Re: [PATCH v4 2/3] c++: Improve constexpr error for dangling local variables [PR110619]

[Jason Merrill <jason@redhat.com>]

On 7/20/23 05:36, Nathaniel Shead wrote:
> Currently, when typeck discovers that a return statement will refer to a
> local variable it rewrites to return a null pointer. This causes the
> error messages for using the return value in a constant expression to be
> unhelpful, especially for reference return values.
> 
> This patch removes this "optimisation".

This isn't an optimization, it's for safety, removing a way for an 
attacker to get a handle on other data on the stack (CWE-562).

But I agree that we need to preserve some element of UB for constexpr 
evaluation to see.

Perhaps we want to move this transformation to 
cp_maybe_instrument_return, so it happens after maybe_save_constexpr_fundef?

> Relying on this raises a warning
> by default and causes UB anyway, so there should be no issue in doing
> so. We also suppress additional warnings from later passes that detect
> this as a dangling pointer, since we've already indicated this anyway.
> 
> 	PR c++/110619
> 
> gcc/cp/ChangeLog:
> 
> 	* semantics.cc (finish_return_stmt): Suppress dangling pointer
> 	reporting on return statement if already reported.
> 	* typeck.cc (check_return_expr): Don't set return expression to
> 	zero for dangling addresses.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/cpp1y/constexpr-lifetime5.C: Test reported message is
> 	correct.
> 	* g++.dg/cpp1y/constexpr-lifetime6.C: Likewise.
> 	* g++.dg/cpp1y/constexpr-110619.C: New test.
> 	* g++.dg/warn/Wreturn-local-addr-6.C: Remove check for return
> 	value optimisation.
> 
> Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
> ---
>   gcc/cp/semantics.cc                              |  5 ++++-
>   gcc/cp/typeck.cc                                 |  5 +++--
>   gcc/testsuite/g++.dg/cpp1y/constexpr-110619.C    | 10 ++++++++++
>   gcc/testsuite/g++.dg/cpp1y/constexpr-lifetime5.C |  4 ++--
>   gcc/testsuite/g++.dg/cpp1y/constexpr-lifetime6.C |  8 ++++----
>   gcc/testsuite/g++.dg/warn/Wreturn-local-addr-6.C |  3 ---
>   6 files changed, 23 insertions(+), 12 deletions(-)
>   create mode 100644 gcc/testsuite/g++.dg/cpp1y/constexpr-110619.C
> 
> diff --git a/gcc/cp/semantics.cc b/gcc/cp/semantics.cc
> index 8fb47fd179e..107407de513 100644
> --- a/gcc/cp/semantics.cc
> +++ b/gcc/cp/semantics.cc
> @@ -1260,7 +1260,10 @@ finish_return_stmt (tree expr)
>   
>     r = build_stmt (input_location, RETURN_EXPR, expr);
>     if (no_warning)
> -    suppress_warning (r, OPT_Wreturn_type);
> +    {
> +      suppress_warning (r, OPT_Wreturn_type);
> +      suppress_warning (r, OPT_Wdangling_pointer_);
> +    }
>     r = maybe_cleanup_point_expr_void (r);
>     r = add_stmt (r);
>   
> diff --git a/gcc/cp/typeck.cc b/gcc/cp/typeck.cc
> index 859b133a18d..47233b3b717 100644
> --- a/gcc/cp/typeck.cc
> +++ b/gcc/cp/typeck.cc
> @@ -11273,8 +11273,9 @@ check_return_expr (tree retval, bool *no_warning)
>         else if (!processing_template_decl
>   	       && maybe_warn_about_returning_address_of_local (retval, loc)
>   	       && INDIRECT_TYPE_P (valtype))
> -	retval = build2 (COMPOUND_EXPR, TREE_TYPE (retval), retval,
> -			 build_zero_cst (TREE_TYPE (retval)));
> +	/* Suppress the Wdangling-pointer warning in the return statement
> +	   that would otherwise occur.  */
> +	*no_warning = true;
>       }
>   
>     /* A naive attempt to reduce the number of -Wdangling-reference false
> diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-110619.C b/gcc/testsuite/g++.dg/cpp1y/constexpr-110619.C
> new file mode 100644
> index 00000000000..cca13302238
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-110619.C
> @@ -0,0 +1,10 @@
> +// { dg-do compile { target c++14 } }
> +// { dg-options "-Wno-return-local-addr" }
> +// PR c++/110619
> +
> +constexpr auto f() {
> +    int i = 0;
> +    return &i;
> +};
> +
> +static_assert( f() != nullptr );
> diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-lifetime5.C b/gcc/testsuite/g++.dg/cpp1y/constexpr-lifetime5.C
> index a4bc71d890a..ad3ef579f63 100644
> --- a/gcc/testsuite/g++.dg/cpp1y/constexpr-lifetime5.C
> +++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-lifetime5.C
> @@ -1,11 +1,11 @@
>   // { dg-do compile { target c++14 } }
>   // { dg-options "-Wno-return-local-addr" }
>   
> -constexpr const int& id(int x) { return x; }
> +constexpr const int& id(int x) { return x; }  // { dg-message "note: declared here" }
>   
>   constexpr bool test() {
>     const int& y = id(3);
>     return y == 3;
>   }
>   
> -constexpr bool x = test();  // { dg-error "" }
> +constexpr bool x = test();  // { dg-error "accessing object outside its lifetime" }
> diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-lifetime6.C b/gcc/testsuite/g++.dg/cpp1y/constexpr-lifetime6.C
> index f358aff4490..b81e89af79c 100644
> --- a/gcc/testsuite/g++.dg/cpp1y/constexpr-lifetime6.C
> +++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-lifetime6.C
> @@ -4,12 +4,12 @@
>   struct Empty {};
>   
>   constexpr const Empty& empty() {
> -  return Empty{};
> +  return Empty{};  // { dg-message "note: declared here" }
>   }
>   
> -constexpr const Empty& empty_parm(Empty e) {
> +constexpr const Empty& empty_parm(Empty e) {  // { dg-message "note: declared here" }
>     return e;
>   }
>   
> -constexpr Empty a = empty();  // { dg-error "" }
> -constexpr Empty b = empty_parm({});  // { dg-error "" }
> +constexpr Empty a = empty();  // { dg-error "accessing object outside its lifetime" }
> +constexpr Empty b = empty_parm({});  // { dg-error "accessing object outside its lifetime" }
> diff --git a/gcc/testsuite/g++.dg/warn/Wreturn-local-addr-6.C b/gcc/testsuite/g++.dg/warn/Wreturn-local-addr-6.C
> index fae8b7e766f..ec8e241d83e 100644
> --- a/gcc/testsuite/g++.dg/warn/Wreturn-local-addr-6.C
> +++ b/gcc/testsuite/g++.dg/warn/Wreturn-local-addr-6.C
> @@ -24,6 +24,3 @@ return_addr_local_as_intref (void)
>   
>     return (const intptr_t&)a;   // { dg-warning "\\\[-Wreturn-local-addr]" } */
>   }
> -
> -/* Verify that the return value has been replaced with zero:
> -  { dg-final { scan-tree-dump-times "return 0;" 2 "optimized" } } */