically speaking undefined behavior since that array
// doesn't go that far. But we have no way to determine its size without
// looking at memory (either scanning it for the null terminator, or
// examining phdrs for PT_DYNAMIC's p_filesz--if phdrs are even available).
// In practice things are fine as long as access past the end of the array
// is not actually attempted through the span. We cap the span at the
// bounds of the overall module image anyway (though a mapped image can
// have whole-page holes so even this provides no guarantee that access
// through the span cannot fault).
auto first = reinterpret_cast<const typename Elf::Dyn*>(kDynamic);
auto last = reinterpret_cast<const typename Elf::Dyn*>(kImageEnd);
return {first, static_cast<size_t>(last - first)};
}
};
// Explicit instantiations are required to convince the compiler that there are
// definitions for the static member variables, which are really acting as
// scoped extern "C" declarations.
extern template class Self<ElfClass::k32>;
extern template class Self<ElfClass::k64>;
// Determine which ELF class is used in this program's own ELF header, in
// case a 64-bit program was converted to ELFCLASS32 at link time.
// Use it like:
// ```
// auto rv = elfldltl::VisitSelf([](auto&& self) { return self.Method(...); });
// ```
template <typename T>
inline decltype(auto) VisitSelf(T&& visitor) {
if constexpr (ElfClass::kNative == ElfClass::k64) {
if (Self<ElfClass::k64>::Match()) {
return std::forward<T>(visitor)(Self<ElfClass::k64>());
}
}
return std::forward<T>(visitor)(Self<ElfClass::k32>());
}
} // namespace elfldltl
#endif // SRC_LIB_ELFLDLTL_INCLUDE_LIB_ELFLDLTL_SELF_H_