From 1cf5b23988ea0086a252a5c8b005b075f1e9b030 Mon Sep 17 00:00:00 2001 From: Andrii Nakryiko Date: Sun, 12 Jan 2020 23:31:41 -0800 Subject: [PATCH] bpftool: Apply preserve_access_index attribute to all types in BTF dump This patch makes structs and unions, emitted through BTF dump, automatically CO-RE-relocatable (unless disabled with `#define BPF_NO_PRESERVE_ACCESS_INDEX`, specified before including generated header file). This effectivaly turns usual bpf_probe_read() call into equivalent of bpf_core_read(), by automatically applying builtin_preserve_access_index to any field accesses of types in generated C types header. This is especially useful for tp_btf/fentry/fexit BPF program types. They allow direct memory access, so BPF C code just uses straightfoward a->b->c access pattern to read data from kernel. But without kernel structs marked as CO-RE relocatable through preserve_access_index attribute, one has to enclose all the data reads into a special __builtin_preserve_access_index code block, like so: __builtin_preserve_access_index(({ x = p->pid; /* where p is struct task_struct *, for example */ })); This is very inconvenient and obscures the logic quite a bit. By marking all auto-generated types with preserve_access_index attribute the above code is reduced to just a clean and natural `x = p->pid;`. Signed-off-by: Andrii Nakryiko Signed-off-by: Alexei Starovoitov Link: https://lore.kernel.org/bpf/20200113073143.1779940-5-andriin@fb.com --- tools/bpf/bpftool/btf.c | 8 ++++++++ 1 file changed, 8 insertions(+) diff --git a/tools/bpf/bpftool/btf.c b/tools/bpf/bpftool/btf.c index e5bc97b71ceb..60c75be0666d 100644 --- a/tools/bpf/bpftool/btf.c +++ b/tools/bpf/bpftool/btf.c @@ -370,6 +370,10 @@ static int dump_btf_c(const struct btf *btf, if (IS_ERR(d)) return PTR_ERR(d); + printf("#ifndef BPF_NO_PRESERVE_ACCESS_INDEX\n"); + printf("#pragma clang attribute push (__attribute__((preserve_access_index)), apply_to = record)\n"); + printf("#endif\n\n"); + if (root_type_cnt) { for (i = 0; i < root_type_cnt; i++) { err = btf_dump__dump_type(d, root_type_ids[i]); @@ -386,6 +390,10 @@ static int dump_btf_c(const struct btf *btf, } } + printf("#ifndef BPF_NO_PRESERVE_ACCESS_INDEX\n"); + printf("#pragma clang attribute pop\n"); + printf("#endif\n"); + done: btf_dump__free(d); return err;