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80 lines
3.0 KiB
C++
80 lines
3.0 KiB
C++
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// Copyright 2017 The Chromium Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style license that can be
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// found in the LICENSE file.
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#include "base/android/library_loader/anchor_functions.h"
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#include "base/logging.h"
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#include "build/build_config.h"
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#if BUILDFLAG(SUPPORTS_CODE_ORDERING)
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// These functions are here to delimit the start and end of the ordered part of
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// .text. They require a suitably constructed orderfile, with these functions at
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// the beginning and end.
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//
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// These functions are weird: this is due to ICF (Identical Code Folding).
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// The linker merges functions that have the same code, which would be the case
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// if these functions were empty, or simple.
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// Gold's flag --icf=safe will *not* alias functions when their address is used
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// in code, but as of November 2017, we use the default setting that
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// deduplicates function in this case as well.
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//
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// Thus these functions are made to be unique, using inline .word in assembly.
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//
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// Note that code |CheckOrderingSanity()| below will make sure that these
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// functions are not aliased, in case the toolchain becomes really clever.
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extern "C" {
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// These functions have a well-defined ordering in this file, see the comment
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// in |IsOrderingSane()|.
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void dummy_function_end_of_ordered_text() {
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asm(".word 0x21bad44d");
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asm(".word 0xb815c5b0");
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}
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void dummy_function_start_of_ordered_text() {
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asm(".word 0xe4a07375");
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asm(".word 0x66dda6dc");
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}
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// These two symbols are defined by anchor_functions.lds and delimit the start
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// and end of .text.
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void linker_script_start_of_text();
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void linker_script_end_of_text();
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} // extern "C"
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namespace base {
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namespace android {
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const size_t kStartOfText =
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reinterpret_cast<size_t>(linker_script_start_of_text);
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const size_t kEndOfText = reinterpret_cast<size_t>(linker_script_end_of_text);
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const size_t kStartOfOrderedText =
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reinterpret_cast<size_t>(dummy_function_start_of_ordered_text);
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const size_t kEndOfOrderedText =
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reinterpret_cast<size_t>(dummy_function_end_of_ordered_text);
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bool IsOrderingSane() {
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size_t here = reinterpret_cast<size_t>(&IsOrderingSane);
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// The symbols linker_script_start_of_text and linker_script_end_of_text
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// should cover all of .text, and dummy_function_start_of_ordered_text and
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// dummy_function_end_of_ordered_text should cover the ordered part of it.
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// This check is intended to catch the lack of ordering.
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//
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// Ordered text can start at the start of text, but should not cover the
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// entire range. Most addresses are distinct nonetheless as the symbols are
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// different, but linker-defined symbols have zero size and therefore the
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// start address could be the same as the address of
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// dummy_function_start_of_ordered_text.
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return kStartOfText < here && here < kEndOfText &&
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kStartOfOrderedText < kEndOfOrderedText &&
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kStartOfText <= kStartOfOrderedText && kEndOfOrderedText < kEndOfText;
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}
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} // namespace android
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} // namespace base
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#endif // BUILDFLAG(SUPPORTS_CODE_ORDERING)
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