Implement the show_fdinfo hook for BTF FDs file operations, and make it
print the id and the size of the BTF object. This allows for a quick
retrieval of the BTF id from its FD; or it can help understanding what
type of object (BTF) the file descriptor points to.

Signed-off-by: Quentin Monnet <redacted>
Reviewed-by: Jakub Kicinski <redacted>
---
  kernel/bpf/btf.c | 16 ++++++++++++++++
  1 file changed, 16 insertions(+)

diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index 5fcc7a17eb5a..39e184f1b27c 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c

@@ -3376,6 +3376,19 @@ void btf_type_seq_show(const struct btf *btf, u32 type_id, void *obj,
  	btf_type_ops(t)->seq_show(btf, t, type_id, obj, 0, m);
  }
  
+#ifdef CONFIG_PROC_FS
+static void bpf_btf_show_fdinfo(struct seq_file *m, struct file *filp)
+{
+	const struct btf *btf = filp->private_data;
+
+	seq_printf(m,
+		   "btf_id:\t%u\n"
+		   "data_size:\t%u\n",
+		   btf->id,
+		   btf->data_size);

+}
+#endif
+
  static int btf_release(struct inode *inode, struct file *filp)
  {
  	btf_put(filp->private_data);

@@ -3383,6 +3396,9 @@ static int btf_release(struct inode *inode, struct file *filp)
  }
  
  const struct file_operations btf_fops = {
+#ifdef CONFIG_PROC_FS
+	.show_fdinfo	= bpf_btf_show_fdinfo,
+#endif
  	.release	= btf_release,
  };