From: Bryce Kahle <bryce.kahle@xxxxxxxxxxxxx>
Enables a user to generate minimized kernel module BTF.
If an eBPF program probes a function within a kernel module or uses
types that come from a kernel module, split BTF is required. The split
module BTF contains only the BTF types that are unique to the module.
It will reference the base/vmlinux BTF types and always starts its type
IDs at X+1 where X is the largest type ID in the base BTF.
Minimization allows a user to ship only the types necessary to do
relocations for the program(s) in the provided eBPF object file(s). A
minimized module BTF will still not contain vmlinux BTF types, so you
should always minimize the vmlinux file first, and then minimize the
kernel module file.
Example:
bpftool gen min_core_btf vmlinux.btf vm-min.btf prog.bpf.o
bpftool -B vm-min.btf gen min_core_btf mod.btf mod-min.btf prog.bpf.o
Signed-off-by: Bryce Kahle <bryce.kahle@xxxxxxxxxxxxx>
---
.../bpf/bpftool/Documentation/bpftool-gen.rst | 18 +++++++++++++++++-
tools/bpf/bpftool/gen.c | 17 ++++++++++++-----
2 files changed, 29 insertions(+), 6 deletions(-)
diff --git a/tools/bpf/bpftool/Documentation/bpftool-gen.rst b/tools/bpf/bpftool/Documentation/bpftool-gen.rst
index 5006e724d..e067d3b05 100644
--- a/tools/bpf/bpftool/Documentation/bpftool-gen.rst
+++ b/tools/bpf/bpftool/Documentation/bpftool-gen.rst
@@ -16,7 +16,7 @@ SYNOPSIS
**bpftool** [*OPTIONS*] **gen** *COMMAND*
- *OPTIONS* := { |COMMON_OPTIONS| | { **-L** | **--use-loader** } }
+ *OPTIONS* := { |COMMON_OPTIONS| | { **-B** | **--base-btf** } | { **-L** | **--use-loader** } }
*COMMAND* := { **object** | **skeleton** | **help** }
@@ -202,6 +202,14 @@ OPTIONS
=======
.. include:: common_options.rst
+ -B, --base-btf *FILE*
+ Pass a base BTF object. Base BTF objects are typically used
+ with BTF objects for kernel modules. To avoid duplicating
+ all kernel symbols required by modules, BTF objects for
+ modules are "split", they are built incrementally on top of
+ the kernel (vmlinux) BTF object. So the base BTF reference
+ should usually point to the kernel BTF.
+
-L, --use-loader
For skeletons, generate a "light" skeleton (also known as "loader"
skeleton). A light skeleton contains a loader eBPF program. It does
@@ -444,3 +452,11 @@ ones given to min_core_btf.
obj = bpf_object__open_file("one.bpf.o", &opts);
...
+
+Kernel module BTF may also be minimized by using the -B option:
+
+**$ bpftool -B 5.4.0-smaller.btf gen min_core_btf 5.4.0-module.btf 5.4.0-module-smaller.btf one.bpf.o**
+
+A minimized module BTF will still not contain vmlinux BTF types, so you
+should always minimize the vmlinux file first, and then minimize the
+kernel module file.
diff --git a/tools/bpf/bpftool/gen.c b/tools/bpf/bpftool/gen.c
index ee3ce2b80..634c809a5 100644
--- a/tools/bpf/bpftool/gen.c
+++ b/tools/bpf/bpftool/gen.c
@@ -1630,6 +1630,7 @@ static int do_help(int argc, char **argv)
" %1$s %2$s help\n"
"\n"
" " HELP_SPEC_OPTIONS " |\n"
+ " {-B|--base-btf} |\n"
" {-L|--use-loader} }\n"
"",
bin_name, "gen");
@@ -1695,14 +1696,14 @@ btfgen_new_info(const char *targ_btf_path)
if (!info)
return NULL;
- info->src_btf = btf__parse(targ_btf_path, NULL);
+ info->src_btf = btf__parse_split(targ_btf_path, base_btf);
if (!info->src_btf) {
err = -errno;
p_err("failed parsing '%s' BTF file: %s", targ_btf_path, strerror(errno));
goto err_out;
}
- info->marked_btf = btf__parse(targ_btf_path, NULL);
+ info->marked_btf = btf__parse_split(targ_btf_path, base_btf);
if (!info->marked_btf) {
err = -errno;
p_err("failed parsing '%s' BTF file: %s", targ_btf_path, strerror(errno));
@@ -2141,10 +2142,16 @@ static struct btf *btfgen_get_btf(struct btfgen_info *info)
{
struct btf *btf_new = NULL;
unsigned int *ids = NULL;
+ const struct btf *base;
unsigned int i, n = btf__type_cnt(info->marked_btf);
+ int start_id = 1;
int err = 0;
- btf_new = btf__new_empty();
+ base = btf__base_btf(info->src_btf);
+ if (base)
+ start_id = btf__type_cnt(base);
+
+ btf_new = btf__new_empty_split((struct btf *)base);
if (!btf_new) {
err = -errno;
goto err_out;
@@ -2157,7 +2164,7 @@ static struct btf *btfgen_get_btf(struct btfgen_info *info)
}
/* first pass: add all marked types to btf_new and add their new ids to the ids map */
- for (i = 1; i < n; i++) {
+ for (i = start_id; i < n; i++) {
const struct btf_type *cloned_type, *type;
const char *name;
int new_id;
@@ -2213,7 +2220,7 @@ static struct btf *btfgen_get_btf(struct btfgen_info *info)
}
/* second pass: fix up type ids */
- for (i = 1; i < btf__type_cnt(btf_new); i++) {
+ for (i = start_id; i < btf__type_cnt(btf_new); i++) {
struct btf_type *btf_type = (struct btf_type *) btf__type_by_id(btf_new, i);
err = btf_type_visit_type_ids(btf_type, btfgen_remap_id, ids);
--
2.25.1
