* [PATCH 4/4] fbt: do not provide untraceable functins (for fprobes)
@ 2026-04-14 6:27 Kris Van Hees
2026-04-17 17:11 ` 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
Ensure that we do not provide FBT probes for functions that cannot be
probed using BPF fentry/fexit probes if fprobes are the default.
Signed-off-by: Kris Van Hees <kris.van.hees@oracle.com>
---
libdtrace/dt_prov_fbt.c | 47 ++++++++++++++++++++++++++---------------
1 file changed, 30 insertions(+), 17 deletions(-)
diff --git a/libdtrace/dt_prov_fbt.c b/libdtrace/dt_prov_fbt.c
index 59e4583b2..a2f20c9bc 100644
--- a/libdtrace/dt_prov_fbt.c
+++ b/libdtrace/dt_prov_fbt.c
@@ -86,14 +86,30 @@ static int populate(dtrace_hdl_t *dtp)
}
/* Create a probe (if it does not exist yet). */
-static int provide_probe(dtrace_hdl_t *dtp, const dtrace_probedesc_t *pdp)
+static int provide_probe(dtrace_hdl_t *dtp, dt_module_t *dmp,
+ dtrace_probedesc_t *pdp)
{
dt_provider_t *prv = dt_provider_lookup(dtp, pdp->prv);
if (prv == NULL)
return 0;
+ pdp->mod = dmp->dm_name;
if (dt_probe_lookup(dtp, pdp) != NULL)
return 0;
+
+ /*
+ * The BPF verifier does not allow fentry/fexit probing of variadic
+ * functions.
+ */
+ if (prv->impl->prog_type == BPF_PROG_TYPE_TRACING) {
+ int32_t btf_id;
+
+ btf_id = dt_btf_lookup_name_kind(dtp, dmp, pdp->fun, BTF_KIND_FUNC);
+ if (btf_id <= 0 ||
+ !dt_btf_func_is_traceable(dtp, dmp->dm_btf, btf_id))
+ return -1;
+ }
+
if (dt_tp_probe_insert(dtp, prv, pdp->prv, pdp->mod, pdp->fun, pdp->prb))
return 1;
@@ -173,16 +189,15 @@ static int provide(dtrace_hdl_t *dtp, const dtrace_probedesc_t *pdp)
pd.id = DTRACE_IDNONE;
pd.prv = pdp->prv;
- pd.mod = dmp->dm_name;
pd.fun = pdp->fun;
if (prb & FBT_ENTRY) {
pd.prb = "entry";
- n += provide_probe(dtp, &pd);
+ n += provide_probe(dtp, dmp, &pd);
}
if (prb & FBT_RETURN) {
pd.prb = "return";
- n += provide_probe(dtp, &pd);
+ n += provide_probe(dtp, dmp, &pd);
}
return n;
@@ -190,22 +205,20 @@ static int provide(dtrace_hdl_t *dtp, const dtrace_probedesc_t *pdp)
sym = dt_symbol_by_name(dtp, pdp->fun);
while (sym != NULL) {
- const char *mod = dt_symbol_module(sym)->dm_name;
-
+ dmp = dt_symbol_module(sym);
if (dt_symbol_traceable(sym) &&
- dt_gmatch(mod, pdp->mod)) {
+ dt_gmatch(dmp->dm_name, pdp->mod)) {
pd.id = DTRACE_IDNONE;
pd.prv = pdp->prv;
- pd.mod = mod;
pd.fun = pdp->fun;
if (prb & FBT_ENTRY) {
pd.prb = "entry";
- n += provide_probe(dtp, &pd);
+ n += provide_probe(dtp, dmp, &pd);
}
if (prb & FBT_RETURN) {
pd.prb = "return";
- n += provide_probe(dtp, &pd);
+ n += provide_probe(dtp, dmp, &pd);
}
}
@@ -242,16 +255,15 @@ static int provide(dtrace_hdl_t *dtp, const dtrace_probedesc_t *pdp)
pd.id = DTRACE_IDNONE;
pd.prv = pdp->prv;
- pd.mod = smp->dm_name;
pd.fun = fun;
if (prb & FBT_ENTRY) {
pd.prb = "entry";
- n += provide_probe(dtp, &pd);
+ n += provide_probe(dtp, smp, &pd);
}
if (prb & FBT_RETURN) {
pd.prb = "return";
