* [PATCH 3/4] btf: add dt_btf_func_is_traceable()
@ 2026-04-14 6:27 Kris Van Hees
2026-04-17 17:07 ` Nick Alcock
0 siblings, 1 reply; 3+ messages in thread
From: Kris Van Hees @ 2026-04-14 6:27 UTC (permalink / raw)
To: dtrace, dtrace-devel
Add a function to determine whether a function (given by BTF id) can be
traced using BPF fentry/fexit probes. It implements checks inspired by
btf_distill_func_proto() in kernel/bpf/btf.c.
Signed-off-by: Kris Van Hees <kris.van.hees@oracle.com>
---
libdtrace/dt_btf.c | 65 ++++++++++++++++++++++++++++++++++++++++++++++
libdtrace/dt_btf.h | 2 ++
2 files changed, 67 insertions(+)
diff --git a/libdtrace/dt_btf.c b/libdtrace/dt_btf.c
index 56eb96a4e..fbf377b4e 100644
--- a/libdtrace/dt_btf.c
+++ b/libdtrace/dt_btf.c
@@ -1020,3 +1020,68 @@ dt_btf_func_is_void(dtrace_hdl_t *dtp, const dt_btf_t *btf, uint32_t id)
return 0;
}
+
+/*
+ * Return 1 if the function referenced by the BTF id can be traced using the
+ * fprobe/fexit facility. Specifically, that the function is not variadic and
+ * that none of its arguments exceeds the acceptable value size (16 bytes).
+ * (See btf_distill_func_proto() in kernel/bpf/btf.c for reference.)
+ */
+#define MAX_BPF_FUNC_REG_REGS 5
+int
+dt_btf_func_is_traceable(dtrace_hdl_t *dtp, const dt_btf_t *btf, uint32_t id)
+{
+ btf_type_t *type = dt_btf_real_type_by_id(dtp, btf, id);
+ int argc;
+
+ /* If no prototype is found, BPF fprobes do not work. */
+ if (!type || BTF_INFO_KIND(type->info) != BTF_KIND_FUNC_PROTO)
+ return 0;
+
+ /* If the return type is a struct or union, BPF fprobes do not work. */
+ if (type->type != 0) {
+ btf_type_t *rtype = dt_btf_real_type_by_id(dtp, btf, type->type);
+
+ if (rtype == NULL)
+ return 0;
+
+ switch (BTF_INFO_KIND(rtype->info)) {
+ case BTF_KIND_STRUCT:
+ case BTF_KIND_UNION:
+ return 0;
+ default:
+ /* fall-through */
+ }
+ }
+
+ /*
+ * BPF fprobes do not support functions that are variadic or that have
+ * any arguments passed by value that are a struct or union of size
+ * greater than 16.
+ */
+ argc = BTF_INFO_VLEN(type->info);
+ if (argc > 0) {
+ btf_param_t *args = (btf_param_t *)(type + 1);
+ int i;
+
+ if (args[argc - 1].type == 0) /* variadic */
+ return 0;
+
+ for (i = 0; i < argc; i++) {
+ type = dt_btf_real_type_by_id(dtp, btf, args[i].type);
+ if (type == NULL)
+ return 0;
+
+ switch (BTF_INFO_KIND(type->info)) {
+ case BTF_KIND_STRUCT:
+ case BTF_KIND_UNION:
+ if (type->size > 16) /* value size > 16 */
+ return 0;
+ default:
+ /* fall-through */
+ }
+ }
+ }
+
+ return 1;
+}
diff --git a/libdtrace/dt_btf.h b/libdtrace/dt_btf.h
index d956ad940..2c921a033 100644
--- a/libdtrace/dt_btf.h
+++ b/libdtrace/dt_btf.h
@@ -28,6 +28,8 @@ extern int dt_btf_func_argc(dtrace_hdl_t *dtp, const dt_btf_t *btf,
uint32_t id);
extern int dt_btf_func_is_void(dtrace_hdl_t *dtp, const dt_btf_t *btf,
uint32_t id);
+extern int dt_btf_func_is_traceable(dtrace_hdl_t *dtp, const dt_btf_t *btf,
+ uint32_t id);
extern int dt_btf_get_module_ids(dtrace_hdl_t *);
extern int dt_btf_module_fd(const dt_module_t *);
--
2.53.0
^ permalink raw reply related [flat|nested] 3+ messages in thread* Re: [PATCH 3/4] btf: add dt_btf_func_is_traceable()
2026-04-14 6:27 [PATCH 3/4] btf: add dt_btf_func_is_traceable() Kris Van Hees
@ 2026-04-17 17:07 ` Nick Alcock
2026-04-17 19:04 ` Kris Van Hees
0 siblings, 1 reply; 3+ messages in thread
From: Nick Alcock @ 2026-04-17 17:07 UTC (permalink / raw)
To: Kris Van Hees; +Cc: dtrace, dtrace-devel
On 14 Apr 2026, Kris Van Hees spake thusly:
> Add a function to determine whether a function (given by BTF id) can be
> traced using BPF fentry/fexit probes. It implements checks inspired by
> btf_distill_func_proto() in kernel/bpf/btf.c.
