[DTrace-devel] [PATCH v2] Make DCTX_RODATA available to BPF functions

[Kris Van Hees]

On Thu, Dec 21, 2023 at 04:37:48PM -0500, eugene.loh at oracle.com wrote:
> 
> Use DCTX_RODATA in inet_ntoa.S.
> 
> Signed-off-by: Eugene Loh <eugene.loh at oracle.com>

Reviewed-by: Kris Van Hees <kris.van.hees at oracle.com>

... though I wish we could just use ldxdw %r6, [%r6 + DCTX_RODATA] and have
    a reloc for DCTX_RODATA so that the actual value can be put in during
    linking.  Oh well.

> ---
>  bpf/inet_ntoa6.S     | 12 +++---------
>  libdtrace/dt_bpf.h   |  9 +++++----
>  libdtrace/dt_cc.c    |  3 +++
>  libdtrace/dt_dlibs.c |  1 +
>  4 files changed, 12 insertions(+), 13 deletions(-)
> 
> diff --git a/bpf/inet_ntoa6.S b/bpf/inet_ntoa6.S
> index 24b7232d..d00acbfe 100644
> --- a/bpf/inet_ntoa6.S
> +++ b/bpf/inet_ntoa6.S
> @@ -5,14 +5,6 @@
>  
>  #define BPF_FUNC_probe_read		4
>  
> -/*
> - * We sadly cannot include struct declarations in assembler input files, so we
> - * cannot use offsetof() to programmatically determine the offset of the rodata
> - * member in the DTrace context (dt_ctx_t).  If that structure changes, the
> - * following define must be updated as well.
> - */
> -#define DCTX_RODATA			56
> -
>  #define OUTPUT_LEN			40
>  #define INPUT_LEN			16
>  
> @@ -225,7 +217,9 @@ dt_inet_ntoa6:
>  	 * have the code below output the address in two parts: a 7 word
>  	 * prefix followed by a 1 word suffix.
>  	 */
> -	ldxdw	%r6, [%r6 + DCTX_RODATA]
> +	lddw	%r1, DCTX_RODATA
> +	add	%r1, %r6
> +	ldxdw	%r6, [%r1 + 0]
>  	ldxw	%r1, [%fp + -4]		/* restore tbloff */
>  	lddw	%r0, RODATA_SIZE
>  	jge	%r1, %r0, .Ldone
> diff --git a/libdtrace/dt_bpf.h b/libdtrace/dt_bpf.h
> index bd87869e..0d96fd43 100644
> --- a/libdtrace/dt_bpf.h
> +++ b/libdtrace/dt_bpf.h
> @@ -37,10 +37,11 @@ extern "C" {
>  #define DT_CONST_TASK_COMM		16
>  #define DT_CONST_MUTEX_OWNER		17
>  #define DT_CONST_RWLOCK_CNTS		18
> -#define DT_CONST_RODATA_OFF		19
> -#define DT_CONST_RODATA_SIZE		20
> -#define DT_CONST_ZERO_OFF		21
> -#define DT_CONST_STACK_OFF		22
> +#define DT_CONST_DCTX_RODATA		19
> +#define DT_CONST_RODATA_OFF		20
> +#define DT_CONST_RODATA_SIZE		21
> +#define DT_CONST_ZERO_OFF		22
> +#define DT_CONST_STACK_OFF		23
>  
>  #define DT_BPF_LOG_SIZE_DEFAULT	(UINT32_MAX >> 8)
>  #define DT_BPF_LOG_SIZE_SMALL	4096
> diff --git a/libdtrace/dt_cc.c b/libdtrace/dt_cc.c
> index b90c42d8..f5632cba 100644
> --- a/libdtrace/dt_cc.c
> +++ b/libdtrace/dt_cc.c
> @@ -1164,6 +1164,9 @@ dt_link_construct(dtrace_hdl_t *dtp, const dt_probe_t *prp, dtrace_difo_t *dp,
>  				nrp->dofr_data = total_offset;
>  				continue;
>  			}
> +			case DT_CONST_DCTX_RODATA:
> +				nrp->dofr_data = DCTX_RODATA;
> +				continue;
>  			case DT_CONST_RODATA_OFF:
>  				nrp->dofr_data = dtp->dt_rooffset;
>  				continue;
> diff --git a/libdtrace/dt_dlibs.c b/libdtrace/dt_dlibs.c
> index 54d96b89..5d82f30c 100644
> --- a/libdtrace/dt_dlibs.c
> +++ b/libdtrace/dt_dlibs.c
> @@ -91,6 +91,7 @@ static const dt_ident_t		dt_bpf_symbols[] = {
>  	DT_BPF_SYMBOL_ID(TASK_COMM, DT_IDENT_SCALAR, DT_CONST_TASK_COMM),
>  	DT_BPF_SYMBOL_ID(MUTEX_OWNER, DT_IDENT_SCALAR, DT_CONST_MUTEX_OWNER),
>  	DT_BPF_SYMBOL_ID(RWLOCK_CNTS, DT_IDENT_SCALAR, DT_CONST_RWLOCK_CNTS),
> +	DT_BPF_SYMBOL_ID(DCTX_RODATA, DT_IDENT_SCALAR, DT_CONST_DCTX_RODATA),
>  	DT_BPF_SYMBOL_ID(RODATA_OFF, DT_IDENT_SCALAR, DT_CONST_RODATA_OFF),
>  	DT_BPF_SYMBOL_ID(RODATA_SIZE, DT_IDENT_SCALAR, DT_CONST_RODATA_SIZE),
>  	DT_BPF_SYMBOL_ID(ZERO_OFF, DT_IDENT_SCALAR, DT_CONST_ZERO_OFF),
> -- 
> 2.18.4
> 
>