RE: [PATCH 2/2] perf probe: Support probing at absolute address

[åæéå / HIRAMATUïMASAMI]

> From: Wang Nan [mailto:wangnan0@xxxxxxxxxx]
> 
> It should be useful to allow 'perf probe' probe at absolute offset of
> a target. For example, when (u)probing at a instruction of a shared
> object in a embedded system where debuginfo is not avaliable but we
> know the offset of that instruction by manually digging.
> 
> This patch enables following perf probe command syntax:
> 
>  # perf probe +0xffffffff811e6615
> 
> And
> 
>  # perf probe /lib/x86_64-linux-gnu/libc-2.19.so +0xeb860

Why do we need "+" for the absolute address?
It seems that we can do it if we find that the given probe point
starts with "0x".

Thanks,

> 
> In the above example, we don't need a anchor symbol, so it is possible
> to compute absolute addresses using other methods and then use
> 'perf probe' to create the probing points.
> 
> Signed-off-by: Wang Nan <wangnan0@xxxxxxxxxx>
> Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Masami Hiramatsu <masami.hiramatsu.pt@xxxxxxxxxxx>
> Cc: Namhyung Kim <namhyung@xxxxxxxxxx>
> ---
>  tools/perf/util/probe-event.c  | 144 +++++++++++++++++++++++++++++++++++++----
>  tools/perf/util/probe-event.h  |   3 +
>  tools/perf/util/probe-finder.c |  21 +-----
>  3 files changed, 138 insertions(+), 30 deletions(-)
> 
> diff --git a/tools/perf/util/probe-event.c b/tools/perf/util/probe-event.c
> index 6c7e538..59de69a4 100644
> --- a/tools/perf/util/probe-event.c
> +++ b/tools/perf/util/probe-event.c
> @@ -1194,9 +1194,13 @@ static int parse_perf_probe_point(char *arg, struct perf_probe_event *pev)
>  		*ptr++ = '\0';
>  	}
> 
> -	tmp = strdup(arg);
> -	if (tmp == NULL)
> -		return -ENOMEM;
> +	if (arg[0] == '\0')
> +		tmp = NULL;
> +	else {
> +		tmp = strdup(arg);
> +		if (tmp == NULL)
> +			return -ENOMEM;
> +	}
> 
>  	if (file_spec)
>  		pp->file = tmp;
> @@ -1283,11 +1287,6 @@ static int parse_perf_probe_point(char *arg, struct perf_probe_event *pev)
>  		return -EINVAL;
>  	}
> 
> -	if (pp->offset && !pp->function) {
> -		semantic_error("Offset requires an entry function.\n");
> -		return -EINVAL;
> -	}
> -
>  	if (pp->retprobe && !pp->function) {
>  		semantic_error("Return probe requires an entry function.\n");
>  		return -EINVAL;
> @@ -1299,6 +1298,11 @@ static int parse_perf_probe_point(char *arg, struct perf_probe_event *pev)
>  		return -EINVAL;
>  	}
> 
> +	if (!pp->function && !pp->offset && !pp->file) {
> +		semantic_error("Absolute address should not be zero.\n");
> +		return -EINVAL;
> +	}
> +
>  	pr_debug("symbol:%s file:%s line:%d offset:%lu return:%d lazy:%s\n",
>  		 pp->function, pp->file, pp->line, pp->offset, pp->retprobe,
>  		 pp->lazy_line);
> @@ -1609,7 +1613,7 @@ error:
>  static char *synthesize_perf_probe_point(struct perf_probe_point *pp)
>  {
>  	char *buf, *tmp;
> -	char offs[32] = "", line[32] = "", file[32] = "";
> +	char offs[32] = "", line[32] = "", file[32] = "", addr[32] = "";
>  	int ret, len;
> 
