1 Malloc Debug 2 ============ 3 4 Malloc debug is a method of debugging native memory problems. It can help 5 detect memory corruption, memory leaks, and use after free issues. 6 7 This documentation describes how to enable this feature on versions of 8 the Android OS, Marshmallow or older. Note: malloc debug was full of bugs 9 and was not fully functional until KitKat, so using it on a version older 10 than that is not guaranteed to work at all. 11 12 The documentation for malloc debug on newer versions of Android is 13 [here](README.md). 14 15 On these old versions of the OS, you must be able to set system properties 16 using the setprop command from the shell. This requires the ability to 17 run as root on the device. 18 19 When malloc debug is enabled, it works by adding a shim layer that replaces 20 the normal allocation calls. The replaced calls are: 21 22 * `malloc` 23 * `free` 24 * `calloc` 25 * `realloc` 26 * `posix_memalign` 27 * `memalign` 28 * `malloc_usable_size` 29 30 On 32 bit systems, these two deprecated functions are also replaced: 31 32 * `pvalloc` 33 * `valloc` 34 35 Any errors detected by the library are reported in the log. 36 37 Controlling Malloc Debug Behavior 38 --------------------------------- 39 Malloc debug is controlled by a system property that takes a numeric value 40 named libc.debug.malloc. It has only a few distinct modes that enables a 41 set of different malloc debug checks at once. 42 43 Value 1 44 -------- 45 When enabled, this value creates a special header to all allocations 46 that contains information about the allocation. 47 48 ### Backtrace at Allocation Creation 49 Enable capturing the backtrace of each allocation site. Only the 50 first 16 frames of the backtrace will be captured. 51 This option will slow down allocations by an order of magnitude, and 52 might cause timeouts when trying to start a device. 53 54 ### Track Live Allocations 55 All of the currently live allocations will be tracked and can be retrieved 56 by a call to get\_malloc\_leak\_info (see README\_api.md for details). 57 58 Note: If multiple allocations have the same exact backtrace, then only one 59 entry is returned in the list. 60 61 Value 5 62 ------- 63 When enabled, this value does not create a special header. It only modifies 64 the content of allocations. 65 66 Whenever an allocation is created, initialize the data with a known 67 pattern (0xeb). This does not happen for the calloc calls. 68 Whenever an allocation is freed, write a known pattern over the data (0xef). 69 70 Value 10 71 -------- 72 When enabled, this value creates a special header to all allocations 73 that contains information about the allocation. 74 75 This value enables everything enabled with value 1 plus these other options. 76 77 ### Allocation Guards 78 A 32 byte buffer is placed before the returned allocation (known as 79 a front guard). This buffer is filled with the pattern (0xaa). In addition, 80 a 32 byte buffer is placed after the data for the returned allocation (known 81 as a rear guard). This buffer is filled with the pattern (0xbb). 82 83 When the allocation is freed, both of these guards are verified to contain 84 the expected patterns. If not, then an error message is printed to the log. 85 86 ### Free Memory Tracking 87 When a pointer is freed, do not free the memory right away, but add it to 88 a list of freed allocations. In addition to being added to the list, the 89 entire allocation is filled with the value 0xef, and the backtrace at 90 the time of the free is recorded. As with the backtrace on allocation, 91 only up to 16 frames will be recorded. 92 93 When the list of freed allocations reaches 100, the oldest allocation 94 on the list is removed and verified that it still contains the pattern 0xef. 95 If the entire allocation is not filled with this value, an error is printed 96 to the log. 97 98 ### Log Leaks 99 When the program completes, all of the allocations that are still live 100 are printed to the log as leaks. This isn't very useful since it tends 101 to display a lot of false positive because many programs do not free 102 everything before terminating. 103 104 Option 20 105 --------- 106 Do not use this option value, it only works on the emulator. It has not 107 been verified, so it may or may not work. 108 109 Enable on Certain Processes 110 --------------------------- 111 Using the special system property, libc.debug.malloc.program, will 112 cause malloc debug to only be used on processes with that name. For example, 113 if the property is set to ls, then only the program named ls will have malloc 114 debug enabled. 115 116 Examples 117 ======== 118 Enable malloc debug for all allocations for all processes: 119 120 adb shell stop 121 adb shell setprop libc.debug.malloc 1 122 adb shell start 123 124 Enable malloc debug for a particular process: 125 126 adb shell setprop libc.debug.malloc.program ls 127 adb shell setprop libc.debug.malloc 10 128 adb shell ls /data/local/tmp 129
