1 // Copyright (c) 2013 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 #ifndef BASE_DEBUG_PROC_MAPS_LINUX_H_ 6 #define BASE_DEBUG_PROC_MAPS_LINUX_H_ 7 8 #include <stdint.h> 9 10 #include <string> 11 #include <vector> 12 13 #include "base/base_export.h" 14 15 namespace base { 16 namespace debug { 17 18 // Describes a region of mapped memory and the path of the file mapped. 19 struct MappedMemoryRegion { 20 enum Permission { 21 READ = 1 << 0, 22 WRITE = 1 << 1, 23 EXECUTE = 1 << 2, 24 PRIVATE = 1 << 3, // If set, region is private, otherwise it is shared. 25 }; 26 27 // The address range [start,end) of mapped memory. 28 uintptr_t start; 29 uintptr_t end; 30 31 // Byte offset into |path| of the range mapped into memory. 32 unsigned long long offset; 33 34 // Image base, if this mapping corresponds to an ELF image. 35 uintptr_t base; 36 37 // Bitmask of read/write/execute/private/shared permissions. 38 uint8_t permissions; 39 40 // Name of the file mapped into memory. 41 // 42 // NOTE: path names aren't guaranteed to point at valid files. For example, 43 // "[heap]" and "[stack]" are used to represent the location of the process' 44 // heap and stack, respectively. 45 std::string path; 46 }; 47 48 // Reads the data from /proc/self/maps and stores the result in |proc_maps|. 49 // Returns true if successful, false otherwise. 50 // 51 // There is *NO* guarantee that the resulting contents will be free of 52 // duplicates or even contain valid entries by time the method returns. 53 // 54 // 55 // THE GORY DETAILS 56 // 57 // Did you know it's next-to-impossible to atomically read the whole contents 58 // of /proc/<pid>/maps? You would think that if we passed in a large-enough 59 // buffer to read() that It Should Just Work(tm), but sadly that's not the case. 60 // 61 // Linux's procfs uses seq_file [1] for handling iteration, text formatting, 62 // and dealing with resulting data that is larger than the size of a page. That 63 // last bit is especially important because it means that seq_file will never 64 // return more than the size of a page in a single call to read(). 65 // 66 // Unfortunately for a program like Chrome the size of /proc/self/maps is 67 // larger than the size of page so we're forced to call read() multiple times. 68 // If the virtual memory table changed in any way between calls to read() (e.g., 69 // a different thread calling mprotect()), it can make seq_file generate 70 // duplicate entries or skip entries. 71 // 72 // Even if seq_file was changed to keep flushing the contents of its page-sized 73 // buffer to the usermode buffer inside a single call to read(), it has to 74 // release its lock on the virtual memory table to handle page faults while 75 // copying data to usermode. This puts us in the same situation where the table 76 // can change while we're copying data. 77 // 78 // Alternatives such as fork()-and-suspend-the-parent-while-child-reads were 79 // attempted, but they present more subtle problems than it's worth. Depending 80 // on your use case your best bet may be to read /proc/<pid>/maps prior to 81 // starting other threads. 82 // 83 // [1] http://kernelnewbies.org/Documents/SeqFileHowTo 84 BASE_EXPORT bool ReadProcMaps(std::string* proc_maps); 85 86 // Parses /proc/<pid>/maps input data and stores in |regions|. Returns true 87 // and updates |regions| if and only if all of |input| was successfully parsed. 88 BASE_EXPORT bool ParseProcMaps(const std::string& input, 89 std::vector<MappedMemoryRegion>* regions); 90 91 } // namespace debug 92 } // namespace base 93 94 #endif // BASE_DEBUG_PROC_MAPS_LINUX_H_ 95