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Subject[PATCH 1/6] perf offcpu: Fix a build failure on old kernels
Date
Old kernels have task_struct which contains "state" field and newer
kernels have "__state". While the get_task_state() in the BPF code
handles that in some way, it assumed the current kernel has the new
definition and it caused a build error on old kernels.

We should not assume anything and access them carefully. Do not use
the task struct directly and access them using new and old definitions
in a row.

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)
--
2.37.0.rc0.161.g10f37bed90-goog
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