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README.md

      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 Currently, malloc debug requires root to enable. When it is enabled, it works
      8 by adding a shim layer that replaces the normal allocation calls. The replaced
      9 calls are:
     10 
     11 <pre>
     12 malloc
     13 free
     14 calloc
     15 realloc
     16 posix_memalign
     17 memalign
     18 malloc_usable_size
     19 </pre>
     20 
     21 On 32 bit systems, these two deprecated functions are also replaced:
     22 
     23 <pre>
     24 pvalloc
     25 valloc
     26 </pre>
     27 
     28 Any errors detected by the library are reported in the log.
     29 
     30 Controlling Malloc Debug Behavior
     31 ---------------------------------
     32 Malloc debug is controlled by individual options. Each option can be enabled
     33 individually, or in a group of other options. Every single option can be
     34 combined with every other option.
     35 
     36 Option Descriptions
     37 -------------------
     38 ### front\_guard[=SIZE\_BYTES]
     39 Enables a small buffer placed before the allocated data. This is an attempt
     40 to find memory corruption occuring to a region before the original allocation.
     41 On first allocation, this front guard is written with a specific pattern (0xaa).
     42 When the allocation is freed, the guard is checked to verify it has not been
     43 modified. If any part of the front guard is modified, an error will be reported
     44 in the log indicating what bytes changed.
     45 
     46 If the backtrace option is also enabled, then any error message will include
     47 the backtrace of the allocation site.
     48 
     49 If SIZE\_BYTES is present, it indicates the number of bytes in the guard.
     50 The default is 32 bytes, the max bytes is 16384. SIZE\_BYTES will be
     51 padded so that it is a multiple of 8 bytes on 32 bit systems and 16 bytes
     52 on 64 bit systems to make sure that the allocation returned is aligned
     53 properly.
     54 
     55 This option adds a special header to all allocations that contains the guard
     56 and information about the original allocation.
     57 
     58 Example error:
     59 
     60 <pre>
     61 04-10 12:00:45.621  7412  7412 E malloc_debug: +++ ALLOCATION 0x12345678 SIZE 100 HAS A CORRUPTED FRONT GUARD
     62 04-10 12:00:45.622  7412  7412 E malloc_debug:   allocation[-32] = 0x00 (expected 0xaa)
     63 04-10 12:00:45.622  7412  7412 E malloc_debug:   allocation[-15] = 0x02 (expected 0xaa)
     64 </pre>
     65 
     66 ### rear\_guard[=SIZE\_BYTES]
     67 Enables a small buffer placed after the allocated data. This is an attempt
     68 to find memory corruption occuring to a region after the original allocation.
     69 On first allocation, this rear guard is written with a specific pattern (0xbb).
     70 When the allocation is freed, the guard is checked to verify it has not been
     71 modified. If any part of the rear guard is modified, an error will be reported
     72 in the log indicating what bytes changed.
     73 
     74 If SIZE\_BYTES is present, it indicates the number of bytes in the guard.
     75 The default is 32 bytes, the max bytes is 16384.
     76 
     77 This option adds a special header to all allocations that contains
     78 information about the original allocation.
     79 
     80 Example error:
     81 
     82 <pre>
     83 04-10 12:00:45.621  7412  7412 E malloc_debug: +++ ALLOCATION 0x12345678 SIZE 100 HAS A CORRUPTED REAR GUARD
     84 04-10 12:00:45.622  7412  7412 E malloc_debug:   allocation[130] = 0xbf (expected 0xbb)
     85 04-10 12:00:45.622  7412  7412 E malloc_debug:   allocation[131] = 0x00 (expected 0xbb)
     86 </pre>
     87 
     88 ### guard[=SIZE\_BYTES]
     89 Enables both a front guard and a rear guard on all allocations.
     90 
     91 If SIZE\_BYTES is present, it indicates the number of bytes in both guards.
     92 The default is 32 bytes, the max bytes is 16384.
     93 
     94 ### backtrace[=MAX\_FRAMES]
     95 Enable capturing the backtrace of each allocation site.
     96 This option will slow down allocations by an order of magnitude. If the
     97 system runs too slowly with this option enabled, decreasing the maximum number
     98 of frames captured will speed the allocations up.
