bpf: Document sockmap '-target bpf' requirement for PROG_TYPE_SK_MSG

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.fastabend@gmail.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
This commit is contained in:
John Fastabend 2018-04-23 12:11:02 -07:00 committed by Daniel Borkmann
parent 6899b32b5b
commit 514d6c1959

View File

@ -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!