linux-kernel - Re: [PATCH] gcc-plugins/stackleak: Avoid assignment for unused macro argument

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Date:   Tue, 7 Apr 2020 18:30:18 +0300
From:   Alexander Popov <alex.popov@...ux.com>
To:     Kees Cook <keescook@...omium.org>,
        kernel-hardening@...ts.openwall.com
Cc:     Emese Revfy <re.emese@...il.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] gcc-plugins/stackleak: Avoid assignment for unused macro
 argument

Hello Kees,

On 02.04.2020 11:10, Kees Cook wrote:
> With GCC version >= 8, the cgraph_create_edge() macro argument using
> "frequency" goes unused. Instead of assigning a temporary variable for
> the argument, pass the compute_call_stmt_bb_frequency() call directly
> as the macro argument so that it will just not be uncalled when it is
> not wanted by the macros.

Do you mean "it will just not be called"?

Thanks!

> Silences the warning:
> 
> scripts/gcc-plugins/stackleak_plugin.c:54:6: warning: variable ‘frequency’ set but not used [-Wunused-but-set-variable]
> 
> Now builds cleanly with gcc-7 and gcc-9. Both boot and pass
> STACKLEAK_ERASING LKDTM test.
> 
> Signed-off-by: Kees Cook <keescook@...omium.org>
> ---
>  scripts/gcc-plugins/stackleak_plugin.c | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/scripts/gcc-plugins/stackleak_plugin.c b/scripts/gcc-plugins/stackleak_plugin.c
> index dbd37460c573..cc75eeba0be1 100644
> --- a/scripts/gcc-plugins/stackleak_plugin.c
> +++ b/scripts/gcc-plugins/stackleak_plugin.c
> @@ -51,7 +51,6 @@ static void stackleak_add_track_stack(gimple_stmt_iterator *gsi, bool after)
>  	gimple stmt;
>  	gcall *stackleak_track_stack;
>  	cgraph_node_ptr node;
> -	int frequency;
>  	basic_block bb;
>  
>  	/* Insert call to void stackleak_track_stack(void) */
> @@ -68,9 +67,9 @@ static void stackleak_add_track_stack(gimple_stmt_iterator *gsi, bool after)
>  	bb = gimple_bb(stackleak_track_stack);
>  	node = cgraph_get_create_node(track_function_decl);
>  	gcc_assert(node);
> -	frequency = compute_call_stmt_bb_frequency(current_function_decl, bb);
>  	cgraph_create_edge(cgraph_get_node(current_function_decl), node,
> -			stackleak_track_stack, bb->count, frequency);
> +			stackleak_track_stack, bb->count,
> +			compute_call_stmt_bb_frequency(current_function_decl, bb));
>  }
>  
>  static bool is_alloca(gimple stmt)
>