[PATCH bpf v2 2/5] bpf: Adjust sk size check for sk in bpf_skb_is_valid_access for CO_RE in 32-bit arch

Yang Jihong
2022-11-07T09:24:09Z

The error code -EACCES is returned when bpf prog is tested in 32-bit arch.
This is because bpf_object__relocate modifies instruction to change memory
size to 4 bytes, as shown in the following messages:

libbpf: prog 'kfunc_call_test1': relo #2: matching candidate #0 <byte_off> [18342] struct __sk_buff.sk (0:30:0 @ offset 168)
libbpf: prog 'kfunc_call_test1': relo #2: patched insn #1 (LDX/ST/STX) off 168 -> 168
libbpf: prog 'kfunc_call_test1': relo #2: patched insn #1 (LDX/ST/STX) mem_sz 8 -> 4

As a result, the bpf_skb_is_valid_access check fails, for 32-bit arch,
adjust check sk size.

Signed-off-by: Yang Jihong <yangjihong1@huawei.com>
---
net/core/filter.c | 8 +++++++-
1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/net/core/filter.c b/net/core/filter.c
index bb0136e7a8e4..47cbad2e609f 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -8269,7 +8269,13 @@ static bool bpf_skb_is_valid_access(int off, int size, enum bpf_access_type type
return false;
break;
case offsetof(struct __sk_buff, sk):
- if (type == BPF_WRITE || size != sizeof(__u64))
+ /* CO_RE adjusts pointer accesses from 8-byte read to
+ * 4-byte reads in 32-bit host arch, so 32-bit can only
+ * read the 32-bit pointer or the full 64-bit value,
+ * and 64-bit can read write the 64-bit pointer.
+ */
+ if (type == BPF_WRITE ||
+ (size != sizeof(struct bpf_sock *) && size != sizeof(__u64)))
return false;
info->reg_type = PTR_TO_SOCK_COMMON_OR_NULL;
break;
--
2.30.GIT