bpftool: support displaying relocated-with-base split BTF

If the -R <base_btf> option is used, we can display BTF that has been
generated with distilled base BTF in its relocated form.  For example
for bpf_testmod.ko (which is built as an out-of-tree module, so has
a distilled .BTF.base section:

bpftool btf dump file bpf_testmod.ko

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

bpftool btf dump -R /sys/kernel/btf/vmlinux bpf_testmod.ko

The latter mirrors how the kernel will handle such split
BTF; it relocates 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>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>

tools/bpf/bpftool/Documentation/bpftool-btf.rst

@@ -16,7 +16,7 @@ SYNOPSIS
 
 **bpftool** [*OPTIONS*] **btf** *COMMAND*
 
-*OPTIONS* := { |COMMON_OPTIONS| | { **-B** | **--base-btf** } }
+*OPTIONS* := { |COMMON_OPTIONS| | { **-B** | **--base-btf** } { **-R** | **relocate-base-btf** } }
 
 *COMMANDS* := { **dump** | **help** }
 
@@ -85,6 +85,19 @@ OPTIONS
     BTF object is passed through other handles, this option becomes
     necessary.
 
+-R, --relocate-base-btf *FILE*
+    When split BTF is generated with distilled base BTF for relocation,
+    the latter is stored in a .BTF.base section and allows us to later
+    relocate split BTF and a potentially-changed base BTF by using
+    information in the .BTF.base section about the base types referenced
+    from split BTF.  Relocation is carried out against the split BTF
+    supplied via this parameter and the split BTF will then refer to
+    the base types supplied in *FILE*.
+
+    If this option is not used, split BTF is shown relative to the
+    .BTF.base, which contains just enough information to support later
+    relocation.
+
 EXAMPLES
 ========
 **# bpftool btf dump id 1226**

tools/bpf/bpftool/bash-completion/bpftool

@@ -262,7 +262,7 @@ _bpftool()
     # Deal with options
     if [[ ${words[cword]} == -* ]]; then
         local c='--version --json --pretty --bpffs --mapcompat --debug \
-            --use-loader --base-btf'
+            --use-loader --base-btf --relocate-base-btf'
         COMPREPLY=( $( compgen -W "$c" -- "$cur" ) )
         return 0
     fi
@@ -283,7 +283,7 @@ _bpftool()
             _sysfs_get_netdevs
             return 0
             ;;
-        file|pinned|-B|--base-btf)
+        file|pinned|-B|-R|--base-btf|--relocate-base-btf)
             _filedir
             return 0
             ;;
@@ -297,7 +297,8 @@ _bpftool()
     local i pprev
     for (( i=1; i < ${#words[@]}; )); do
         if [[ ${words[i]::1} == - ]] &&
-            [[ ${words[i]} != "-B" ]] && [[ ${words[i]} != "--base-btf" ]]; then
+            [[ ${words[i]} != "-B" ]] && [[ ${words[i]} != "--base-btf" ]] &&
+            [[ ${words[i]} != "-R" ]] && [[ ${words[i]} != "--relocate-base-btf" ]]; then
             words=( "${words[@]:0:i}" "${words[@]:i+1}" )
             [[ $i -le $cword ]] && cword=$(( cword - 1 ))
         else

tools/bpf/bpftool/btf.c

@@ -638,6 +638,14 @@ 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 && relocate_base_btf) {
+				err = btf__relocate(btf, relocate_base_btf);
+				if (err) {
+					p_err("could not relocate BTF from '%s' with base BTF '%s': %s\n",
+					      *argv, relocate_base_btf_path, strerror(-err));
+					goto done;
+				}
+			}
 		}
 		if (!btf) {
 			err = -errno;
@@ -1075,7 +1083,8 @@ 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|--relocate-base-btf} }\n"
 		"",
 		bin_name, "btf");
 

tools/bpf/bpftool/main.c

@@ -32,6 +32,8 @@ bool verifier_logs;
 bool relaxed_maps;
 bool use_loader;
 struct btf *base_btf;
+struct btf *relocate_base_btf;
+const char *relocate_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' },
+		{ "relocate-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':
@@ -519,6 +522,15 @@ int main(int argc, char **argv)
 		case 'L':
 			use_loader = true;
 			break;
+		case 'R':
+			relocate_base_btf_path = optarg;
+			relocate_base_btf = btf__parse(optarg, NULL);
+			if (!relocate_base_btf) {
+				p_err("failed to parse base BTF for relocation at '%s': %d\n",
+				      optarg, -errno);
+				return -1;
+			}
+			break;
 		default:
 			p_err("unrecognized option '%s'", argv[optind - 1]);
 			if (json_output)

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 *relocate_base_btf;
+extern const char *relocate_base_btf_path;
 extern struct hashmap *refs_table;
 
 void __printf(1, 2) p_err(const char *fmt, ...);