[bpf-next] bpf: fix build without CONFIG_STACKTRACE

[Song Liu]

Without CONFIG_STACKTRACE stack_trace_save_tsk() is not defined. Let
get_callchain_entry_for_task() to always return NULL in such cases.

Fixes: fa28dcb82a38 ("bpf: Introduce helper bpf_get_task_stack()")
Reported-by: kernel test robot <l...@intel.com>
Signed-off-by: Song Liu <songliubrav...@fb.com>
---
 kernel/bpf/stackmap.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c
index 5ad72ab2276b3..a6c361ed7937b 100644
--- a/kernel/bpf/stackmap.c
+++ b/kernel/bpf/stackmap.c
@@ -351,6 +351,7 @@ static void stack_map_get_build_id_offset(struct 
bpf_stack_build_id *id_offs,
 static struct perf_callchain_entry *
 get_callchain_entry_for_task(struct task_struct *task, u32 init_nr)
 {
+#ifdef CONFIG_STACKTRACE
        struct perf_callchain_entry *entry;
        int rctx;
 
@@ -380,6 +381,9 @@ get_callchain_entry_for_task(struct task_struct *task, u32 
init_nr)
        put_callchain_entry(rctx);
 
        return entry;
+#else /* CONFIG_STACKTRACE */
+       return NULL;
+#endif
 }
 
 BPF_CALL_3(bpf_get_stackid, struct pt_regs *, regs, struct bpf_map *, map,
-- 
2.24.1