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@xxxxxx>
---
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;
--
2.17.1
