[PATCHv4 bpf-next 2/4] bpf: Add bpf_vma_build_id_parse function and kfunc

[Jiri Olsa <jolsa@xxxxxxxxxx>]

Adding bpf_vma_build_id_parse function to retrieve build id from
passed vma object and making it available as bpf kfunc.

We can't use build_id_parse directly as kfunc, because we would
not have control over the build id buffer size provided by user.

Instead we are adding new bpf_vma_build_id_parse function with
'build_id__sz' argument that instructs verifier to check for the
available space in build_id buffer.

This way we check that there's always available memory space
behind build_id pointer. We also check that the build_id__sz is
at least BUILD_ID_SIZE_MAX so we can place any buildid in.

The bpf_vma_build_id_parse kfunc is marked as KF_TRUSTED_ARGS,
so it can be only called with trusted vma objects. These are
currently provided only by find_vma callback function and
task_vma iterator program.

Signed-off-by: Jiri Olsa <jolsa@xxxxxxxxxx>
---
 include/linux/bpf.h      |  4 ++++
 kernel/trace/bpf_trace.c | 31 +++++++++++++++++++++++++++++++
 2 files changed, 35 insertions(+)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index c6aa6912ea16..359c8fe11779 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -2839,4 +2839,8 @@ static inline bool type_is_alloc(u32 type)
 	return type & MEM_ALLOC;
 }
 
+int bpf_vma_build_id_parse(struct vm_area_struct *vma,
+			   unsigned char *build_id,
+			   size_t build_id__sz);
+
 #endif /* _LINUX_BPF_H */
diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index 3bbd3f0c810c..7340de74531a 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -23,6 +23,7 @@
 #include <linux/sort.h>
 #include <linux/key.h>
 #include <linux/verification.h>
+#include <linux/buildid.h>
 
 #include <net/bpf_sk_storage.h>
 
@@ -1383,6 +1384,36 @@ static int __init bpf_key_sig_kfuncs_init(void)
 late_initcall(bpf_key_sig_kfuncs_init);
 #endif /* CONFIG_KEYS */
 
+int bpf_vma_build_id_parse(struct vm_area_struct *vma,
+			   unsigned char *build_id,
+			   size_t build_id__sz)
+{
+	__u32 size;
+	int err;
+
+	if (build_id__sz < BUILD_ID_SIZE_MAX)
+		return -EINVAL;
+
+	err = build_id_parse(vma, build_id, &size);
+	return err ?: (int) size;
+}
+
+BTF_SET8_START(tracing_btf_ids)
+BTF_ID_FLAGS(func, bpf_vma_build_id_parse, KF_TRUSTED_ARGS)
+BTF_SET8_END(tracing_btf_ids)
+
+static const struct btf_kfunc_id_set tracing_kfunc_set = {
+	.owner = THIS_MODULE,
+	.set   = &tracing_btf_ids,
+};
+
+static int __init kfunc_tracing_init(void)
+{
+	return register_btf_kfunc_id_set(BPF_PROG_TYPE_TRACING, &tracing_kfunc_set);
+}
+
+late_initcall(kfunc_tracing_init);
+
 static const struct bpf_func_proto *
 bpf_tracing_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
 {
-- 
2.38.1