[PATCH bpf-next] bpf: avoid clang-specific push/pop attribute pragmas in bpftool

["Jose E. Marchesi" <jose.marchesi@xxxxxxxxxx>]

The vmlinux.h file generated by bpftool makes use of compiler pragmas
in order to install the CO-RE preserve_access_index in all the struct
types derived from the BTF info:

  #ifndef __VMLINUX_H__
  #define __VMLINUX_H__

  #ifndef BPF_NO_PRESERVE_ACCESS_INDEX
  #pragma clang attribute push (__attribute__((preserve_access_index)), apply_t = record
  #endif

  [... type definitions generated from kernel BTF ... ]

  #ifndef BPF_NO_PRESERVE_ACCESS_INDEX
  #pragma clang attribute pop
  #endif

The `clang attribute push/pop' pragmas are specific to clang/llvm and
are not supported by GCC.

This patch modifies bpftool in order to, instead of using the pragmas,
define ATTR_PRESERVE_ACCESS_INDEX to conditionally expand to the CO-RE
attribute:

  #ifndef __VMLINUX_H__
  #define __VMLINUX_H__

  #ifndef BPF_NO_PRESERVE_ACCESS_INDEX
  #define ATTR_PRESERVE_ACCESS_INDEX __attribute__((preserve_access_index))
  #else
  #define ATTR_PRESERVE_ACCESS_INDEX
  #endif

  [... type definitions generated from kernel BTF ... ]

  #undef ATTR_PRESERVE_ACCESS_INDEX

To make this possible the btf_dump_opts have been expanded with a new
configurable field `record_attrs_str', which is a string that gets
inserted at the point compiler attributes are expected for each
emmited C struct and union type:

  DECLARE_LIBBPF_OPTS(btf_dump_opts, opts);
  [...]
  opts.record_attrs_str = "ATTR_PRESERVE_ACCESS_INDEX";
  d = btf_dump__new(btf, btf_dump_printf, NULL, &opts);
  [...]
  err = btf_dump__dump_type(d, root_type_ids[i]);

Tested in bpf-next master.
No regressions.

Signed-off-by: Jose E. Marchesi <jose.marchesi@xxxxxxxxxx>
Cc: Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx>
Cc: Yonghong Song <yonghong.song@xxxxxxxxx>
Cc: Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx>
Cc: david.faust@xxxxxxxxxx
Cc: cupertino.miranda@xxxxxxxxxx
---
 tools/bpf/bpftool/btf.c  | 15 +++++++++------
 tools/lib/bpf/btf.h      |  9 ++++++++-
 tools/lib/bpf/btf_dump.c |  8 ++++++++
 3 files changed, 25 insertions(+), 7 deletions(-)

diff --git a/tools/bpf/bpftool/btf.c b/tools/bpf/bpftool/btf.c
index 91fcb75babe3..c548dc3b4bd7 100644
--- a/tools/bpf/bpftool/btf.c
+++ b/tools/bpf/bpftool/btf.c
@@ -463,19 +463,24 @@ static void __printf(2, 0) btf_dump_printf(void *ctx,
 static int dump_btf_c(const struct btf *btf,
 		      __u32 *root_type_ids, int root_type_cnt)
 {
+	DECLARE_LIBBPF_OPTS(btf_dump_opts, opts);
 	struct btf_dump *d;
 	int err = 0, i;
 
-	d = btf_dump__new(btf, btf_dump_printf, NULL, NULL);
+	opts.record_attrs_str = "ATTR_PRESERVE_ACCESS_INDEX";
+	d = btf_dump__new(btf, btf_dump_printf, NULL, &opts);
 	if (!d)
 		return -errno;
 
 	printf("#ifndef __VMLINUX_H__\n");
 	printf("#define __VMLINUX_H__\n");
 	printf("\n");
-	printf("#ifndef BPF_NO_PRESERVE_ACCESS_INDEX\n");
-	printf("#pragma clang attribute push (__attribute__((preserve_access_index)), apply_to = record)\n");
+	printf("#ifdef BPF_NO_PRESERVE_ACCESS_INDEX\n");
+	printf("#define ATTR_PRESERVE_ACCESS_INDEX\n");
+	printf("#else\n");
+	printf("#define ATTR_PRESERVE_ACCESS_INDEX __attribute__((preserve_access_index))\n");
 	printf("#endif\n\n");
+	printf("\n");
 
 	if (root_type_cnt) {
 		for (i = 0; i < root_type_cnt; i++) {
@@ -493,9 +498,7 @@ 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");
+	printf("#undef ATTR_PRESERVE_ACCESS_INDEX\n");
 	printf("\n");
 	printf("#endif /* __VMLINUX_H__ */\n");
 
diff --git a/tools/lib/bpf/btf.h b/tools/lib/bpf/btf.h
index 8e6880d91c84..f7f9a2c7f96e 100644
--- a/tools/lib/bpf/btf.h
+++ b/tools/lib/bpf/btf.h
@@ -234,9 +234,16 @@ LIBBPF_API int btf__dedup(struct btf *btf, const struct btf_dedup_opts *opts);
 struct btf_dump;
 
 struct btf_dump_opts {
+	/* size of this struct, for forward/backward compatibility */
 	size_t sz;
+	/* String to be inserted at the point where compiler
+	 * attributes are expected in each emitted C struct and
+	 * union type.
+	 */
+	const char *record_attrs_str;
+	size_t :0;
 };
-#define btf_dump_opts__last_field sz
+#define btf_dump_opts__last_field record_attrs_str
 
 typedef void (*btf_dump_printf_fn_t)(void *ctx, const char *fmt, va_list args);
 
diff --git a/tools/lib/bpf/btf_dump.c b/tools/lib/bpf/btf_dump.c
index 5dbca76b953f..fbc112f31d33 100644
--- a/tools/lib/bpf/btf_dump.c
+++ b/tools/lib/bpf/btf_dump.c
@@ -116,6 +116,11 @@ struct btf_dump {
 	 * data for typed display; allocated if needed.
 	 */
 	struct btf_dump_data *typed_dump;
+	/*
+	 * string with C attributes to be used in record
+	 * types.
+         */
+	const char *record_attrs_str;
 };
 
 static size_t str_hash_fn(long key, void *ctx)
@@ -167,6 +172,7 @@ struct btf_dump *btf_dump__new(const struct btf *btf,
 	d->printf_fn = printf_fn;
 	d->cb_ctx = ctx;
 	d->ptr_sz = btf__pointer_size(btf) ? : sizeof(void *);
+	d->record_attrs_str = OPTS_GET(opts, record_attrs_str, NULL);
 
 	d->type_names = hashmap__new(str_hash_fn, str_equal_fn, NULL);
 	if (IS_ERR(d->type_names)) {
@@ -1024,6 +1030,8 @@ static void btf_dump_emit_struct_def(struct btf_dump *d,
 	}
 	if (packed)
 		btf_dump_printf(d, " __attribute__((packed))");
+	if (d->record_attrs_str)
+		btf_dump_printf(d, " %s", d->record_attrs_str);
 }
 
 static const char *missing_base_types[][2] = {
-- 
2.30.2