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authorJohn Fastabend <john.fastabend@gmail.com>2018-04-23 12:11:02 -0700
committerDaniel Borkmann <daniel@iogearbox.net>2018-04-23 23:42:21 +0200
commit514d6c1959f9b396f1b51850925900adedffb951 (patch)
treef806288e530b2d6623b599123bea3d229a37d68d /Documentation
parent6899b32b5b2dee358936b82b8363b716607a138f (diff)
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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>
Diffstat (limited to 'Documentation')
-rw-r--r--Documentation/bpf/bpf_devel_QA.txt10
1 files changed, 9 insertions, 1 deletions
diff --git a/Documentation/bpf/bpf_devel_QA.txt b/Documentation/bpf/bpf_devel_QA.txt
index 1a0b704e1a38..da57601153a0 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!