- n += provide_probe(dtp, &pd);
+ n += provide_probe(dtp, smp, &pd);
}
}
@@ -323,10 +335,11 @@ static int fprobe_trampoline(dt_pcb_t *pcb, uint_t exitlbl)
*/
dmp = dt_module_lookup_by_name(dtp, prp->desc->mod);
if (dmp && prp->argc == 2) {
- int32_t btf_id = dt_tp_probe_get_id(prp);
- int i = dt_btf_func_argc(dtp, dmp->dm_btf, btf_id);
-
- emit(dlp, BPF_LOAD(BPF_DW, BPF_REG_0, BPF_REG_8, i * 8));
+ emit(dlp, BPF_MOV_REG(BPF_REG_1, BPF_REG_8));
+ emit(dlp, BPF_MOV_REG(BPF_REG_2, BPF_REG_FP));
+ emit(dlp, BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, DT_TRAMP_SP_SLOT(0)));
+ emit(dlp, BPF_CALL_HELPER(dtp->dt_bpfhelper[BPF_FUNC_get_func_ret]));
+ emit(dlp, BPF_LOAD(BPF_DW, BPF_REG_0, BPF_REG_FP, DT_TRAMP_SP_SLOT(0)));
emit(dlp, BPF_STORE(BPF_DW, BPF_REG_7, DMST_ARG(1), BPF_REG_0));
}
}
--
2.53.0
^ permalink raw reply related [flat|nested] 3+ messages in thread* Re: [PATCH 4/4] fbt: do not provide untraceable functins (for fprobes)
2026-04-14 6:27 [PATCH 4/4] fbt: do not provide untraceable functins (for fprobes) Kris Van Hees
@ 2026-04-17 17:11 ` Nick Alcock
2026-04-17 19:28 ` Kris Van Hees
0 siblings, 1 reply; 3+ messages in thread
From: Nick Alcock @ 2026-04-17 17:11 UTC (permalink / raw)
To: Kris Van Hees; +Cc: dtrace, dtrace-devel
On 14 Apr 2026, Kris Van Hees uttered the following:
> Subject: Re: [PATCH 4/4] fbt: do not provide untraceable functins (for fprobes)
Typo.
> Ensure that we do not provide FBT probes for functions that cannot be
> probed using BPF fentry/fexit probes if fprobes are the default.
I guess falling back to kprobes for those single functions alone is
either too complicated or has unpleasantly visible side-effects?
> Signed-off-by: Kris Van Hees <kris.van.hees@oracle.com>
> ---
> libdtrace/dt_prov_fbt.c | 47 ++++++++++++++++++++++++++---------------
> 1 file changed, 30 insertions(+), 17 deletions(-)
>
> diff --git a/libdtrace/dt_prov_fbt.c b/libdtrace/dt_prov_fbt.c
> index 59e4583b2..a2f20c9bc 100644
> --- a/libdtrace/dt_prov_fbt.c
> +++ b/libdtrace/dt_prov_fbt.c
> @@ -86,14 +86,30 @@ static int populate(dtrace_hdl_t *dtp)
> }
>
> /* Create a probe (if it does not exist yet). */
> -static int provide_probe(dtrace_hdl_t *dtp, const dtrace_probedesc_t *pdp)
> +static int provide_probe(dtrace_hdl_t *dtp, dt_module_t *dmp,
> + dtrace_probedesc_t *pdp)
> {
> dt_provider_t *prv = dt_provider_lookup(dtp, pdp->prv);
>
> if (prv == NULL)
> return 0;
> + pdp->mod = dmp->dm_name;
> if (dt_probe_lookup(dtp, pdp) != NULL)
> return 0;
> +
> + /*
> + * The BPF verifier does not allow fentry/fexit probing of variadic
> + * functions.
This comment is wrong now that this is being implemented via another
function: the fact that dt_btf_func_is_traceable() checks variadicity
belongs above that function (and is indeed there). Something like
/*
* Do not provide probes for functions with BTF that indicates that they
* are not traceable.
