[PATCH] Fortran: Avoid accessing gfc_charlen when not looking at BT_CHARACTER (PR 110677)

[Martin Jambor]

Hello,

this patch addresses an issue uncovered by the undefined behavior
sanitizer.  In function resolve_structure_cons in resolve.cc there is
a test starting with:

      if (cons->expr->ts.type == BT_CHARACTER && comp->ts.u.cl
          && comp->ts.u.cl->length
          && comp->ts.u.cl->length->expr_type == EXPR_CONSTANT

and UBSAN complained of loads from comp->ts.u.cl->length->expr_type of
integer value 1818451807 which is outside of the value range expr_t
enum.  If I understand the code correctly it the entire load was
unwanted because comp->ts.type in those cases is BT_CLASS and not
BT_CHARACTER.  This patch simply adds a check to make sure it is only
accessed in those cases.

I have verified that the UPBSAN failure goes away with this patch, it
also passes bootstrap and testing on x86_64-linux.  OK for master?

Thanks,

Martin



gcc/fortran/ChangeLog:

2023-08-14  Martin Jambor  <mjam...@suse.cz>

        PR fortran/110677
        * resolve.cc (resolve_structure_cons): Check comp->ts is character
        type before accessing stuff through comp->ts.u.cl.
---
 gcc/fortran/resolve.cc | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/gcc/fortran/resolve.cc b/gcc/fortran/resolve.cc
index e7c8d919bef..5b4dfc5fcd2 100644
--- a/gcc/fortran/resolve.cc
+++ b/gcc/fortran/resolve.cc
@@ -1396,8 +1396,9 @@ resolve_structure_cons (gfc_expr *expr, int init)
         the one of the structure, ensure this if the lengths are known at
         compile time and when we are dealing with PARAMETER or structure
         constructors.  */
-      if (cons->expr->ts.type == BT_CHARACTER && comp->ts.u.cl
-         && comp->ts.u.cl->length
+      if (cons->expr->ts.type == BT_CHARACTER
+         && comp->ts.type == BT_CHARACTER
+         && comp->ts.u.cl && comp->ts.u.cl->length
          && comp->ts.u.cl->length->expr_type == EXPR_CONSTANT
          && cons->expr->ts.u.cl && cons->expr->ts.u.cl->length
          && cons->expr->ts.u.cl->length->expr_type == EXPR_CONSTANT
-- 
2.41.0