[RFC bpf-next 13/13] bpftool: support displaying reconciled-with-base split BTF

[Alan Maguire <alan.maguire@oracle.com>]

If the -R <base_btf> option is used, we can display BTF
that has been generated with base reference BTF in its
reconciled form.  For example if we build a module with
base reference BTF:

BTF_BASE_REF=1 make -C. M=path/2/module

...we can display its content relative to that base
reference BTF via

bpftool btf dump file path/2/module

Alternatively, we can display content reconciled with
(a possibly changed) base BTF via

bpftool btf dump -R vmlinux path/2/module

The latter mirrors how the kernel will handle such split
BTF; it reconciles its representation with the running
kernel, and if successful, renumbers BTF ids to reference
the current vmlinux BTF.

Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
---
 tools/bpf/bpftool/Documentation/bpftool-btf.rst | 17 +++++++++++++++++
 tools/bpf/bpftool/btf.c                         | 14 ++++++++++++--
 tools/bpf/bpftool/main.c                        | 14 +++++++++++++-
 tools/bpf/bpftool/main.h                        |  2 ++
 4 files changed, 44 insertions(+), 3 deletions(-)

diff --git a/tools/bpf/bpftool/Documentation/bpftool-btf.rst b/tools/bpf/bpftool/Documentation/bpftool-btf.rst
index 342716f74ec4..97b6426367e6 100644
--- a/tools/bpf/bpftool/Documentation/bpftool-btf.rst
+++ b/tools/bpf/bpftool/Documentation/bpftool-btf.rst
@@ -90,6 +90,23 @@ OPTIONS
 		  to autodetect the path for the base object, and passing
 		  this option is optional. When the main BTF object is passed
 		  through other handles, this option becomes necessary.
+        -R, --reconcile-base-btf *FILE*
+                  When split BTF is generated with base reference BTF,
+                  the latter is stored in a .BTF.base_ref section and allows
+                  us to later reconcile split BTF and a potentially-changed
+                  base BTF by noting that type ids in the split BTF that
+                  refer to base BTF refer to specific integers, structs etc.
+                  By passing base BTF via the -R option, we can carry out
+                  that reconciliation operation between split/base reference
+                  BTF and base BTF.  If successful, the split BTF will then
+                  refer to the base BTF types from *reconcile-base-btf*.
+
+                  If this argument is not used, split BTF is shown relative
+                  to .BTF.base_ref data as a base; however in this mode
+                  it is not possible to generate "C" output, since the
+                  information retained in .BTF.base_ref is just enough to
+                  clarify what a type id refers to, but does not contain
+                  full type details for all types.
 
 EXAMPLES
 ========
diff --git a/tools/bpf/bpftool/btf.c b/tools/bpf/bpftool/btf.c
index f1e0ea06f05f..355b56eff9f2 100644
--- a/tools/bpf/bpftool/btf.c
+++ b/tools/bpf/bpftool/btf.c
@@ -644,8 +644,17 @@ static int do_dump(int argc, char **argv)
 			base_btf = btf__parse_opts(*argv, &optp);
 			if (base_btf)
 				btf = btf__parse_split(*argv, base_btf);
-			if (btf)
+			if (btf && reconcile_base_btf) {
+				err = btf__reconcile(btf, reconcile_base_btf);
+				if (err) {
+					p_err("coud not reconcile BTF from '%s' with base BTF '%s': %s\n",
+					      *argv, reconcile_base_btf_path,
+					      strerror(-err));
+					goto done;
+				}
+			} else {
 				c_compat = false;
+			}
 		}
 		if (!btf) {
 			err = -errno;
@@ -1088,7 +1097,8 @@ static int do_help(int argc, char **argv)
 		"       " HELP_SPEC_MAP "\n"
 		"       " HELP_SPEC_PROGRAM "\n"
 		"       " HELP_SPEC_OPTIONS " |\n"
-		"                    {-B|--base-btf} }\n"
+		"                    {-B|--base-btf}\n"
+		"                    {-R|--reconcile-base-btf}}\n"
 		"",
 		bin_name, "btf");
 
diff --git a/tools/bpf/bpftool/main.c b/tools/bpf/bpftool/main.c
index 08d0ac543c67..33ac0ca77162 100644
--- a/tools/bpf/bpftool/main.c
+++ b/tools/bpf/bpftool/main.c
@@ -32,6 +32,8 @@ bool verifier_logs;
 bool relaxed_maps;
 bool use_loader;
 struct btf *base_btf;
+struct btf *reconcile_base_btf;
+const char *reconcile_base_btf_path;
 struct hashmap *refs_table;
 
 static void __noreturn clean_and_exit(int i)
@@ -448,6 +450,7 @@ int main(int argc, char **argv)
 		{ "debug",	no_argument,	NULL,	'd' },
 		{ "use-loader",	no_argument,	NULL,	'L' },
 		{ "base-btf",	required_argument, NULL, 'B' },
+		{ "reconcile-base-btf", required_argument, NULL, 'R' },
 		{ 0 }
 	};
 	bool version_requested = false;
@@ -473,7 +476,7 @@ int main(int argc, char **argv)
 	bin_name = "bpftool";
 
 	opterr = 0;
-	while ((opt = getopt_long(argc, argv, "VhpjfLmndB:l",
+	while ((opt = getopt_long(argc, argv, "VhpjfLmndB:lR:",
 				  options, NULL)) >= 0) {
 		switch (opt) {
 		case 'V':
@@ -516,6 +519,15 @@ int main(int argc, char **argv)
 				return -1;
 			}
 			break;
+		case 'R':
+			reconcile_base_btf_path = optarg;
+			reconcile_base_btf = btf__parse(optarg, NULL);
+			if (!reconcile_base_btf) {
+				p_err("failed to parse base BTF for reconciliation at '%s': %d\n",
+				      optarg, -errno);
+				return -1;
+			}
+			break;
 		case 'L':
 			use_loader = true;
 			break;
diff --git a/tools/bpf/bpftool/main.h b/tools/bpf/bpftool/main.h
index b8bb08d10dec..c84bfc1d9135 100644
--- a/tools/bpf/bpftool/main.h
+++ b/tools/bpf/bpftool/main.h
@@ -83,6 +83,8 @@ extern bool verifier_logs;
 extern bool relaxed_maps;
 extern bool use_loader;
 extern struct btf *base_btf;
+extern struct btf *reconcile_base_btf;
+extern const char *reconcile_base_btf_path;
 extern struct hashmap *refs_table;
 
 void __printf(1, 2) p_err(const char *fmt, ...);
-- 
2.39.3