*/
maybe?
> + */
> + if (prv->impl->prog_type == BPF_PROG_TYPE_TRACING) {
> + int32_t btf_id;
> +
> + btf_id = dt_btf_lookup_name_kind(dtp, dmp, pdp->fun, BTF_KIND_FUNC);
> + if (btf_id <= 0 ||
> + !dt_btf_func_is_traceable(dtp, dmp->dm_btf, btf_id))
> + return -1;
> + }
> +
> if (dt_tp_probe_insert(dtp, prv, pdp->prv, pdp->mod, pdp->fun, pdp->prb))
> return 1;
>
The hunks below appear to be doing something not called out in the
commit log. I think this is getting the symbol module right?
> @@ -173,16 +189,15 @@ static int provide(dtrace_hdl_t *dtp, const dtrace_probedesc_t *pdp)
>
> pd.id = DTRACE_IDNONE;
> pd.prv = pdp->prv;
> - pd.mod = dmp->dm_name;
> pd.fun = pdp->fun;
>
> if (prb & FBT_ENTRY) {
> pd.prb = "entry";
> - n += provide_probe(dtp, &pd);
> + n += provide_probe(dtp, dmp, &pd);
> }
> if (prb & FBT_RETURN) {
> pd.prb = "return";
> - n += provide_probe(dtp, &pd);
> + n += provide_probe(dtp, dmp, &pd);
> }
>
> return n;
> @@ -190,22 +205,20 @@ static int provide(dtrace_hdl_t *dtp, const dtrace_probedesc_t *pdp)
>
> sym = dt_symbol_by_name(dtp, pdp->fun);
> while (sym != NULL) {
> - const char *mod = dt_symbol_module(sym)->dm_name;
> -
> + dmp = dt_symbol_module(sym);
> if (dt_symbol_traceable(sym) &&
> - dt_gmatch(mod, pdp->mod)) {
> + dt_gmatch(dmp->dm_name, pdp->mod)) {
> pd.id = DTRACE_IDNONE;
> pd.prv = pdp->prv;
> - pd.mod = mod;
> pd.fun = pdp->fun;
>
> if (prb & FBT_ENTRY) {
> pd.prb = "entry";
> - n += provide_probe(dtp, &pd);
> + n += provide_probe(dtp, dmp, &pd);
> }
> if (prb & FBT_RETURN) {
> pd.prb = "return";
> - n += provide_probe(dtp, &pd);
> + n += provide_probe(dtp, dmp, &pd);
> }
>
> }
> @@ -242,16 +255,15 @@ static int provide(dtrace_hdl_t *dtp, const dtrace_probedesc_t *pdp)
>
> pd.id = DTRACE_IDNONE;
> pd.prv = pdp->prv;
> - pd.mod = smp->dm_name;
> pd.fun = fun;
>
> if (prb & FBT_ENTRY) {
> pd.prb = "entry";
> - n += provide_probe(dtp, &pd);
> + n += provide_probe(dtp, smp, &pd);
> }
> if (prb & FBT_RETURN) {
> pd.prb = "return";
> - n += provide_probe(dtp, &pd);
> + n += provide_probe(dtp, smp, &pd);
> }
> }
>
The need for this trampoline change is also rather opaque.