     99 
    100 Note that any backtrace frames that occur within the malloc backtrace library
    101 itself are not recorded.
    102 
    103 If MAX\_FRAMES is present, it indicates the maximum number of frames to
    104 capture in a backtrace. The default is 16 frames, the maximumum value
    105 this can be set to is 256.
    106 
    107 This option adds a special header to all allocations that contains the
    108 backtrace and information about the original allocation.
    109 
    110 ### backtrace\_enable\_on\_signal[=MAX\_FRAMES]
    111 Enable capturing the backtrace of each allocation site. If the
    112 backtrace capture is toggled when the process receives the signal
    113 SIGRTMAX - 19 (which is 45 on most Android devices). When this
    114 option is used alone, backtrace capture starts out disabled until the signal
    115 is received. If both this option and the backtrace option are set, then
    116 backtrace capture is enabled until the signal is received.
    117 
    118 If MAX\_FRAMES is present, it indicates the maximum number of frames to
    119 capture in a backtrace. The default is 16 frames, the maximumum value
    120 this can be set to is 256.
    121 
    122 This option adds a special header to all allocations that contains the
    123 backtrace and information about the original allocation.
    124 
    125 ### fill\_on\_alloc[=MAX\_FILLED\_BYTES]
    126 Any allocation routine, other than calloc, will result in the allocation being
    127 filled with the value 0xeb. When doing a realloc to a larger size, the bytes
    128 above the original usable size will be set to 0xeb.
    129 
    130 If MAX\_FILLED\_BYTES is present, it will only fill up to the specified number
    131 of bytes in the allocation. The default is to fill the entire allocation.
    132 
    133 ### fill\_on\_free[=MAX\_FILLED\_BYTES]
    134 When an allocation is freed, fill it with 0xef.
    135 
    136 If MAX\_FILLED\_BYTES is present, it will only fill up to the specified number
    137 of bytes in the allocation. The default is to fill the entire allocation.
    138 
    139 ### fill[=MAX\_FILLED\_BYTES]
    140 This enables both the fill\_on\_alloc option and the fill\_on\_free option.
    141 
    142 If MAX\_FILLED\_BYTES is present, it will only fill up to the specified number
    143 of bytes in the allocation. The default is to fill the entire allocation.
    144 
    145 ### expand\_alloc[=EXPAND\_BYTES]
    146 Add an extra amount to allocate for every allocation.
    147 
    148 If XX is present, it is the number of bytes to expand the allocation by.
    149 The default is 16 bytes, the max bytes is 16384.
    150 
    151 ### free\_track[=ALLOCATION\_COUNT]
    152 When a pointer is freed, do not free the memory right away, but add it to
    153 a list of freed allocations. In addition to being added to the list, the
    154 entire allocation is filled with the value 0xef, and the backtrace at
    155 the time of the free is recorded. The backtrace recording is completely
    156 separate from the backtrace option, and happens automatically if this
    157 option is enabled. By default, a maximum of 16 frames will be recorded,
    158 but this value can be changed using the free\_track\_backtrace\_num\_frames
    159 option. It can also be completely disabled by setting the option to zero.
    160 See the full description of this option below.
    161 
    162 When the list is full, an allocation is removed from the list and is
    163 checked to make sure that none of the contents have been modified since
    164 being placed on the list. When the program terminates, all of the allocations
    165 left on the list are verified.
    166 
    167 If ALLOCATION\_COUNT is present, it indicates the total number of allocations
    168 in the list. The default is to record 100 freed allocations, the max
    169 allocations to record is 16384.
    170 
    171 This option adds a special header to all allocations that contains
    172 information about the original allocation.
