[PATCH bpf-next v2] bpftool: btf: Support dumping a single type from file

[Daniel Xu <dxu@xxxxxxxxx>]

Some projects, for example xdp-tools [0], prefer to check in a minimized
vmlinux.h rather than the complete file which can get rather large.

However, when you try to add a minimized version of a complex struct (eg
struct xfrm_state), things can get quite complex if you're trying to
manually untangle and deduplicate the dependencies.

This commit teaches bpftool to do a minimized dump of a single type by
providing an optional root_id argument.

Example usage:

    $ ./bpftool btf dump file ~/dev/linux/vmlinux | rg "STRUCT 'xfrm_state'"
    [12643] STRUCT 'xfrm_state' size=912 vlen=58

    $ ./bpftool btf dump file ~/dev/linux/vmlinux root_id 12643 format c
    #ifndef __VMLINUX_H__
    #define __VMLINUX_H__

    [..]

    struct xfrm_type_offload;

    struct xfrm_sec_ctx;

    struct xfrm_state {
            possible_net_t xs_net;
            union {
                    struct hlist_node gclist;
                    struct hlist_node bydst;
            };
            union {
                    struct hlist_node dev_gclist;
                    struct hlist_node bysrc;
            };
            struct hlist_node byspi;
    [..]

[0]: https://github.com/xdp-project/xdp-tools/blob/master/headers/bpf/vmlinux.h

Signed-off-by: Daniel Xu <dxu@xxxxxxxxx>
---
Changes in v2:
* Add early error check for invalid BTF ID

 .../bpf/bpftool/Documentation/bpftool-btf.rst |  5 +++--
 tools/bpf/bpftool/btf.c                       | 19 +++++++++++++++++++
 2 files changed, 22 insertions(+), 2 deletions(-)

diff --git a/tools/bpf/bpftool/Documentation/bpftool-btf.rst b/tools/bpf/bpftool/Documentation/bpftool-btf.rst
index 3f6bca03ad2e..5abd0e99022f 100644
--- a/tools/bpf/bpftool/Documentation/bpftool-btf.rst
+++ b/tools/bpf/bpftool/Documentation/bpftool-btf.rst
@@ -27,7 +27,7 @@ BTF COMMANDS
 | **bpftool** **btf dump** *BTF_SRC* [**format** *FORMAT*]
 | **bpftool** **btf help**
 |
-| *BTF_SRC* := { **id** *BTF_ID* | **prog** *PROG* | **map** *MAP* [{**key** | **value** | **kv** | **all**}] | **file** *FILE* }
+| *BTF_SRC* := { **id** *BTF_ID* | **prog** *PROG* | **map** *MAP* [{**key** | **value** | **kv** | **all**}] | **file** *FILE* [**root_id** *ROOT_ID*] }
 | *FORMAT* := { **raw** | **c** [**unsorted**] }
 | *MAP* := { **id** *MAP_ID* | **pinned** *FILE* }
 | *PROG* := { **id** *PROG_ID* | **pinned** *FILE* | **tag** *PROG_TAG* | **name** *PROG_NAME* }
@@ -60,7 +60,8 @@ bpftool btf dump *BTF_SRC*
 
     When specifying *FILE*, an ELF file is expected, containing .BTF section
     with well-defined BTF binary format data, typically produced by clang or
-    pahole.
+    pahole. You can choose to dump a single type and all its dependent types
+    by providing an optional *ROOT_ID*.
 
     **format** option can be used to override default (raw) output format. Raw
     (**raw**) or C-syntax (**c**) output formats are supported. With C-style
diff --git a/tools/bpf/bpftool/btf.c b/tools/bpf/bpftool/btf.c
index d005e4fd6128..a75e17efaf5e 100644
--- a/tools/bpf/bpftool/btf.c
+++ b/tools/bpf/bpftool/btf.c
@@ -953,6 +953,8 @@ static int do_dump(int argc, char **argv)
 		NEXT_ARG();
 	} else if (is_prefix(src, "file")) {
 		const char sysfs_prefix[] = "/sys/kernel/btf/";
+		__u32 root_id;
+		char *end;
 
 		if (!base_btf &&
 		    strncmp(*argv, sysfs_prefix, sizeof(sysfs_prefix) - 1) == 0 &&
@@ -967,6 +969,23 @@ static int do_dump(int argc, char **argv)
 			goto done;
 		}
 		NEXT_ARG();
+
+		if (argc && is_prefix(*argv, "root_id")) {
+			NEXT_ARG();
+			root_id = strtoul(*argv, &end, 0);
+			if (*end) {
+				err = -1;
+				p_err("can't parse %s as root ID", *argv);
+				goto done;
+			}
+			if (root_id >= btf__type_cnt(btf)) {
+				err = -EINVAL;
+				p_err("invalid root ID: %u", root_id);
+				goto done;
+			}
+			root_type_ids[root_type_cnt++] = root_id;
+			NEXT_ARG();
+		}
 	} else {
 		err = -1;
 		p_err("unrecognized BTF source specifier: '%s'", src);
-- 
2.46.0