Re: [PATCH v2] Fix segfault when invoking -var-info-path-expression on a dynamic varobj

[Simon Marchi <simark@simark.ca>]

On 2018-06-25 04:59 PM, Jan Vrany wrote:
> Invoking -var-info-path-expression on a dynamic varobj lead either in wrong
> (nonsense) result or to a segmentation fault in cplus_describe_child().
> This was caused by the fact that varobj_get_path_expr() called
> cplus_path_expr_of_child() ignoring the fact the parent of the variable
> is dynamic. Then, cplus_describe_child() accessed the underlaying C type
> members by index, causing (i) either wrong (nonsense) expression being
> returned (since dynamic child may be completely arbibtrary value)
> or (ii) segmentation fault (in case the index higher than number of
> underlaying C type members.
> 
> This fixes the problem by checking whether a varobj is a child of a dynamic
> varobj and, if so, reporting an error to MI consumer as described in
> the documentation.

Hi Jan,

The patch does not compile for me:

  CXX    varobj.o
In file included from /home/simark/src/binutils-gdb/gdb/common/common-defs.h:90,
                 from /home/simark/src/binutils-gdb/gdb/defs.h:28,
                 from /home/simark/src/binutils-gdb/gdb/varobj.c:18:
/home/simark/src/binutils-gdb/gdb/varobj.c: In function ‘const char* varobj_get_path_expr(const varobj*)’:
/home/simark/src/binutils-gdb/gdb/varobj.c:969:41: error: ‘cur’ was not declared in this scope
       gdb_assert (!varobj_is_dynamic_p (cur));
                                         ^~~
/home/simark/src/binutils-gdb/gdb/common/gdb_assert.h:33:13: note: in definition of macro ‘gdb_assert’
   ((void) ((expr) ? 0 :                                                       \
             ^~~~
> diff --git a/gdb/mi/mi-cmd-var.c b/gdb/mi/mi-cmd-var.c
> index 38c59c7e66..fa47387357 100644
> --- a/gdb/mi/mi-cmd-var.c
> +++ b/gdb/mi/mi-cmd-var.c
> @@ -438,7 +438,15 @@ mi_cmd_var_info_path_expression (const char *command, char **argv, int argc)
>  
>    /* Get varobj handle, if a valid var obj name was specified.  */
>    var = varobj_get_handle (argv[0]);
> -  
> +
> +  /* -var-info-path-expression is currently not valid for children of
> +     a dynamic varobj.  */
> +  for (struct varobj *cur = var->parent; cur != nullptr; cur = cur->parent)
> +    {
> +      if (varobj_is_dynamic_p (cur))
> +        error (_("Invalid variable object (child of a dynamic varobj)"));
> +    }
> +

To make it simpler, instead of checking all the parents recursively here, have you
considered adding a check to the varobj_get_path_expr_parent, like this?  I haven't
given it a very long though, but it does pass your test :).

diff --git a/gdb/varobj.c b/gdb/varobj.c
index f2c10ddc57ff..e601cf0b9780 100644
--- a/gdb/varobj.c
+++ b/gdb/varobj.c
@@ -948,6 +948,9 @@ varobj_get_path_expr_parent (const struct varobj *var)
   while (!is_root_p (parent) && !is_path_expr_parent (parent))
     parent = parent->parent;

+  if (varobj_is_dynamic_p (parent))
+    error (_("Invalid variable object (child of a dynamic varobj)"));
+
   return parent;
 }


Thanks,

Simon