Re: [PATCH 2/6] perf record: Enable off-cpu analysis with BPF

[Namhyung Kim <namhyung@kernel.org>]

On Tue, May 31, 2022 at 11:01:14PM -0700, Namhyung Kim wrote:
> On Tue, May 31, 2022 at 5:00 PM Ian Rogers <irogers@google.com> wrote:
> >
> > On Wed, May 18, 2022 at 3:47 PM Namhyung Kim <namhyung@kernel.org> wrote:
> [SNIP]
> > > +
> > > +/*
> > > + * Old kernel used to call it task_struct->state and now it's '__state'.
> > > + * Use BPF CO-RE "ignored suffix rule" to deal with it like below:
> > > + *
> > > + * https://nakryiko.com/posts/bpf-core-reference-guide/#handling-incompatible-field-and-type-changes
> > > + */
> > > +static inline int get_task_state(struct task_struct *t)
> > > +{
> > > +       if (bpf_core_field_exists(t->__state))
> > > +               return BPF_CORE_READ(t, __state);
> > > +
> >
> > When building against a pre-5.14 kernel I'm running into a build issue here:
> >
> > tools/perf/util/bpf_skel/off_cpu.bpf.c:96:31: error: no member named '__
> > state' in 'struct task_struct'; did you mean 'state'?
> >        if (bpf_core_field_exists(t->__state))
> >                                     ^~~~~~~
> >                                     state
> >
> > This isn't covered by Andrii's BPF CO-RE reference guide. I have an
> > #iffy workaround below,but this will be brittle if the 5.14+ kernel
> > code is backported. Suggestions welcomed :-)
> 
> Thanks for the fix.  I think we should not guess the field name
> in the current task struct and check both versions separately.
> I'm afraid the version check won't work with some backported
> kernels.  But do we care?


What about this instead?

----8<----
From a621f836f00e11942e5d39a735ec8f7a21962d6a Mon Sep 17 00:00:00 2001
From: Namhyung Kim <namhyung@kernel.org>
Date: Tue, 31 May 2022 14:25:05 -0700
Subject: [PATCH] perf tools: Fix a build failure in off-cpu BPF program on old kernels

Old kernels have task_struct which contains "state" field.  While the
get_task_state() in the BPF code handles that, it assumed the kernel
has the new definition and caused a build error on old kernels.

We should not assume anything and access them carefully.

Reported-by: Ian Rogers <irogers@google.com>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
---
 tools/perf/util/bpf_skel/off_cpu.bpf.c | 20 ++++++++++++++------
 1 file changed, 14 insertions(+), 6 deletions(-)

diff --git a/tools/perf/util/bpf_skel/off_cpu.bpf.c b/tools/perf/util/bpf_skel/off_cpu.bpf.c
index 792ae2847080..cc6d7fd55118 100644
--- a/tools/perf/util/bpf_skel/off_cpu.bpf.c
+++ b/tools/perf/util/bpf_skel/off_cpu.bpf.c
@@ -71,6 +71,11 @@ struct {
 	__uint(max_entries, 1);
 } cgroup_filter SEC(".maps");
 
+/* new kernel task_struct definition */
+struct task_struct___new {
+	long __state;
+} __attribute__((preserve_access_index));
+
 /* old kernel task_struct definition */
 struct task_struct___old {
 	long state;
@@ -93,14 +98,17 @@ const volatile bool uses_cgroup_v1 = false;
  */
 static inline int get_task_state(struct task_struct *t)
 {
-	if (bpf_core_field_exists(t->__state))
-		return BPF_CORE_READ(t, __state);
+	/* recast pointer to capture new type for compiler */
+	struct task_struct___new *t_new = (void *)t;
 
-	/* recast pointer to capture task_struct___old type for compiler */
-	struct task_struct___old *t_old = (void *)t;
+	if (bpf_core_field_exists(t_new->__state)) {
+		return BPF_CORE_READ(t_new, __state);
+	} else {
+		/* recast pointer to capture old type for compiler */
+		struct task_struct___old *t_old = (void *)t;
 
-	/* now use old "state" name of the field */
-	return BPF_CORE_READ(t_old, state);
+		return BPF_CORE_READ(t_old, state);
+	}
 }
 
 static inline __u64 get_cgroup_id(struct task_struct *t)
-- 
2.36.1.255.ge46751e96f-goog