>  	buf = zalloc(MAX_CMDLEN);
> @@ -1622,6 +1626,11 @@ static char *synthesize_perf_probe_point(struct perf_probe_point *pp)
>  		if (ret <= 0)
>  			goto error;
>  	}
> +	if (!pp->function) {
> +		ret = e_snprintf(addr, 32, "0x%lx", pp->offset);
> +		if (ret <= 0)
> +			goto error;
> +	}
>  	if (pp->line) {
>  		ret = e_snprintf(line, 32, ":%d", pp->line);
>  		if (ret <= 0)
> @@ -1639,9 +1648,11 @@ static char *synthesize_perf_probe_point(struct perf_probe_point *pp)
>  			goto error;
>  	}
> 
> -	if (pp->function)
> -		ret = e_snprintf(buf, MAX_CMDLEN, "%s%s%s%s%s", pp->function,
> -				 offs, pp->retprobe ? "%return" : "", line,
> +	if (pp->function || pp->offset)
> +		ret = e_snprintf(buf, MAX_CMDLEN, "%s%s%s%s%s",
> +				 pp->function ? : addr,
> +				 pp->function ? offs : "",
> +				 pp->retprobe ? "%return" : "", line,
>  				 file);
>  	else
>  		ret = e_snprintf(buf, MAX_CMDLEN, "%s%s", file, line);
> @@ -1786,6 +1797,11 @@ char *synthesize_probe_trace_command(struct probe_trace_event *tev)
>  	if (tev->uprobes)
>  		ret = e_snprintf(buf + len, MAX_CMDLEN - len, "%s:0x%lx",
>  				 tp->module, tp->address);
> +	else if (tp->symbol[0] == '0' && tp->symbol[1] == 'x')
> +		/* Absolute address. See try_to_find_absolute_address() */
> +		ret = e_snprintf(buf + len, MAX_CMDLEN - len, "%s%s0x%lx",
> +				 tp->module ?: "", tp->module ? ":" : "",
> +				 tp->address);
>  	else
>  		ret = e_snprintf(buf + len, MAX_CMDLEN - len, "%s%s%s+%lu",
>  				 tp->module ?: "", tp->module ? ":" : "",
> @@ -2572,6 +2588,87 @@ err_out:
>  	goto out;
>  }
> 
> +static int try_to_find_absolute_address(struct perf_probe_event *pev,
> +					struct probe_trace_event **tevs)
> +{
> +	struct perf_probe_point *pp = &pev->point;
> +	struct probe_trace_event *tev;
> +	struct probe_trace_point *tp;
> +	int i, err;
> +
> +	if (perf_probe_event_need_dwarf(pev) || pev->point.function)
> +		return -EINVAL;
> +
> +	/*
> +	 * This is 'perf probe /lib/libc.so +0xabcd'. Try to probe at
> +	 * absolute address.
> +	 *
> +	 * Only one tev can be generated by this.
> +	 */
> +	*tevs = zalloc(sizeof(*tev));
> +	if (!*tevs)
> +		return -ENOMEM;
> +
> +	tev = *tevs;
> +	tp = &tev->point;
> +
> +	/*
> +	 * Don't use tp->offset, use address directly, because
> +	 * in synthesize_probe_trace_command() address cannot be
> +	 * zero.
> +	 */
> +	tp->address = pev->point.offset;
> +	tp->retprobe = pp->retprobe;
> +	tev->uprobes = pev->uprobes;
> +
> +	err = -ENOMEM;
> +	/* Give it a '0x' leading symbol name */
> +	if (asprintf(&tp->symbol, "0x%lx", tp->address) < 0)
> +		goto errout;
> +
> +	/* For kprobe, check range */
> +	if ((!tev->uprobes) &&
> +	    (kprobe_warn_out_range(tev->point.symbol,
> +				   tev->point.address))) {
> +		err = -EACCES;
> +		goto errout;
> +	}
> +
> +	if (asprintf(&tp->realname, "abs_%lx", tp->address) < 0)
> +		goto errout;
> +
> +	if (pev->target) {
> +		tp->module = strdup(pev->target);
> +		if (!tp->module)
> +			goto errout;
> +	}
> +
> +	if (tev->group) {
