[Bug c/82599] New: Assignments from statically initialized flexible arrays copy too much

[karzes at sonic dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82599

            Bug ID: 82599
           Summary: Assignments from statically initialized flexible
                    arrays copy too much
           Product: gcc
           Version: 5.4.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: karzes at sonic dot net
  Target Milestone: ---

Created attachment 42388
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=42388&action=edit
C program that demonstrates the bug

I am running on a 32-bit Ubuntu system.

The attached file, fa4.c, shows a problem with the gcc extension which allows
static initialization of flexible array members.

The problem is in main, with the declaration and initialization of second:

    s second = first;

The compiler isn't allocating space for the flexible array member in second,
which makes sense.  However, it is copying *all* of first into second,
including the space that was allocated for the flexible array member.  The
result is that it writes past the end of second, and continues writing into the
array v.  When v is printed, instead of printing <xxxxxxxx>, it instead prints
the tail of the string that was copied from first, which comes out as <defgh>.

The same behavior is seen if the declaration of and assignment to s are split
into:

    s second;
    second = first;

I believe the assignment should only copy sizeof(s) bytes, rather than
including the storage allocated for the flexible array member.

Note that if the declaration and static initialization of first is moved to a
separate file, the problem disappears, and the structure copy behaves as it
should.  But when gcc sees the true allocated size of first, it copies too much
in the assignment.  It should always copy the same amount regardless of where
first is defined.