1 // Copyright 2017 The Chromium Authors. All rights reserved. 2 // Use of this source code is governed by a BSD-style license that can be 3 // found in the LICENSE file. 4 5 #include "base/android/library_loader/anchor_functions.h" 6 7 #include "base/logging.h" 8 #include "build/build_config.h" 9 10 #if BUILDFLAG(SUPPORTS_CODE_ORDERING) 11 12 // These functions are here to delimit the start and end of the ordered part of 13 // .text. They require a suitably constructed orderfile, with these functions at 14 // the beginning and end. 15 // 16 // These functions are weird: this is due to ICF (Identical Code Folding). 17 // The linker merges functions that have the same code, which would be the case 18 // if these functions were empty, or simple. 19 // Gold's flag --icf=safe will *not* alias functions when their address is used 20 // in code, but as of November 2017, we use the default setting that 21 // deduplicates function in this case as well. 22 // 23 // Thus these functions are made to be unique, using inline .word in assembly. 24 // 25 // Note that code |CheckOrderingSanity()| below will make sure that these 26 // functions are not aliased, in case the toolchain becomes really clever. 27 extern "C" { 28 29 // These functions have a well-defined ordering in this file, see the comment 30 // in |IsOrderingSane()|. 31 void dummy_function_end_of_ordered_text() { 32 asm(".word 0x21bad44d"); 33 asm(".word 0xb815c5b0"); 34 } 35 36 void dummy_function_start_of_ordered_text() { 37 asm(".word 0xe4a07375"); 38 asm(".word 0x66dda6dc"); 39 } 40 41 // These two symbols are defined by anchor_functions.lds and delimit the start 42 // and end of .text. 43 void linker_script_start_of_text(); 44 void linker_script_end_of_text(); 45 46 } // extern "C" 47 48 namespace base { 49 namespace android { 50 51 const size_t kStartOfText = 52 reinterpret_cast<size_t>(linker_script_start_of_text); 53 const size_t kEndOfText = reinterpret_cast<size_t>(linker_script_end_of_text); 54 const size_t kStartOfOrderedText = 55 reinterpret_cast<size_t>(dummy_function_start_of_ordered_text); 56 const size_t kEndOfOrderedText = 57 reinterpret_cast<size_t>(dummy_function_end_of_ordered_text); 58 59 bool IsOrderingSane() { 60 size_t here = reinterpret_cast<size_t>(&IsOrderingSane); 61 // The symbols linker_script_start_of_text and linker_script_end_of_text 62 // should cover all of .text, and dummy_function_start_of_ordered_text and 63 // dummy_function_end_of_ordered_text should cover the ordered part of it. 64 // This check is intended to catch the lack of ordering. 65 // 66 // Ordered text can start at the start of text, but should not cover the 67 // entire range. Most addresses are distinct nonetheless as the symbols are 68 // different, but linker-defined symbols have zero size and therefore the 69 // start address could be the same as the address of 70 // dummy_function_start_of_ordered_text. 71 return kStartOfText < here && here < kEndOfText && 72 kStartOfOrderedText < kEndOfOrderedText && 73 kStartOfText <= kStartOfOrderedText && kEndOfOrderedText < kEndOfText; 74 } 75 76 } // namespace android 77 } // namespace base 78 79 #endif // BUILDFLAG(SUPPORTS_CODE_ORDERING) 80
