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      1 //===-- sanitizer_coverage_win_sections.cpp -------------------------------===//
      2 //
      3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
      4 // See https://llvm.org/LICENSE.txt for license information.
      5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
      6 //
      7 //===----------------------------------------------------------------------===//
      8 //
      9 // This file defines delimiters for Sanitizer Coverage's section. It contains
     10 // Windows specific tricks to coax the linker into giving us the start and stop
     11 // addresses of a section, as ELF linkers can do, to get the size of certain
     12 // arrays. According to https://msdn.microsoft.com/en-us/library/7977wcck.aspx
     13 // sections with the same name before "$" are sorted alphabetically by the
     14 // string that comes after "$" and merged into one section. We take advantage
     15 // of this by putting data we want the size of into the middle (M) of a section,
     16 // by using the letter "M" after "$". We get the start of this data (ie:
     17 // __start_section_name) by making the start variable come at the start of the
     18 // section (using the letter A after "$"). We do the same to get the end of the
     19 // data by using the letter "Z" after "$" to make the end variable come after
     20 // the data. Note that because of our technique the address of the start
     21 // variable is actually the address of data that comes before our middle
     22 // section. We also need to prevent the linker from adding any padding. Each
     23 // technique we use for this is explained in the comments below.
     24 //===----------------------------------------------------------------------===//
     25 
     26 #include "sanitizer_platform.h"
     27 #if SANITIZER_WINDOWS
     28 #include <stdint.h>
     29 
     30 extern "C" {
     31 // Use uint64_t so the linker won't need to add any padding if it tries to word
     32 // align the start of the 8-bit counters array. The array will always start 8
     33 // bytes after __start_sancov_cntrs.
     34 #pragma section(".SCOV$CA", read, write)
     35 __declspec(allocate(".SCOV$CA")) uint64_t __start___sancov_cntrs = 0;
     36 
     37 // Even though we said not to align __stop__sancov_cntrs (using the "align"
     38 // declspec), MSVC's linker may try to align the section, .SCOV$CZ, containing
     39 // it. This can cause a mismatch between the number of PCs and counters since
     40 // each PCTable element is 8 bytes (unlike counters which are 1 byte) so no
     41 // padding would be added to align .SCOVP$Z, However, if .SCOV$CZ section is 1
     42 // byte, the linker won't try to align it on an 8-byte boundary, so use a
     43 // uint8_t for __stop_sancov_cntrs.
     44 #pragma section(".SCOV$CZ", read, write)
     45 __declspec(allocate(".SCOV$CZ")) __declspec(align(1)) uint8_t
     46     __stop___sancov_cntrs = 0;
     47 
     48 #pragma section(".SCOV$GA", read, write)
     49 __declspec(allocate(".SCOV$GA")) uint64_t __start___sancov_guards = 0;
     50 #pragma section(".SCOV$GZ", read, write)
     51 __declspec(allocate(".SCOV$GZ")) __declspec(align(1)) uint8_t
     52     __stop___sancov_guards = 0;
     53 
     54 // The guard array and counter array should both be merged into the .data
     55 // section to reduce the number of PE sections. However, because PCTable is
     56 // constant it should be merged with the .rdata section.
     57 #pragma comment(linker, "/MERGE:.SCOV=.data")
     58 
     59 #pragma section(".SCOVP$A", read)
     60 __declspec(allocate(".SCOVP$A")) uint64_t __start___sancov_pcs = 0;
     61 #pragma section(".SCOVP$Z", read)
     62 __declspec(allocate(".SCOVP$Z")) __declspec(align(1)) uint8_t
     63     __stop___sancov_pcs = 0;
     64 
     65 #pragma comment(linker, "/MERGE:.SCOVP=.rdata")
     66 }
     67 #endif  // SANITIZER_WINDOWS
     68