> +		tev->group = strdup(pev->group);
> +		if (!tev->group)
> +			goto errout;
> +	}
> +
> +	if (pev->event) {
> +		tev->event = strdup(pev->event);
> +		if (!tev->event)
> +			goto errout;
> +	}
> +
> +	tev->nargs = pev->nargs;
> +	for (i = 0; i < tev->nargs; i++)
> +		copy_to_probe_trace_arg(&tev->args[i], &pev->args[i]);
> +
> +	return 1;
> +
> +errout:
> +	if (*tevs) {
> +		clear_probe_trace_events(*tevs, 1);
> +		*tevs = NULL;
> +	}
> +	return err;
> +}
> +
>  bool __weak arch__prefers_symtab(void) { return false; }
> 
>  static int convert_to_probe_trace_events(struct perf_probe_event *pev,
> @@ -2588,6 +2685,10 @@ static int convert_to_probe_trace_events(struct perf_probe_event *pev,
>  		}
>  	}
> 
> +	ret = try_to_find_absolute_address(pev, tevs);
> +	if (ret > 0)
> +		return ret;
> +
>  	if (arch__prefers_symtab() && !perf_probe_event_need_dwarf(pev)) {
>  		ret = find_probe_trace_events_from_map(pev, tevs);
>  		if (ret > 0)
> @@ -2758,3 +2859,22 @@ end:
>  	return ret;
>  }
> 
> +int copy_to_probe_trace_arg(struct probe_trace_arg *tvar,
> +			    struct perf_probe_arg *pvar)
> +{
> +	tvar->value = strdup(pvar->var);
> +	if (tvar->value == NULL)
> +		return -ENOMEM;
> +	if (pvar->type) {
> +		tvar->type = strdup(pvar->type);
> +		if (tvar->type == NULL)
> +			return -ENOMEM;
> +	}
> +	if (pvar->name) {
> +		tvar->name = strdup(pvar->name);
> +		if (tvar->name == NULL)
> +			return -ENOMEM;
> +	} else
> +		tvar->name = NULL;
> +	return 0;
> +}
> diff --git a/tools/perf/util/probe-event.h b/tools/perf/util/probe-event.h
> index 83ee95e..174a3cf 100644
> --- a/tools/perf/util/probe-event.h
> +++ b/tools/perf/util/probe-event.h
> @@ -156,4 +156,7 @@ int e_snprintf(char *str, size_t size, const char *format, ...)
>  /* Maximum index number of event-name postfix */
>  #define MAX_EVENT_INDEX	1024
> 
> +int copy_to_probe_trace_arg(struct probe_trace_arg *tvar,
> +			    struct perf_probe_arg *pvar);
> +
>  #endif /*_PROBE_EVENT_H */
> diff --git a/tools/perf/util/probe-finder.c b/tools/perf/util/probe-finder.c
> index 7b80f8c..29c43c068 100644
> --- a/tools/perf/util/probe-finder.c
> +++ b/tools/perf/util/probe-finder.c
> @@ -553,24 +553,9 @@ static int find_variable(Dwarf_Die *sc_die, struct probe_finder *pf)
>  	char buf[32], *ptr;
>  	int ret = 0;
> 
> -	if (!is_c_varname(pf->pvar->var)) {
> -		/* Copy raw parameters */
> -		pf->tvar->value = strdup(pf->pvar->var);
> -		if (pf->tvar->value == NULL)
> -			return -ENOMEM;
> -		if (pf->pvar->type) {
> -			pf->tvar->type = strdup(pf->pvar->type);
> -			if (pf->tvar->type == NULL)
> -				return -ENOMEM;
> -		}
> -		if (pf->pvar->name) {
> -			pf->tvar->name = strdup(pf->pvar->name);
> -			if (pf->tvar->name == NULL)
> -				return -ENOMEM;
> -		} else
> -			pf->tvar->name = NULL;
> -		return 0;
> -	}
> +	/* Copy raw parameters */
> +	if (!is_c_varname(pf->pvar->var))
> +		return copy_to_probe_trace_arg(pf->tvar, pf->pvar);
> 
>  	if (pf->pvar->name)
>  		pf->tvar->name = strdup(pf->pvar->name);
> --
> 1.8.3.4