> @@ -323,10 +335,11 @@ static int fprobe_trampoline(dt_pcb_t *pcb, uint_t exitlbl)
> */
> dmp = dt_module_lookup_by_name(dtp, prp->desc->mod);
> if (dmp && prp->argc == 2) {
> - int32_t btf_id = dt_tp_probe_get_id(prp);
> - int i = dt_btf_func_argc(dtp, dmp->dm_btf, btf_id);
> -
> - emit(dlp, BPF_LOAD(BPF_DW, BPF_REG_0, BPF_REG_8, i * 8));
> + emit(dlp, BPF_MOV_REG(BPF_REG_1, BPF_REG_8));
> + emit(dlp, BPF_MOV_REG(BPF_REG_2, BPF_REG_FP));
> + emit(dlp, BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, DT_TRAMP_SP_SLOT(0)));
> + emit(dlp, BPF_CALL_HELPER(dtp->dt_bpfhelper[BPF_FUNC_get_func_ret]));
> + emit(dlp, BPF_LOAD(BPF_DW, BPF_REG_0, BPF_REG_FP, DT_TRAMP_SP_SLOT(0)));
> emit(dlp, BPF_STORE(BPF_DW, BPF_REG_7, DMST_ARG(1), BPF_REG_0));
> }
> }
--
NULL && (void)
^ permalink raw reply [flat|nested] 3+ messages in thread* Re: [PATCH 4/4] fbt: do not provide untraceable functins (for fprobes)
2026-04-17 17:11 ` Nick Alcock
@ 2026-04-17 19:28 ` Kris Van Hees
0 siblings, 0 replies; 3+ messages in thread
From: Kris Van Hees @ 2026-04-17 19:28 UTC (permalink / raw)
To: Nick Alcock; +Cc: Kris Van Hees, dtrace, dtrace-devel
On Fri, Apr 17, 2026 at 06:11:48PM +0100, Nick Alcock wrote:
> On 14 Apr 2026, Kris Van Hees uttered the following:
>
> > Subject: Re: [PATCH 4/4] fbt: do not provide untraceable functins (for fprobes)
>
> Typo.
Thanks.
> > Ensure that we do not provide FBT probes for functions that cannot be
> > probed using BPF fentry/fexit probes if fprobes are the default.
>
> I guess falling back to kprobes for those single functions alone is
> either too complicated or has unpleasantly visible side-effects?
Someday, perhaps. But right now the infrastructure cannot handle that. The
implementation of probes for a provider is at the level of the provider and not
at the level of the probe.
> > Signed-off-by: Kris Van Hees <kris.van.hees@oracle.com>
> > ---
> > libdtrace/dt_prov_fbt.c | 47 ++++++++++++++++++++++++++---------------
> > 1 file changed, 30 insertions(+), 17 deletions(-)
> >
> > diff --git a/libdtrace/dt_prov_fbt.c b/libdtrace/dt_prov_fbt.c
> > index 59e4583b2..a2f20c9bc 100644
> > --- a/libdtrace/dt_prov_fbt.c
> > +++ b/libdtrace/dt_prov_fbt.c
> > @@ -86,14 +86,30 @@ static int populate(dtrace_hdl_t *dtp)
> > }
> >
> > /* Create a probe (if it does not exist yet). */
> > -static int provide_probe(dtrace_hdl_t *dtp, const dtrace_probedesc_t *pdp)
> > +static int provide_probe(dtrace_hdl_t *dtp, dt_module_t *dmp,
> > + dtrace_probedesc_t *pdp)
> > {
> > dt_provider_t *prv = dt_provider_lookup(dtp, pdp->prv);
> >
> > if (prv == NULL)
> > return 0;
> > + pdp->mod = dmp->dm_name;
> > if (dt_probe_lookup(dtp, pdp) != NULL)
> > return 0;
> > +
> > + /*
> > + * The BPF verifier does not allow fentry/fexit probing of variadic
> > + * functions.
>
> This comment is wrong now that this is being implemented via another
> function: the fact that dt_btf_func_is_traceable() checks variadicity
> belongs above that function (and is indeed there). Something like
Oops, forgot to update that.
> /*
> * Do not provide probes for functions with BTF that indicates that they
> * are not traceable.
> */
>
> maybe?
>
> > + */
> > + if (prv->impl->prog_type == BPF_PROG_TYPE_TRACING) {
> > + int32_t btf_id;
> > +
> > + btf_id = dt_btf_lookup_name_kind(dtp, dmp, pdp->fun, BTF_KIND_FUNC);
> > + if (btf_id <= 0 ||
> > + !dt_btf_func_is_traceable(dtp, dmp->dm_btf, btf_id))
> > + return -1;
> > + }
> > +
> > if (dt_tp_probe_insert(dtp, prv, pdp->prv, pdp->mod, pdp->fun, pdp->prb))
> > return 1;
> >
>
> The hunks below appear to be doing something not called out in the
> commit log. I think this is getting the symbol module right?
This is necessary because the code above needs access to dmp.
