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

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Message-ID: <202004070933.5112AA6@keescook>
Date:   Tue, 7 Apr 2020 09:33:16 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Alexander Popov <alex.popov@...ux.com>
Cc:     kernel-hardening@...ts.openwall.com,
        Emese Revfy <re.emese@...il.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] gcc-plugins/stackleak: Avoid assignment for unused macro
 argument

On Tue, Apr 07, 2020 at 06:30:18PM +0300, Alexander Popov wrote:
> 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"?

I really did. ;) Thanks, I'll adjust this.

-Kees

> 
> 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)
> > 
> 

-- 
Kees Cook