This is a tool to interpet and show information generated b y my experimental
changes to OpenBSD's malloc.
First step is to apply the diff to src/lib/libc/stdlib/malloc.c, recompile and install libc:
$ cd /usr/src/lib/libc
$ patch < malloc.diff
$ make obj
$ MALLOC_STATS=1 make
$ doas make install
After this, malloc will generate utrace(2) records when instructed to do so.
These ulog records are written to a file on regular exit of your program
The trace file is generated by running your program with
MALLOC_OPTIONS=DT ktrace -tu program
The MALLOC_OPTIONS instruct malloc to write the infomration and the ktrace invocation
takes care of catching the utrace records and writing themn to a file, by
default called ktrace.out. After that you can use the mdump program to
display the information.
Remeber to build your program and libraries with debug information.
On OpenBSD system libraries are installed with debug info, so that's
convenient. To get deeper stack trace information on leaks use
more T's. Note that the machanism used (builtin_return_address)
is not guaranteed to work for more than one T, some executablea
on some platforms will crash when too many T's are used.
To compile mdump, you'll need to elftoolchain package.
# doas pkg_add elftoolchain
$ make obj
$ make
$ doas make install
This wil install the tool and man page into /usr/local.
Basic usage is
mdump
to show leak info and
mdump -D
to show a dump of malloc's internal state at program exit.
To produce readable stack traces, the program and its libraries should be
compiled with debug information, typically -g.
Statically linked programs must be compiled with the
-nopie option.
High optimization levels can produce debug information that
mdump
cannot interpret.
While running (and if enabled by MALLOC_OPTIONS) malloc
stores backtrace information for allocations.
Per stackframe the return address is recorded.
Any allocation equal or larger than a page is tracked. For smaller allocations,
only a sample is recorded. This means that not all leaks will be shown for smaller allocations.
At program exit, malloc will check which allocations are not freed,
translate the addresses into library/executable plus offset information
using dladdr(3) and construct utrace records to send out.
The mdump program then takes this information and translates the library
plus offset information into function name + file + linenumber information using
the debug information embedded in the program and its libraries.