    173 
    174 Example error:
    175 
    176 <pre>
    177 04-15 12:00:31.304  7412  7412 E malloc_debug: +++ ALLOCATION 0x12345678 USED AFTER FREE
    178 04-15 12:00:31.305  7412  7412 E malloc_debug:   allocation[20] = 0xaf (expected 0xef)
    179 04-15 12:00:31.305  7412  7412 E malloc_debug:   allocation[99] = 0x12 (expected 0xef)
    180 04-15 12:00:31.305  7412  7412 E malloc_debug: Backtrace at time of free:
    181 04-15 12:00:31.305  7412  7412 E malloc_debug:           #00  pc 00029310  /system/lib/libc.so
    182 04-15 12:00:31.305  7412  7412 E malloc_debug:           #01  pc 00021438  /system/lib/libc.so (newlocale+160)
    183 04-15 12:00:31.305  7412  7412 E malloc_debug:           #02  pc 000a9e38  /system/lib/libc++.so
    184 04-15 12:00:31.305  7412  7412 E malloc_debug:           #03  pc 000a28a8  /system/lib/libc++.so
    185 </pre>
    186 
    187 In addition, there is another type of error message that can occur if
    188 an allocation has a special header applied, and the header is corrupted
    189 before the verification occurs. This is the error message that will be found
    190 in the log:
    191 
    192 <pre>
    193 +++ ALLOCATION 0x12345678 HAS CORRUPTED HEADER TAG 0x1cc7dc00 AFTER FREE
    194 </pre>
    195 
    196 ### free\_track\_backtrace\_num\_frames[=MAX\_FRAMES]
    197 This option only has meaning if free\_track is set. It indicates how many
    198 backtrace frames to capture when an allocation is freed.
    199 
    200 If MAX\_FRAMES is present, it indicates the number of frames to capture.
    201 If the value is set to zero, then no backtrace will be captured when the
    202 allocation is freed. The default is to record 16 frames, the max number of
    203 frames to to record is 256.
    204 
    205 ### leak\_track
    206 Track all live allocations. When the program terminates, all of the live
    207 allocations will be dumped to the log. If the backtrace option was enabled,
    208 then the log will include the backtrace of the leaked allocations. This
    209 option is not useful when enabled globally because a lot of programs do not
    210 free everything before the program terminates.
    211 
    212 This option adds a special header to all allocations that contains
    213 information about the original allocation.
    214 
    215 Example leak error found in the log:
    216 
    217 <pre>
    218 04-15 12:35:33.304  7412  7412 E malloc_debug: +++ APP leaked block of size 100 at 0x2be3b0b0 (leak 1 of 2)
    219 04-15 12:35:33.304  7412  7412 E malloc_debug: Backtrace at time of allocation:
    220 04-15 12:35:33.305  7412  7412 E malloc_debug:           #00  pc 00029310  /system/lib/libc.so
    221 04-15 12:35:33.305  7412  7412 E malloc_debug:           #01  pc 00021438  /system/lib/libc.so (newlocale+160)
    222 04-15 12:35:33.305  7412  7412 E malloc_debug:           #02  pc 000a9e38  /system/lib/libc++.so
    223 04-15 12:35:33.305  7412  7412 E malloc_debug:           #03  pc 000a28a8  /system/lib/libc++.so
    224 04-15 12:35:33.305  7412  7412 E malloc_debug: +++ APP leaked block of size 24 at 0x7be32380 (leak 2 of 2)
    225 04-15 12:35:33.305  7412  7412 E malloc_debug: Backtrace at time of allocation:
    226 04-15 12:35:33.305  7412  7412 E malloc_debug:           #00  pc 00029310  /system/lib/libc.so
    227 04-15 12:35:33.305  7412  7412 E malloc_debug:           #01  pc 00021438  /system/lib/libc.so (newlocale+160)
    228 04-15 12:35:33.305  7412  7412 E malloc_debug:           #02  pc 000a9e38  /system/lib/libc++.so
    229 04-15 12:35:33.305  7412  7412 E malloc_debug:           #03  pc 000a28a8  /system/lib/libc++.so
