linux-kernel - Re: [PATCH 3/8] objtool: Rework allocating stack

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Message-ID: <20200423155425.GW20730@hirez.programming.kicks-ass.net>
Date:   Thu, 23 Apr 2020 17:54:25 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Alexandre Chartre <alexandre.chartre@...cle.com>
Cc:     jpoimboe@...hat.com, linux-kernel@...r.kernel.org,
        jthierry@...hat.com, tglx@...utronix.de, x86@...nel.org,
        mbenes@...e.cz
Subject: Re: [PATCH 3/8] objtool: Rework allocating stack_ops on decode

On Thu, Apr 23, 2020 at 05:40:38PM +0200, Alexandre Chartre wrote:

> > @@ -77,6 +77,17 @@ unsigned long arch_jump_destination(stru
> >   	return insn->offset + insn->len + insn->immediate;
> >   }
> > +#define PUSH_OP(op) \
> > +({ \
> > +	list_add_tail(&op->list, ops_list); \
> > +	NULL; \
> > +})
> > +
> > +#define ADD_OP(op) \
> > +	if (!(op = calloc(1, sizeof(*op)))) \
> > +		return -1; \
> > +	else for (; op; op = PUSH_OP(op))
> > +
> 
> I would better have a function to alloc+add op instead of weird macros,
> for example:
> 
> static struct stack_op *add_op(void)
> {
>         struct stack *op;
> 
>         op = calloc(1, sizeof(*op));
>         if (!op)
>                 return NULL;
>         list_add_tail(&op->list, ops_list);
> }
> 
> Then it requires two more lines when using it but I think the code is much
> cleaner and clearer, e.g.:
> 
>                         op = add_op();
>                         if (!op)
>                                 return -1;
>                         op->src.type = OP_SRC_ADD;
>                         op->src.reg = op_to_cfi_reg[modrm_reg][rex_r];
>                         op->dest.type = OP_DEST_REG;
>                         op->dest.reg = CFI_SP;

The 'problem' which this is that it doesn't NULL op again, so any later
use will do 'funny' things instead of crashing sensibly. Also, I'm
mightly lazy, I don't like endlessly repeating the same things.