How OpenBSD Leverages Malloc to Find Bugs

http://os-blog.com/how-openbsd-leverages-malloc-to-find-bugs/

by adbge • 15 years ago

88 32 15 years ago

nodata• 15 years ago

Can this be configured at runtime using an environment variable? It would be nice to test for bugs like this in dev.

pmjordan• 15 years ago

You could easily do this on Linux, where the executable format allows you to override library-defined symbols, including malloc(). Implement the new malloc/calloc/realloc/free as a static library and link it to your dev builds. Don't link it for releases.

Or just use valgrind, if you can.

piotrSikora• 15 years ago

You seem to miss the difference between "developer testing his/her app" and "everyone testing every app".

To the point, I'm using "/etc/malloc.conf -> AFGJPRX" on most of my systems, which means that virtually everything that runs there is checked. Can you easily do that on Linux? How many apps are you running via valgrind on a daily basis?

lflux4• 15 years ago

Yes, by setting the MALLOC_OPTIONS environment variable. See http://www.openbsd.org/cgi-bin/man.cgi?query=malloc

munin• 15 years ago

the windows heap manager can be configured to do this on a per-application basis using the 'gflags' utility.

it's a useful debugging tool. you're not assured that use-after-free is going to be tightly temporally coupled with the free (a lot of the time it is though) and as more time passes after free() the odds increase that the dangling virtual address is re-allocated.

it is also the enemy of performance. you should probably not run production things with the heap configured in this way.

smutticus• 15 years ago

How is this different from efence on Linux?

lflux4• 15 years ago

It comes with the operating system, no need to re-link against a different library or a different LD_PRELOAD path.

eis3• 15 years ago

Without having looked at OpenBSD's implementation, just unmapping free pages does not mean you catch all writes to free()d memory. If a block is smaller than the pagesize, which many times is the case, a page will likely still hold other objects so it wont be unmapped and therefor not trigger a segfault.

Valgrind is my tool of choice to find invalid reads or writes. It catches all of them plus it gives you nice information like a stacktrace.

What OpenBSD's implementation of malloc() does get you though is improved coping with memory fragmentation. Allocating memory in a continously heap brings lots of pain - jemalloc (which is the default allocator in FreeBSD and NetBSD) solves this nicely by also only using mmap() instead of sbrk() on Linux.

Note: I should have said "resource reclamation" instead of "fragmentation". See discussion with ajross below for details.

ajross3• 15 years ago

Or simply if the address in question has been remapped since the original free. For big 32 bit processes that's very likely. There's nothing wrong with the implementation really, it's just oversold.

Obviously the valgrind advice is great. But I don't follow your point about mmap vs. sbrk regarding fragmentation. Fragmentation is a property of memory addresses, not the syscall used to allocate them. A big linear heap allocated with sbrk will fragment just as badly as the same heap allocated in a single block, even if it's allowed to have "holes" in it.

Maybe you're claiming that jemalloc deliberatly leaves gaps between allocations? That can help a little in practice, though the real effect is just to change the size of the fragmented blocks.

eis3• 15 years ago

Actually also with a 64 bit address space, you can easily get the same address if the allocator returns a cached block.

The problem with sbrk() is that if you allocate a few megabytes of memory + a tiny chunk, even after freeing everything apart from the tiny chunk near the "program break", nothing will be given back to the OS. If you use mmap() instead though, all pages can be given back apart from the one where the tiny chunk resides in. This makes for a tremendous difference sometimes.

xradionut• 15 years ago

"Valgrind is my tool of choice to find invalid reads or writes. It catches all of them plus it gives you nice information like a stacktrace."

Be careful! It's easy to depend on -Wall and Valgrind and still overlook subtle legal but "doh!" errors in array or pointer logic. (Humbly speaking from experience...)

16s• 15 years ago

-Wextra may help some too.

Spider• 15 years ago

Is this really different from the glibc malloc() implementation with MALLOC_CHECK_ set to 3?

> MALLOC_CHECK_ is designed to be tolerant against simple errors, such as double calls of free() with the same argument, or overruns of a single byte (off-by-one bugs). Not all such errors can be protected against, however, and memory leaks can result.

> If MALLOC_CHECK_ is set to 0, any detected heap corruption is silently ignored;

> if set to 1, a diagnostic message is printed on stderr;

> if set to 2, abort(3) is called immediately;

> if set to 3, a diagnostic message is printed on stderr and the program is aborted.

marshray• 15 years ago

Yes, the OpenBSD technique of completely unmapping the memory rather than re-using it will be able to catch even some read accesses, not just write accesses that happen to corrupt malloc's guards.

Tharkun• 15 years ago

Why is this on hacker news? OpenBSD has been doing this for years? About as news worthy as Windows 95's BSOD.

koenigdavidmj• 15 years ago

HN is about anything that hackers might find interesting. We're even kind of tolerant of reposts with a sufficient time gap since the last post.

chollida1• 15 years ago

Not everyone spends all day working in OpenBSD, I learned something from the article.

jeffreymcmanus2• 15 years ago

Stop saying "leverages" unless you're describing lifting a heavy rock using a stick. The word you're looking for is "uses".

jemfinch• 15 years ago

Your comment contributes nothing to this thread or this website. Stop wasting your time and others' posting about matters of stylistic opinion.

jeffreymcmanus2• 15 years ago

Pot, meet kettle.

dfc• 15 years ago

In addition to adding nothing to this thread your statement is incorrect. The oxford dictionary explicitly defines a figurative definition of leverage.

If this usage bothers you I must caution you against working in a finance related field. One or two days would expose you to countless instances of using leverage in a figurative sense.

bartwe• 15 years ago

I have a number of custom allocators that use the same techniques, with some improvements.

When space is cheap: All allocations are two 4k pages The returned pointer is alligned with the end of the buffer to detect overruns. (with a 16byte allignment) The page following the alloc is always denied. The space around the allocation is filled with flag values, and these are checked on free. After free the pages are held in storage for a few thousand following allocations.

The variants of this allocator does things like only doing this for specific ranges of allocation sizes or only after a certain number of allocations.

With this a good number of overruns and use after free bugs have been found.

Mostly used this technique on windows with delphi, on linux i prefer valgrind.

eridius2• 15 years ago

Why 2 4k pages? If I'm allocating 1k of memory, what does having an entire extra page get me?

jsnell2• 15 years ago

The second 4k is a guard page, to catch buffer overflows. So the heap looks like:

3k empty, 1k data, 4k empty and read/write-protected.

The extra page is needed since you can't set the memory protection flags at a higher granularity.

surki• 15 years ago

What you describe is implemented in these libraries: libduma (http://duma.sourceforge.net/) and efence (http://linux.die.net/man/3/efence)

They also have buffer underflow protection (either overflow or underflow at a time).

But in 32 bit systems, for applications that heavily use memory, you may run out of address space

How OpenBSD Leverages Malloc to Find Bugs

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