> > @@ -173,16 +189,15 @@ static int provide(dtrace_hdl_t *dtp, const dtrace_probedesc_t *pdp)
> >
> > pd.id = DTRACE_IDNONE;
> > pd.prv = pdp->prv;
> > - pd.mod = dmp->dm_name;
> > pd.fun = pdp->fun;
> >
> > if (prb & FBT_ENTRY) {
> > pd.prb = "entry";
> > - n += provide_probe(dtp, &pd);
> > + n += provide_probe(dtp, dmp, &pd);
> > }
> > if (prb & FBT_RETURN) {
> > pd.prb = "return";
> > - n += provide_probe(dtp, &pd);
> > + n += provide_probe(dtp, dmp, &pd);
> > }
> >
> > return n;
> > @@ -190,22 +205,20 @@ static int provide(dtrace_hdl_t *dtp, const dtrace_probedesc_t *pdp)
> >
> > sym = dt_symbol_by_name(dtp, pdp->fun);
> > while (sym != NULL) {
> > - const char *mod = dt_symbol_module(sym)->dm_name;
> > -
> > + dmp = dt_symbol_module(sym);
> > if (dt_symbol_traceable(sym) &&
> > - dt_gmatch(mod, pdp->mod)) {
> > + dt_gmatch(dmp->dm_name, pdp->mod)) {
> > pd.id = DTRACE_IDNONE;
> > pd.prv = pdp->prv;
> > - pd.mod = mod;
> > pd.fun = pdp->fun;
> >
> > if (prb & FBT_ENTRY) {
> > pd.prb = "entry";
> > - n += provide_probe(dtp, &pd);
> > + n += provide_probe(dtp, dmp, &pd);
> > }
> > if (prb & FBT_RETURN) {
> > pd.prb = "return";
> > - n += provide_probe(dtp, &pd);
> > + n += provide_probe(dtp, dmp, &pd);
> > }
> >
> > }
> > @@ -242,16 +255,15 @@ static int provide(dtrace_hdl_t *dtp, const dtrace_probedesc_t *pdp)
> >
> > pd.id = DTRACE_IDNONE;
> > pd.prv = pdp->prv;
> > - pd.mod = smp->dm_name;
> > pd.fun = fun;
> >
> > if (prb & FBT_ENTRY) {
> > pd.prb = "entry";
> > - n += provide_probe(dtp, &pd);
> > + n += provide_probe(dtp, smp, &pd);
> > }
> > if (prb & FBT_RETURN) {
> > pd.prb = "return";
> > - n += provide_probe(dtp, &pd);
> > + n += provide_probe(dtp, smp, &pd);
> > }
> > }
> >
>
> The need for this trampoline change is also rather opaque.
Oops, that was meant to be in a different patch. Will separate it out.
> > @@ -323,10 +335,11 @@ static int fprobe_trampoline(dt_pcb_t *pcb, uint_t exitlbl)
> > */
> > dmp = dt_module_lookup_by_name(dtp, prp->desc->mod);
> > if (dmp && prp->argc == 2) {
> > - int32_t btf_id = dt_tp_probe_get_id(prp);
> > - int i = dt_btf_func_argc(dtp, dmp->dm_btf, btf_id);
> > -
> > - emit(dlp, BPF_LOAD(BPF_DW, BPF_REG_0, BPF_REG_8, i * 8));
> > + emit(dlp, BPF_MOV_REG(BPF_REG_1, BPF_REG_8));
> > + emit(dlp, BPF_MOV_REG(BPF_REG_2, BPF_REG_FP));
> > + emit(dlp, BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, DT_TRAMP_SP_SLOT(0)));
> > + emit(dlp, BPF_CALL_HELPER(dtp->dt_bpfhelper[BPF_FUNC_get_func_ret]));
> > + emit(dlp, BPF_LOAD(BPF_DW, BPF_REG_0, BPF_REG_FP, DT_TRAMP_SP_SLOT(0)));
> > emit(dlp, BPF_STORE(BPF_DW, BPF_REG_7, DMST_ARG(1), BPF_REG_0));
> > }
> > }
>
> --
> NULL && (void)
^ permalink raw reply [flat|nested] 3+ messages in thread
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2026-04-17 17:11 ` Nick Alcock
2026-04-17 19:28 ` Kris Van Hees
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