    230 </pre>
    231 
    232 Additional Errors
    233 -----------------
    234 There are a few other error messages that might appear in the log.
    235 
    236 ### Use After Free
    237 <pre>
    238 04-15 12:00:31.304  7412  7412 E malloc_debug: +++ ALLOCATION 0x12345678 USED AFTER FREE (free)
    239 04-15 12:00:31.305  7412  7412 E malloc_debug: Backtrace of original free:
    240 04-15 12:00:31.305  7412  7412 E malloc_debug:           #00  pc 00029310  /system/lib/libc.so
    241 04-15 12:00:31.305  7412  7412 E malloc_debug:           #01  pc 00021438  /system/lib/libc.so (newlocale+160)
    242 04-15 12:00:31.305  7412  7412 E malloc_debug:           #02  pc 000a9e38  /system/lib/libc++.so
    243 04-15 12:00:31.305  7412  7412 E malloc_debug:           #03  pc 000a28a8  /system/lib/libc++.so
    244 04-15 12:00:31.305  7412  7412 E malloc_debug: Backtrace at time of failure:
    245 04-15 12:00:31.305  7412  7412 E malloc_debug:           #00  pc 00029310  /system/lib/libc.so
    246 04-15 12:00:31.305  7412  7412 E malloc_debug:           #01  pc 00021438  /system/lib/libc.so (newlocale+160)
    247 04-15 12:00:31.305  7412  7412 E malloc_debug:           #02  pc 000a9e38  /system/lib/libc++.so
    248 04-15 12:00:31.305  7412  7412 E malloc_debug:           #03  pc 000a28a8  /system/lib/libc++.so
    249 </pre>
    250 
    251 This indicates that code is attempting to free an already freed pointer. The
    252 name in parenthesis indicates that the application called the function
    253 <i>free</i> with the bad pointer.
    254 
    255 For example, this message:
    256 
    257 <pre>
    258 04-15 12:00:31.304  7412  7412 E malloc_debug: +++ ALLOCATION 0x12345678 USED AFTER FREE (realloc)
    259 </pre>
    260 
    261 Would indicate that the application called the <i>realloc</i> function
    262 with an already freed pointer.
    263 
    264 ### Invalid Tag
    265 <pre>
    266 04-15 12:00:31.304  7412  7412 E malloc_debug: +++ ALLOCATION 0x12345678 HAS INVALID TAG 1ee7d000 (malloc_usable_size)
    267 04-15 12:00:31.305  7412  7412 E malloc_debug: Backtrace at time of failure:
    268 04-15 12:00:31.305  7412  7412 E malloc_debug:           #00  pc 00029310  /system/lib/libc.so
    269 04-15 12:00:31.305  7412  7412 E malloc_debug:           #01  pc 00021438  /system/lib/libc.so (newlocale+160)
    270 04-15 12:00:31.305  7412  7412 E malloc_debug:           #02  pc 000a9e38  /system/lib/libc++.so
    271 04-15 12:00:31.305  7412  7412 E malloc_debug:           #03  pc 000a28a8  /system/lib/libc++.so
    272 </pre>
    273 
    274 This indicates that a function (malloc\_usable\_size) was called with
    275 a pointer that is either not allocated memory, or that the memory of
    276 the pointer has been corrupted.
    277 
    278 As with the other error message, the function in parenthesis is the
    279 function that was called with the bad pointer.
