[PATCH bpf v1 1/3] bpf: fix sysfs export of empty BTF section

[Tony Ambardar <tony.ambardar@xxxxxxxxx>]

If BTF data is missing or removed from the ELF section it is still exported
via sysfs as a zero-length file:

  root@OpenWrt:/# ls -l /sys/kernel/btf/vmlinux
  -r--r--r--    1 root    root    0 Jul 18 02:59 /sys/kernel/btf/vmlinux

Moreover, reads from this file succeed and leak kernel data:

  root@OpenWrt:/# hexdump -C /sys/kernel/btf/vmlinux|head -10
  000000 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
  *
  000cc0 00 00 00 00 00 00 00 00 00 00 00 00 80 83 b0 80 |................|
  000cd0 00 10 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
  000ce0 00 00 00 00 00 00 00 00 00 00 00 00 57 ac 6e 9d |............W.n.|
  000cf0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
  *
  002650 00 00 00 00 00 00 00 10 00 00 00 01 00 00 00 01 |................|
  002660 80 82 9a c4 80 85 97 80 81 a9 51 68 00 00 00 02 |..........Qh....|
  002670 80 25 44 dc 80 85 97 80 81 a9 50 24 81 ab c4 60 |.%D.......P$...`|

This situation was first observed with kernel 5.4.x, cross-compiled for a
MIPS target system. Fix by adding a sanity-check for export of zero-length
data sections.

Fixes: 341dfcf8d78e ("btf: expose BTF info through sysfs")

Signed-off-by: Tony Ambardar <Tony.Ambardar@xxxxxxxxx>
---
 kernel/bpf/sysfs_btf.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/bpf/sysfs_btf.c b/kernel/bpf/sysfs_btf.c
index 3b495773de5a..11b3380887fa 100644
--- a/kernel/bpf/sysfs_btf.c
+++ b/kernel/bpf/sysfs_btf.c
@@ -30,15 +30,15 @@ static struct kobject *btf_kobj;
 
 static int __init btf_vmlinux_init(void)
 {
-	if (!__start_BTF)
+	bin_attr_btf_vmlinux.size = __stop_BTF - __start_BTF;
+
+	if (!__start_BTF || bin_attr_btf_vmlinux.size == 0)
 		return 0;
 
 	btf_kobj = kobject_create_and_add("btf", kernel_kobj);
 	if (!btf_kobj)
 		return -ENOMEM;
 
-	bin_attr_btf_vmlinux.size = __stop_BTF - __start_BTF;
-
 	return sysfs_create_bin_file(btf_kobj, &bin_attr_btf_vmlinux);
 }
 
-- 
2.25.1