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Date: Sat, 21 Mar 2015 14:28:10 +0100
From: Szabolcs Nagy <>
To: Konstantin Serebryany <>
Cc: Rich Felker <>,
Subject: Re: buffer overflow in regcomp and a way to find more of those

* Konstantin Serebryany <> [2015-03-20 23:05:13 -0700]:
> On Fri, Mar 20, 2015 at 7:20 PM, Rich Felker <> wrote:
> > On Fri, Mar 20, 2015 at 07:14:33PM -0700, Konstantin Serebryany wrote:
> >> If you build the source with "-fsanitize=leak -fsanitize-coverage=4
> >> -O1" the compiler will not insert any of the asan instrumentation
> >> and only insert calls to a couple of functions needed for coverage.
> >> Then, instead of linking with the full asan+coverage run-time, you
> >> will need a very simple re-implementation of coverage-only runtime.
> >
> > Could the existing runtime be used, just stripped down?
> Yes, but for the basic functionality needed by the fuzzer it's simpler
> to write it from scratch, see below:
> ========================================================
> svn co
> cat <<EOF >cov-minimal-rt.c
> static long counter;
> void __sanitizer_cov_with_check(int *guard) {
>   if (*guard == 0) {
>     counter++;
>     *guard=1;
>   }
> }
> long __sanitizer_get_total_unique_coverage() { return counter; }
> void __sanitizer_cov_module_init() {}
> void __sanitizer_reset_coverage(){}
> void __sanitizer_get_coverage_guards(){}
> void __sanitizer_get_number_of_counters(){}
> void __sanitizer_update_counter_bitset_and_clear_counters(){}
> void __sanitizer_set_death_callback(){}
> clang -std=c++11 -c Fuzzer/Fuzzer*.cpp -I Fuzzer
> clang -std=c++11  -fsanitize=leak -fsanitize-coverage=3 -mllvm
> -sanitizer-coverage-block-threshold=0  Fuzzer/test/SimpleTest.cpp -c
> clang -c cov-minimal-rt.c
> clang++ *.o
> ./a.out
> ========================================================

with this i could run the fuzzer against libc.a

it's a bit more work to link to libc.a than adding
a -L so i attached the scripts i used (and an example)
so others can reproduce it

c++ headers cannot be used in the test (that would
require cleaning up the libstdc++ header mess)

but i think there is no reason to use c++ for these
libc api tests anyway

you may need to adjust the directories the scripts use

(the linking may need to change when compiler-rt is
used instead of libgcc)


cd workdir
./ reg.c
./ reg.o

of course to make it useful the malloc magic is needed for
more likely crashes

> The recently added afl-style counters
> (
> are a bit more involved, but the basic bool-per-edge is quite enough
> in most cases.


> The fuzzer itself is written in C++ and uses STL (probably, not the
> best idea, but it makes the experiments simpler).
> Can't tell if it will be a problem with musl, but after all the fuzzer
> itself is also trivial (as well as the entire concept)

c++ happens to work because musl is (almost) abi compatible with
glibc on x86 so we can just link to the glibc linked libstdc++

(this can eg fail when the c++ thread local storage destructor
abi is used, that is not implemented in musl yet)

so yes c++ makes things more painful: you need to recompile the
entire toolchain to make it work reliably (and then both gcc
and clang have broken assumptions about the libc so you have to
patch them) which is too much work for running tests

> > Well static linking with musl does not impose any constraint on
> > redefining functions, so you could easily use a debugging malloc that
> > lines up each allocation to end on a page boundary with a guard page
> > after it.
> Yea... This will slowdown fuzzing and guard pages only protect you
> from overflow in one direction (ether left, of right, but not both).
> But this is better than nothing.

you can run the tests twice (for left and right) :)

> > This would of course be slow and use lots of memory but
> > would catch all heap overflows. And -fstack-protector-all would catch
> > most stack-based overflows.
> Only stack-overflow-write by a small amount, but yes, better than nothing.
> BTW, writing a minimalistic asan run-time as part of musl should be a
> matter of a couple of hours.
> Probably much faster than making the current monster work with static linking.
> I'd be happy to help with such.

how would this look?

compile the tests and libc with asan, but instead of linking the
asan runtime from clang use a musl specific one?

i assume for that we still need to change the libc startup code, malloc
functions and may be some things around thread stacks

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