    280 
    281 Examples
    282 ========
    283 Enable backtrace tracking of all allocation for all processes:
    284 
    285 <pre>
    286   adb shell stop
    287   adb shell setprop libc.debug.malloc.options backtrace
    288   adb shell start
    289 </pre>
    290 
    291 Enable backtrace tracking for a specific process (ls):
    292 
    293 <pre>
    294   adb shell setprop libc.debug.malloc.options backtrace
    295   adb shell setprop libc.debug.malloc.program ls
    296   adb shell ls
    297 </pre>
    298 
    299 Enable backtrace tracking for the zygote and zygote based processes:
    300 
    301 <pre>
    302   adb shell stop
    303   adb shell setprop libc.debug.malloc.program app_process
    304   adb shell setprop libc.debug.malloc.options backtrace
    305   adb shell start
    306 </pre>
    307 
    308 Enable multiple options (backtrace and guards):
    309 
    310 <pre>
    311   adb shell stop
    312   adb shell setprop libc.debug.malloc.options "\"backtrace guards\""
    313   adb shell start
    314 </pre>
    315 
    316 Enable malloc debug when multiple processes have the same name. This method
    317 can be used to enable malloc debug for only a very specific process if
    318 multiple processes have the same name.
    319 
    320 Note: The double quotes in the adb shell command are necessary. Otherwise,
    321 the setprop command will fail since the backtrace guards options will look
    322 like two arguments instead of one.
    323 
    324 <pre>
    325   adb shell
    326   # setprop libc.debug.malloc.env_enabled
    327   # setprop libc.debug.malloc.options backtrace
    328   # export LIBC_DEBUG_MALLOC_ENABLE 1
    329   # ls
    330 </pre>
    331

README_api.md

      1 Native Memory Tracking using libc Callbacks
      2 -------------------------------------------
      3 Malloc debug can be used to get information on all of the live allocations
      4 in a process. The libc library in Android exports two calls that can be
      5 used to gather this data from a process. This tracking can be enabled using
      6 either the backtrace option or the backtrace\_enabled\_on\_signal option.
      7 
      8 The function to gather the data:
      9 
     10 <pre>
     11 <b>
     12 extern "C" void get_malloc_leak_info(uint8_t** info, size_t* overall_size, size_t* info_size, size_t* total_memory, size_t* backtrace_size);
     13 </b>
     14 </pre>
     15 
     16 <i>info</i> is set to a buffer allocated by the call that contains all of
     17 the allocation information.
     18 <i>overall\_size</i> is set to the total size of the buffer returned. If this
     19 <i>info\_size</i>
     20 value is zero, then there are no allocation being tracked.
     21 <i>total\_memory</i> is set to the sum of all allocation sizes that are live at
     22 the point of the function call. This does not include the memory allocated
     23 by the malloc debug library itself.
     24 <i>backtrace\_size</i> is set to the maximum number of backtrace entries
     25 that are present for each allocation.
     26 
     27 In order to free the buffer allocated by the function, call:
     28 
     29 <pre>
     30 <b>
     31 extern "C" void free_malloc_leak_info(uint8_t* info);
     32 </b>
     33 </pre>
     34 
     35 ### Format of info Buffer
     36 <pre>
     37 size_t size_of_original_allocation
     38 size_t num_backtrace_frames
     39 uintptr_t pc1
     40 uintptr_t pc2
     41 uintptr_t pc3
     42 .
     43 .
     44 .
     45 </pre>
     46 
     47 The number of <i>uintptr\_t</i> values is determined by the value
     48 <i>backtrace\_size</i> as returned by the original call to
     49 <i>get\_malloc\_leak\_info</i>. This value is not variable, it is the same
     50 for all the returned data. The value
     51 <i>num\_backtrace\_frames</i> contains the real number of frames found. The
     52 extra frames are set to zero. Each <i>uintptr\_t</i> is a pc of the callstack.
     53 The calls from within the malloc debug library are automatically removed.
     54 
     55 For 32 bit systems, <i>size\_t</i> and <i>uintptr\_t</i> are both 4 byte values.
     56 
     57 For 64 bit systems, <i>size\_t</i> and <i>uintptr\_t</i> are both 8 byte values.
     58 
     59 The total number of these structures returned in <i>info</i> is
     60 <i>overall\_size</i> divided by <i>info\_size</i>.
     61 
     62 Note, the size value in each allocation data structure will have bit 31 set
     63 if this allocation was created by the Zygote process. This helps to distinguish
     64 between native allocations created by the application.
     65