Subject: Re: [PATCH v3 2/3] commit-graph: fix buffer read-overflow

From: Josh Steadmon <[email-masked]>
Date: Mon, 10 Dec 2018 13:56:49 -0800

On 2018.12.09 13:01, Junio C Hamano wrote:
> Josh Steadmon <[email-masked]> writes:
> 
> > diff --git a/t/t5318-commit-graph.sh b/t/t5318-commit-graph.sh
> > index 5fe21db99f..5b6b44b78e 100755
> > --- a/t/t5318-commit-graph.sh
> > +++ b/t/t5318-commit-graph.sh
> > @@ -366,24 +366,30 @@ GRAPH_OCTOPUS_DATA_OFFSET=$(($GRAPH_COMMIT_DATA_OFFSET + \
> >  GRAPH_BYTE_OCTOPUS=$(($GRAPH_OCTOPUS_DATA_OFFSET + 4))
> >  GRAPH_BYTE_FOOTER=$(($GRAPH_OCTOPUS_DATA_OFFSET + 4 * $NUM_OCTOPUS_EDGES))
> >  
> > -# usage: corrupt_graph_and_verify <position> <data> <string>
> > +# usage: corrupt_graph_and_verify <position> <data> <string> [<zero_pos>]
> >  # Manipulates the commit-graph file at the position
> > -# by inserting the data, then runs 'git commit-graph verify'
> > +# by inserting the data, optionally zeroing the file
> > +# starting at <zero_pos>, then runs 'git commit-graph verify'
> >  # and places the output in the file 'err'. Test 'err' for
> >  # the given string.
> >  corrupt_graph_and_verify() {
> >  	pos=$1
> >  	data="${2:-\0}"
> >  	grepstr=$3
> > +	orig_size=$(stat --format=%s $objdir/info/commit-graph)
> 
> "stat(1)" is not so portable, so you'll get complaints from minority
> platform users later.  So is "truncate(1)".

Ack, thanks for the catch. I have a workaround for stat in the form of
"wc -c", and for truncate with a combination of dd and /dev/zero.
However, I'm finding conflicting information about whether or not
/dev/zero exists on macOS. At the least, it sounds like it might not
work on very old versions. Would this be acceptable, or should I add a
new test function to do this?

> > +	zero_pos=${4:-${orig_size}}
> >  	cd "$TRASH_DIRECTORY/full" &&
> >  	test_when_finished mv commit-graph-backup $objdir/info/commit-graph &&
> >  	cp $objdir/info/commit-graph commit-graph-backup &&
> >  	printf "$data" | dd of="$objdir/info/commit-graph" bs=1 seek="$pos" conv=notrunc &&
> > +	truncate --size=$zero_pos $objdir/info/commit-graph &&
> > +	truncate --size=$orig_size $objdir/info/commit-graph &&
> >  	test_must_fail git commit-graph verify 2>test_err &&
> >  	grep -v "^+" test_err >err
> >  	test_i18ngrep "$grepstr" err
> >  }
> >  
> > +
> >  test_expect_success 'detect bad signature' '
> >  	corrupt_graph_and_verify 0 "\0" \
> >  		"graph signature"
> > @@ -484,6 +490,11 @@ test_expect_success 'detect invalid checksum hash' '
> >  		"incorrect checksum"
> >  '
> >  
> > +test_expect_success 'detect incorrect chunk count' '
> > +	corrupt_graph_and_verify $GRAPH_BYTE_CHUNK_COUNT "\xff" \
> 
> Implementations of printf(1) may not grok "\xff" as a valid
> representation of "\377".  The shell built-in of dash(1) for example
> would not work with this.

Ack, will fix in V4. Thanks.

> > +		"chunk lookup table entry missing" $GRAPH_CHUNK_LOOKUP_OFFSET
> > +'
> > +
> >  test_expect_success 'git fsck (checks commit-graph)' '
> >  	cd "$TRASH_DIRECTORY/full" &&
> >  	git fsck &&