[DTrace-devel] [PATCH] Unify NULL pointer checking in dt_cg_check_notnull()

[Eugene Loh]

Reviewed-by: Eugene Loh <eugene.loh at oracle.com>

What will be the ordering of patches?  If this goes in first, I can 
tweak dt_cg_act_[u]stack() to use it.  If those patches go in first, 
this patch should include those two functions.

What about DIVZERO in dt_cg_arithmetic_op()?  How about extending the 
new function to allow for use with DIVZERO?

On 6/18/21 2:07 PM, Kris Van Hees wrote:
> We need to generate code in multiple places to validate that a pointer
> value is not NULL.  This patch introduces the dt_cg_check_notnull()
> function to do just that,  Existing checks are replaced by a call to
> this function.
>
> Signed-off-by: Kris Van Hees <kris.van.hees at oracle.com>
> ---
>   libdtrace/dt_cg.c | 40 +++++++++++++++++-----------------------
>   1 file changed, 17 insertions(+), 23 deletions(-)
>
> diff --git a/libdtrace/dt_cg.c b/libdtrace/dt_cg.c
> index 3d705a11..22f97404 100644
> --- a/libdtrace/dt_cg.c
> +++ b/libdtrace/dt_cg.c
> @@ -612,6 +612,21 @@ dt_cg_probe_error(dt_pcb_t *pcb, uint32_t off, uint32_t fault, uint64_t illval)
>   	emit(dlp,  BPF_RETURN());
>   }
>   
> +/*
> + * Generate code to validate that the value in the given register 'reg' is not
> + * the NULL pointer.
> + */
> +static void
> +dt_cg_check_notnull(dt_irlist_t *dlp, dt_regset_t *drp, int reg)
> +{
> +	uint_t	lbl_notnull = dt_irlist_label(dlp);
> +
> +	emit(dlp,  BPF_BRANCH_IMM(BPF_JNE, reg, 0, lbl_notnull));
> +	dt_cg_probe_error(yypcb, -1, DTRACEFLT_BADADDR, 0);
> +	emitl(dlp, lbl_notnull,
> +		   BPF_NOP());
> +}
> +
>   /*
>    * Check whether mst->fault indicates a fault was triggered.  If so, abort the
>    * current clause by means of a straight jump to the exit labal.
> @@ -3191,7 +3206,6 @@ dt_cg_node(dt_node_t *dnp, dt_irlist_t *dlp, dt_regset_t *drp)
>   
>   		if (!(dnp->dn_flags & DT_NF_REF)) {
>   			uint_t	ubit;
> -			uint_t	lbl_valid = dt_irlist_label(dlp);
>   
>   			/*
>   			 * Save and restore DT_NF_USERLAND across dt_cg_load():
> @@ -3202,12 +3216,7 @@ dt_cg_node(dt_node_t *dnp, dt_irlist_t *dlp, dt_regset_t *drp)
>   			dnp->dn_flags |=
>   			    (dnp->dn_child->dn_flags & DT_NF_USERLAND);
>   
> -			/* if NULL pointer, report BADARR */
> -			emit(dlp,  BPF_BRANCH_IMM(BPF_JNE, dnp->dn_reg, 0,
> -						  lbl_valid));
> -			dt_cg_probe_error(yypcb, -1, DTRACEFLT_BADADDR, 0);
> -			emitl(dlp, lbl_valid,
> -				   BPF_NOP());
> +			dt_cg_check_notnull(dlp, drp, dnp->dn_reg);
>   
>   			/* FIXME: Does not handled signed or userland */
>   			emit(dlp, BPF_LOAD(dt_cg_load(dnp, ctfp, dnp->dn_type), dnp->dn_reg, dnp->dn_reg, 0));
> @@ -3293,25 +3302,10 @@ dt_cg_node(dt_node_t *dnp, dt_irlist_t *dlp, dt_regset_t *drp)
>   
>   	case DT_TOK_PTR:
>   	case DT_TOK_DOT: {
> -		uint_t	lbl_valid = dt_irlist_label(dlp);
>   
>   		assert(dnp->dn_right->dn_kind == DT_NODE_IDENT);
>   		dt_cg_node(dnp->dn_left, dlp, drp);
> -
> -		/*
> -		 * If the lvalue is the NULL pointer, we must report a BADARR
> -		 * fault.
> -		 *
> -		 *	if (left != 0)		// jne %lreg, 0, valid
> -		 *		goto valid;
> -		 *				//     (report BADADDR fault)
> -		 * valid:
> -		 */
> -		emit(dlp,  BPF_BRANCH_IMM(BPF_JNE, dnp->dn_left->dn_reg, 0,
> -					  lbl_valid));
> -		dt_cg_probe_error(yypcb, -1, DTRACEFLT_BADADDR, 0);
> -		emitl(dlp, lbl_valid,
> -			   BPF_NOP());
> +		dt_cg_check_notnull(dlp, drp, dnp->dn_left->dn_reg);
>   
>   		/*
>   		 * If the left-hand side of PTR or DOT is a dynamic variable,