BPF_PROG_TYPE_SK_MSG programs use a 'void *' for both data and the
data_end pointers. Additionally, the verifier ensures that every
accesses into the values is a __u64 read. This correctly maps on
to the BPF 64-bit architecture.
However, to ensure that when building on 32bit architectures that
clang uses correct types the '-target bpf' option _must_ be
specified. To make this clear add a note to the Documentation.
Signed-off-by: John Fastabend <john.fastab...@gmail.com>
---
0 files changed
diff --git a/Documentation/bpf/bpf_devel_QA.txt
b/Documentation/bpf/bpf_devel_QA.txt
index 1a0b704..da57601 100644
--- a/Documentation/bpf/bpf_devel_QA.txt
+++ b/Documentation/bpf/bpf_devel_QA.txt
@@ -557,6 +557,14 @@ A: Although LLVM IR generation and optimization try to
stay architecture
pulls in some header files containing file scope host assembly codes.
- You can add "-fno-jump-tables" to work around the switch table issue.
- Otherwise, you can use bpf target.
+ Otherwise, you can use bpf target. Additionally, you _must_ use bpf target
+ when:
+
+ - Your program uses data structures with pointer or long / unsigned long
+ types that interface with BPF helpers or context data structures. Access
+ into these structures is verified by the BPF verifier and may result
+ in verification failures if the native architecture is not aligned with
+ the BPF architecture, e.g. 64-bit. An example of this is
+ BPF_PROG_TYPE_SK_MSG require '-target bpf'
Happy BPF hacking!