... looks reasonable.
> Signed-off-by: Kris Van Hees <kris.van.hees@oracle.com>
Reviewed-by: Nick Alcock <nick.alcock@oracle.com>
(though you might possibly want to implement the suggestion below, it's
very much optional.)
> ---
> libdtrace/dt_btf.c | 65 ++++++++++++++++++++++++++++++++++++++++++++++
> libdtrace/dt_btf.h | 2 ++
> 2 files changed, 67 insertions(+)
>
> diff --git a/libdtrace/dt_btf.c b/libdtrace/dt_btf.c
> index 56eb96a4e..fbf377b4e 100644
> --- a/libdtrace/dt_btf.c
> +++ b/libdtrace/dt_btf.c
> @@ -1020,3 +1020,68 @@ dt_btf_func_is_void(dtrace_hdl_t *dtp, const dt_btf_t *btf, uint32_t id)
>
> return 0;
> }
> +
> +/*
> + * Return 1 if the function referenced by the BTF id can be traced using the
> + * fprobe/fexit facility. Specifically, that the function is not variadic and
> + * that none of its arguments exceeds the acceptable value size (16 bytes).
> + * (See btf_distill_func_proto() in kernel/bpf/btf.c for reference.)
> + */
> +#define MAX_BPF_FUNC_REG_REGS 5
A shame we need to hardwire this. More of a shame that we need to
hardwire it deep in a .c file where it'll almost certainly be overlooked
when the time comes to change it. Stick it in a suitable header, maybe?
> +int
> +dt_btf_func_is_traceable(dtrace_hdl_t *dtp, const dt_btf_t *btf, uint32_t id)
> +{
> + btf_type_t *type = dt_btf_real_type_by_id(dtp, btf, id);
> + int argc;
> +
> + /* If no prototype is found, BPF fprobes do not work. */
> + if (!type || BTF_INFO_KIND(type->info) != BTF_KIND_FUNC_PROTO)
> + return 0;
> +
> + /* If the return type is a struct or union, BPF fprobes do not work. */
> + if (type->type != 0) {
> + btf_type_t *rtype = dt_btf_real_type_by_id(dtp, btf, type->type);
> +
> + if (rtype == NULL)
> + return 0;
> +
> + switch (BTF_INFO_KIND(rtype->info)) {
> + case BTF_KIND_STRUCT:
> + case BTF_KIND_UNION:
> + return 0;
> + default:
> + /* fall-through */
> + }
> + }
> +
> + /*
> + * BPF fprobes do not support functions that are variadic or that have
> + * any arguments passed by value that are a struct or union of size
> + * greater than 16.
> + */
> + argc = BTF_INFO_VLEN(type->info);
> + if (argc > 0) {
> + btf_param_t *args = (btf_param_t *)(type + 1);
> + int i;
> +
> + if (args[argc - 1].type == 0) /* variadic */
> + return 0;
> +
> + for (i = 0; i < argc; i++) {
> + type = dt_btf_real_type_by_id(dtp, btf, args[i].type);
> + if (type == NULL)
> + return 0;
> +
> + switch (BTF_INFO_KIND(type->info)) {
> + case BTF_KIND_STRUCT:
> + case BTF_KIND_UNION:
> + if (type->size > 16) /* value size > 16 */
> + return 0;
> + default:
> + /* fall-through */
> + }
> + }
> + }
> +
> + return 1;
> +}
> diff --git a/libdtrace/dt_btf.h b/libdtrace/dt_btf.h
> index d956ad940..2c921a033 100644
> --- a/libdtrace/dt_btf.h
> +++ b/libdtrace/dt_btf.h
> @@ -28,6 +28,8 @@ extern int dt_btf_func_argc(dtrace_hdl_t *dtp, const dt_btf_t *btf,
> uint32_t id);
> extern int dt_btf_func_is_void(dtrace_hdl_t *dtp, const dt_btf_t *btf,
> uint32_t id);
> +extern int dt_btf_func_is_traceable(dtrace_hdl_t *dtp, const dt_btf_t *btf,
> + uint32_t id);
> extern int dt_btf_get_module_ids(dtrace_hdl_t *);
> extern int dt_btf_module_fd(const dt_module_t *);
--
NULL && (void)
^ permalink raw reply [flat|nested] 3+ messages in thread* Re: [PATCH 3/4] btf: add dt_btf_func_is_traceable()
2026-04-17 17:07 ` Nick Alcock
@ 2026-04-17 19:04 ` Kris Van Hees
0 siblings, 0 replies; 3+ messages in thread
From: Kris Van Hees @ 2026-04-17 19:04 UTC (permalink / raw)
To: Nick Alcock; +Cc: Kris Van Hees, dtrace, dtrace-devel
On Fri, Apr 17, 2026 at 06:07:12PM +0100, Nick Alcock wrote:
> On 14 Apr 2026, Kris Van Hees spake thusly:
>
> > Add a function to determine whether a function (given by BTF id) can be
> > traced using BPF fentry/fexit probes. It implements checks inspired by
> > btf_distill_func_proto() in kernel/bpf/btf.c.
