[DTrace-devel] [PATCH] cg: add a function to scalarize pointers

[Kris Van Hees]

Withdrawing this for now...  No longer needed (for now).

On Fri, Feb 24, 2023 at 11:11:22AM -0500, Kris Van Hees via DTrace-devel wrote:
> The logic to scalarize a pointer was already present in dt_cg_typecast()
> for ALLOCA and DPTR pointers, but new use cases are emerging.  Move the
> logic to dt_cg_scalarize().
> 
> Signed-off-by: Kris Van Hees <kris.van.hees at oracle.com>
> ---
>  libdtrace/dt_cg.c | 37 ++++++++++++++++++++++++++-----------
>  1 file changed, 26 insertions(+), 11 deletions(-)
> 
> diff --git a/libdtrace/dt_cg.c b/libdtrace/dt_cg.c
> index 6cf311be..60e05642 100644
> --- a/libdtrace/dt_cg.c
> +++ b/libdtrace/dt_cg.c
> @@ -2739,6 +2739,31 @@ dt_cg_store(dt_node_t *src, dt_irlist_t *dlp, dt_regset_t *drp, dt_node_t *dst)
>  		dt_regset_free(drp, dreg);
>  }
>  
> +/*
> + * Scalarize a pointer (turn it into a regular scalar).  This is often needed
> + * to ensure that the BPF verifier will no longer treat it as a pointer and
> + * therefore enforce boundary checking.
> + *
> + * There is no way back - once a value is scalarized it cannot be returned to
> + * being a pointer that is recognized by the BPF verifier.
> + */
> +static void
> +dt_cg_scalarize(const dt_node_t *src, const dt_node_t *dst,
> +		dt_irlist_t *dlp, dt_regset_t *drp)
> +{
> +	int	mst;
> +
> +	if ((mst = dt_regset_alloc(drp)) == -1)
> +		longjmp(yypcb->pcb_jmpbuf, EDT_NOREG);
> +
> +	emit(dlp,  BPF_LOAD(BPF_DW, mst, BPF_REG_FP, DT_STK_DCTX));
> +	emit(dlp,  BPF_LOAD(BPF_DW, mst, mst, DCTX_MST));
> +	emit(dlp,  BPF_STORE(BPF_DW, mst, DMST_SCALARIZER, src->dn_reg));
> +	emit(dlp,  BPF_LOAD(BPF_DW, dst->dn_reg, mst, DMST_SCALARIZER));
> +
> +	dt_regset_free(drp, mst);
> +}
> +
>  /*
>   * Generate code for a typecast or for argument promotion from the type of the
>   * actual to the type of the formal.  We need to generate code for casts when
> @@ -2761,17 +2786,7 @@ dt_cg_typecast(const dt_node_t *src, const dt_node_t *dst,
>  
>  	if (dt_node_is_arith(dst) && dt_node_is_pointer(src) &&
>  	    (src->dn_flags & (DT_NF_ALLOCA | DT_NF_DPTR))) {
> -		int	mst;
> -
> -		if ((mst = dt_regset_alloc(drp)) == -1)
> -			longjmp(yypcb->pcb_jmpbuf, EDT_NOREG);
> -
> -		emit(dlp,  BPF_LOAD(BPF_DW, mst, BPF_REG_FP, DT_STK_DCTX));
> -		emit(dlp,  BPF_LOAD(BPF_DW, mst, mst, DCTX_MST));
> -		emit(dlp,  BPF_STORE(BPF_DW, mst, DMST_SCALARIZER, src->dn_reg));
> -		emit(dlp,  BPF_LOAD(BPF_DW, dst->dn_reg, mst, DMST_SCALARIZER));
> -
> -		dt_regset_free(drp, mst);
> +		dt_cg_scalarize(src, dst, dlp, drp);
>  	} else {
>  		int	srcsigned = src->dn_flags & DT_NF_SIGNED;
>  		int	dstsigned = dst->dn_flags & DT_NF_SIGNED;
> -- 
> 2.39.1
> 
> 
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