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// Copyright 2017 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#include "base/android/library_loader/anchor_functions.h"
#include "build/build_config.h"
#if BUILDFLAG(SUPPORTS_CODE_ORDERING)
// These functions are here to delimit the start and end of the ordered part of
// .text. They require a suitably constructed orderfile, with these functions at
// the beginning and end.
//
// These functions are weird: this is due to ICF (Identical Code Folding).
// The linker merges functions that have the same code, which would be the case
// if these functions were empty, or simple.
// Gold's flag --icf=safe will *not* alias functions when their address is used
// in code, but as of November 2017, we use the default setting that
// deduplicates function in this case as well.
//
// Thus these functions are made to be unique, using inline .word in assembly,
// or the equivalent directive depending on the architecture.
//
// Note that code |CheckOrderingSanity()| below will make sure that these
// functions are not aliased, in case the toolchain becomes really clever.
extern "C" {
// The assembler has a different syntax depending on the architecture.
#if defined(ARCH_CPU_ARMEL) || defined(ARCH_CPU_ARM64)
// These functions have a well-defined ordering in this file, see the comment
// in |IsOrderingSane()|.
void dummy_function_end_of_ordered_text() {
asm(".word 0x21bad44d");
asm(".word 0xb815c5b0");
}
void dummy_function_start_of_ordered_text() {
asm(".word 0xe4a07375");
asm(".word 0x66dda6dc");
}
#elif defined(ARCH_CPU_X86_FAMILY)
void dummy_function_end_of_ordered_text() {
asm(".4byte 0x21bad44d");
asm(".4byte 0xb815c5b0");
}
void dummy_function_start_of_ordered_text() {
asm(".4byte 0xe4a07375");
asm(".4byte 0x66dda6dc");
}
#endif
// These two symbols are defined by anchor_functions.lds and delimit the start
// and end of .text.
void linker_script_start_of_text();
void linker_script_end_of_text();
} // extern "C"
namespace base {
namespace android {
const size_t kStartOfText =
reinterpret_cast<size_t>(linker_script_start_of_text);
const size_t kEndOfText = reinterpret_cast<size_t>(linker_script_end_of_text);
const size_t kStartOfOrderedText =
reinterpret_cast<size_t>(dummy_function_start_of_ordered_text);
const size_t kEndOfOrderedText =
reinterpret_cast<size_t>(dummy_function_end_of_ordered_text);
bool AreAnchorsSane() {
size_t here = reinterpret_cast<size_t>(&IsOrderingSane);
return kStartOfText < here && here < kEndOfText;
}
bool IsOrderingSane() {
// The symbols linker_script_start_of_text and linker_script_end_of_text
// should cover all of .text, and dummy_function_start_of_ordered_text and
// dummy_function_end_of_ordered_text should cover the ordered part of it.
// This check is intended to catch the lack of ordering.
//
// Ordered text can start at the start of text, but should not cover the
// entire range. Most addresses are distinct nonetheless as the symbols are
// different, but linker-defined symbols have zero size and therefore the
// start address could be the same as the address of
// dummy_function_start_of_ordered_text.
return AreAnchorsSane() && kStartOfOrderedText < kEndOfOrderedText &&
kStartOfText <= kStartOfOrderedText && kEndOfOrderedText < kEndOfText;
}
} // namespace android
} // namespace base
#endif // BUILDFLAG(SUPPORTS_CODE_ORDERING)
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