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 <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200113073143.1779940-5-andriin@fb.com

diff --git a/tools/bpf/bpftool/btf.c b/tools/bpf/bpftool/btf.c
index e5bc97b..60c75be 100644 (file)
--- 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;