>
> ... looks reasonable.
>
> > Signed-off-by: Kris Van Hees <kris.van.hees@oracle.com>
>
> Reviewed-by: Nick Alcock <nick.alcock@oracle.com>
>
> (though you might possibly want to implement the suggestion below, it's
> very much optional.)
>
> > ---
> > libdtrace/dt_btf.c | 65 ++++++++++++++++++++++++++++++++++++++++++++++
> > libdtrace/dt_btf.h | 2 ++
> > 2 files changed, 67 insertions(+)
> >
> > diff --git a/libdtrace/dt_btf.c b/libdtrace/dt_btf.c
> > index 56eb96a4e..fbf377b4e 100644
> > --- a/libdtrace/dt_btf.c
> > +++ b/libdtrace/dt_btf.c
> > @@ -1020,3 +1020,68 @@ dt_btf_func_is_void(dtrace_hdl_t *dtp, const dt_btf_t *btf, uint32_t id)
> >
> > return 0;
> > }
> > +
> > +/*
> > + * Return 1 if the function referenced by the BTF id can be traced using the
> > + * fprobe/fexit facility. Specifically, that the function is not variadic and
> > + * that none of its arguments exceeds the acceptable value size (16 bytes).
> > + * (See btf_distill_func_proto() in kernel/bpf/btf.c for reference.)
> > + */
> > +#define MAX_BPF_FUNC_REG_REGS 5
>
> A shame we need to hardwire this. More of a shame that we need to
> hardwire it deep in a .c file where it'll almost certainly be overlooked
> when the time comes to change it. Stick it in a suitable header, maybe?
It is an internal (non-UAPI) aspect of BPF that is only relevant here, which is
why I chose to define it here. I think it would be confusing to have it in a
more generic header file.
> > +int
> > +dt_btf_func_is_traceable(dtrace_hdl_t *dtp, const dt_btf_t *btf, uint32_t id)
> > +{
> > + btf_type_t *type = dt_btf_real_type_by_id(dtp, btf, id);
> > + int argc;
> > +
> > + /* If no prototype is found, BPF fprobes do not work. */
> > + if (!type || BTF_INFO_KIND(type->info) != BTF_KIND_FUNC_PROTO)
> > + return 0;
> > +
> > + /* If the return type is a struct or union, BPF fprobes do not work. */
> > + if (type->type != 0) {
> > + btf_type_t *rtype = dt_btf_real_type_by_id(dtp, btf, type->type);
> > +
> > + if (rtype == NULL)
> > + return 0;
> > +
> > + switch (BTF_INFO_KIND(rtype->info)) {
> > + case BTF_KIND_STRUCT:
> > + case BTF_KIND_UNION:
> > + return 0;
> > + default:
> > + /* fall-through */
> > + }
> > + }
> > +
> > + /*
> > + * BPF fprobes do not support functions that are variadic or that have
> > + * any arguments passed by value that are a struct or union of size
> > + * greater than 16.
> > + */
> > + argc = BTF_INFO_VLEN(type->info);
> > + if (argc > 0) {
> > + btf_param_t *args = (btf_param_t *)(type + 1);
> > + int i;
> > +
> > + if (args[argc - 1].type == 0) /* variadic */
> > + return 0;
> > +
> > + for (i = 0; i < argc; i++) {
> > + type = dt_btf_real_type_by_id(dtp, btf, args[i].type);
> > + if (type == NULL)
> > + return 0;
> > +
> > + switch (BTF_INFO_KIND(type->info)) {
> > + case BTF_KIND_STRUCT:
> > + case BTF_KIND_UNION:
> > + if (type->size > 16) /* value size > 16 */
> > + return 0;
> > + default:
> > + /* fall-through */
> > + }
> > + }
> > + }
> > +
> > + return 1;
> > +}
> > diff --git a/libdtrace/dt_btf.h b/libdtrace/dt_btf.h
> > index d956ad940..2c921a033 100644
> > --- a/libdtrace/dt_btf.h
> > +++ b/libdtrace/dt_btf.h
> > @@ -28,6 +28,8 @@ extern int dt_btf_func_argc(dtrace_hdl_t *dtp, const dt_btf_t *btf,
> > uint32_t id);
> > extern int dt_btf_func_is_void(dtrace_hdl_t *dtp, const dt_btf_t *btf,
> > uint32_t id);
> > +extern int dt_btf_func_is_traceable(dtrace_hdl_t *dtp, const dt_btf_t *btf,
> > + uint32_t id);
> > extern int dt_btf_get_module_ids(dtrace_hdl_t *);
> > extern int dt_btf_module_fd(const dt_module_t *);
>
> --
> NULL && (void)
^ permalink raw reply [flat|nested] 3+ messages in thread
end of thread, other threads:[~2026-04-17 19:04 UTC | newest]
Thread overview: 3+ messages (download: mbox.gz follow: Atom feed
-- links below jump to the message on this page --
2026-04-14 6:27 [PATCH 3/4] btf: add dt_btf_func_is_traceable() Kris Van Hees
2026-04-17 17:07 ` Nick Alcock
2026-04-17 19:04 ` Kris Van Hees
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox