[DTrace-devel] [PATCH 2/4] Store strings in the trace output buffer without length prefixes

[Eugene Loh]

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

On 1/26/22 11:28 AM, Kris Van Hees via DTrace-devel wrote:
> diff --git a/libdtrace/dt_cg.c b/libdtrace/dt_cg.c
> @@ -932,34 +932,29 @@ dt_cg_store_val(dt_pcb_t *pcb, dt_node_t *dnp, dtrace_actkind_t kind,
>   	} else if (dt_node_is_string(dnp)) {
> +		size_t	strsize = pcb->pcb_hdl->dt_options[DTRACEOPT_STRSIZE];
> +

Isn't strsize==size?  That is, no need to define strsize... just use 
size instead?

>   		dt_cg_check_notnull(dlp, drp, dnp->dn_reg);
>   
>   		TRACE_REGSET("store_val(): Begin ");
> -		off = dt_rec_add(pcb->pcb_hdl, dt_cg_fill_gap, kind,
> -				 size + DT_STRLEN_BYTES + 1, 1, pfp, arg);
> -
> -		/*
> -		 * Copy the length of the string from the source string as a
> -		 * half-word (2 bytes) into the buffer at [%r9 + off].
> -		 */
> -		emit(dlp, BPF_LOAD(BPF_H, BPF_REG_0, dnp->dn_reg, 0));
> -		emit(dlp, BPF_STORE(BPF_H, BPF_REG_9, off, BPF_REG_0));
> +		off = dt_rec_add(pcb->pcb_hdl, dt_cg_fill_gap, kind, size + 1,
> +				 1, pfp, arg);
>   
>   		/*
>   		 * Copy the string data (no more than STRSIZE + 1 bytes) to the
> -		 * buffer at [%r9 + off + DT_STRLEN_BYTES].  We (ab)use the
> -		 * fact that probe_read_str) stops at the terminating NUL byte.
> +		 * buffer at (%r9 + off).  We depend on the fact that
> +		 * probe_read_str() stops at the terminating NUL byte.
>   		 */
>   		if (dt_regset_xalloc_args(drp) == -1)
>   			longjmp(yypcb->pcb_jmpbuf, EDT_NOREG);
>   
>   		emit(dlp, BPF_MOV_REG(BPF_REG_1, BPF_REG_9));
> -		emit(dlp, BPF_ALU64_IMM(BPF_ADD, BPF_REG_1, off + DT_STRLEN_BYTES));
> -		emit(dlp, BPF_MOV_IMM(BPF_REG_2, size + 1));
> +		emit(dlp, BPF_ALU64_IMM(BPF_ADD, BPF_REG_1, off));
> +		emit(dlp, BPF_MOV_IMM(BPF_REG_2, strsize + 1));
>   		emit(dlp, BPF_MOV_REG(BPF_REG_3, dnp->dn_reg));
> +		emit(dlp, BPF_ALU64_IMM(BPF_ADD, BPF_REG_3, DT_STRLEN_BYTES));
>   		dt_regset_free(drp, dnp->dn_reg);
>   		dt_cg_tstring_free(pcb, dnp);
> -		emit(dlp, BPF_ALU64_IMM(BPF_ADD, BPF_REG_3, DT_STRLEN_BYTES));
>   		dt_regset_xalloc(drp, BPF_REG_0);
>   		emit(dlp, BPF_CALL_HELPER(BPF_FUNC_probe_read_str));
>   		dt_regset_free_args(drp);