Subject: Re: [PATCH] grep: report missing left operand of --and

From: Ævar Arnfjörð Bjarmason <[email-masked]>
Date: Tue, 29 Jun 2021 19:52:19 +0200

On Mon, Jun 28 2021, René Scharfe wrote:

> Git grep allows combining two patterns with --and.  It checks and
> reports if the second pattern is missing when compiling the expression.
> A missing first pattern, however, is only reported later at match time.
> Thus no error is returned if no matching is done, e.g. because no file
> matches the also given pathspec.
>
> When that happens we get an expression tree with an GREP_NODE_AND node
> and a NULL pointer to the missing left child.  free_pattern_expr()
> tries to dereference it during the cleanup at the end, which result in
> a segmentation fault.
>
> Fix this by verifying the presence of the left operand at expression
> compilation time.
>
> Reported-by: Matthew Hughes <[email-masked]>
> Signed-off-by: René Scharfe <[email-masked]>
> ---
> Whether the check in match_expr_eval() can now be turned into a BUG is
> left as an exercise for the reader. ;-)
>
>  grep.c          |  2 ++
>  t/t7810-grep.sh | 10 ++++++++++
>  2 files changed, 12 insertions(+)
>
> diff --git a/grep.c b/grep.c
> index 8f91af1cb0..7d0ea4e956 100644
> --- a/grep.c
> +++ b/grep.c
> @@ -655,6 +655,8 @@ static struct grep_expr *compile_pattern_and(struct grep_pat **list)
>  	struct grep_expr *x, *y, *z;
>
>  	x = compile_pattern_not(list);
> +	if (!x)
> +		die("Not a valid grep expression");
>  	p = *list;
>  	if (p && p->token == GREP_AND) {
>  		if (!p->next)
> diff --git a/t/t7810-grep.sh b/t/t7810-grep.sh
> index 5830733f3d..c581239674 100755
> --- a/t/t7810-grep.sh
> +++ b/t/t7810-grep.sh
> @@ -11,6 +11,13 @@ export GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME
>
>  . ./test-lib.sh
>
> +test_invalid_grep_expression() {
> +	params="$@" &&
> +	test_expect_success "invalid expression: grep $params" '
> +		test_must_fail git grep $params -- nonexisting
> +	'
> +}
> +
>  cat >hello.c <<EOF
>  #include <assert.h>
>  #include <stdio.h>
> @@ -89,6 +96,9 @@ test_expect_success 'grep should not segfault with a bad input' '
>  	test_must_fail git grep "("
>  '
>
> +test_invalid_grep_expression -e A --and
> +test_invalid_grep_expression --and -e A
> +
>  for H in HEAD ''
>  do
>  	case "$H" in

This seems like an incomplete fix, for the exact same thing with --or we
silently return 1, as we would if we exited early in free_pattern_expr
on !x, which aside from the segfault I think we should probably make a
habit in our own free()-like functions.

Whatever we're doing about the --and segfault it seems like we should do
the same under --or, no?

Your first test also passes before your fix, it's only the latter that
segfaults. The first one emits:

    fatal: --and not followed by pattern expression

So having that in a leading patch to indicate no behavior was changed
would be better.

Instead of the "Not a valid grep expression" error let's instead say
something like:

    fatal: --[and|or] must follow a pattern expression

The error (which I know you just copied from elsewhere) is misleading,
it's not the pattern that's not valid (as to me it implies), but our own
--and/--or option usage.

And the "excercise for the reader" is a bit flippant, do we actually hit
that condition now? If not and we're sure we won't now seems like the
time to add a BUG() there, and to change the "Not a valid grep
expression" to "internal error in --and/--or parsing" or something.

Thanks for the patch!