Re: [Qemu-devel] [PATCH v2] tests: set MALLOC_PERTURB_ to expose memory bugs

[Stefan Hajnoczi <stefanha@redhat.com>]

On Tue, May 21, 2013 at 06:56:07AM -0600, Eric Blake wrote:
> On 05/21/2013 06:43 AM, Stefan Hajnoczi wrote:
> > glibc wipes malloc(3) memory when the MALLOC_PERTURB_ environment
> > variable is set.  The value of the environment variable determines the
> > bit pattern used to wipe memory.  For more information, see
> > http://udrepper.livejournal.com/11429.html.
> > 
> > Set MALLOC_PERTURB_ for gtester and qemu-iotests.  Note we pick a random
> > value from 1 to 255 to expose more bugs.  If you need to reproduce a
> > crash use 'show environment' in gdb to extract the MALLOC_PERTURB_
> > value from a core dump.
> > 
> > Both make check and qemu-iotests pass with MALLOC_PERTURB_ enabled.
> > 
> > Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> > ---
> >  tests/Makefile           | 5 ++++-
> >  tests/qemu-iotests/check | 3 ++-
> >  2 files changed, 6 insertions(+), 2 deletions(-)
> > 
> > diff --git a/tests/Makefile b/tests/Makefile
> > index a307d5a..24880c6 100644
> > --- a/tests/Makefile
> > +++ b/tests/Makefile
> > @@ -171,6 +171,7 @@ GCOV_OPTIONS = -n $(if $(V),-f,)
> >  $(patsubst %, check-qtest-%, $(QTEST_TARGETS)): check-qtest-%: $(check-qtest-y)
> >  	$(if $(CONFIG_GCOV),@rm -f *.gcda */*.gcda */*/*.gcda */*/*/*.gcda,)
> >  	$(call quiet-command,QTEST_QEMU_BINARY=$*-softmmu/qemu-system-$* \
> > +		MALLOC_PERTURB_=${MALLOC_PERTURB_:-$(($RANDOM % 255 + 1))} \
> 
> This is a Makefile; don't you need to use $$ instead of $ (three instances)?
> 
> $RANDOM is a bash-ism.  If make is run with SHELL as /bin/sh on a
> platform where dash is the primary shell, it will fail:
> 
> $ dash -c 'echo $(($RANDOM % 255))'
> dash: 1: arithmetic expression: expecting primary: " % 255"
> 
> HOWEVER: you can exploit the fact that inside $(()), you don't need $ to
> use the value of a defined variable, and also the fact that unless set
> -u is in effect, an undefined variable name silently evaluates as 0:
> 
> $ dash -c 'echo $((RANDOM % 255))'
> 0
> 
> then you could write the shell code:
> 
> 	MALLOC_PERTURB_=${MALLOC_PERTURB_:-$((RANDOM % 255 + 1))}
> 
> or the Makefile code:
> 
> 	MALLOC_PERTURB_=$${MALLOC_PERTURB_:-$$((RANDOM % 255 + 1))}
> 
> and things will at least work on /bin/sh as dash (even though there will
> be no randomness and you are always testing with 1 in that case).

Silly me.  I did test it but it silently "worked".

Will resend.

Stefan