From b719879138254a4f4a526ff274f7b3104f21317a Mon Sep 17 00:00:00 2001 From: Bryce Kahle Date: Fri, 12 Jan 2024 17:32:48 -0800 Subject: [PATCH] bpftool: add support for split BTF to gen min_core_btf 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 vmlinux-min.btf prog.bpf.o bpftool -B vmlinux-min.btf gen min_core_btf module.btf module-min.btf prog.bpf.o Signed-off-by: Bryce Kahle --- docs/bpftool-gen.rst | 20 +++++++++++++++++++- src/gen.c | 17 ++++++++++++----- 2 files changed, 31 insertions(+), 6 deletions(-) diff --git a/docs/bpftool-gen.rst b/docs/bpftool-gen.rst index 5006e724..59f983df 100644 --- a/docs/bpftool-gen.rst +++ b/docs/bpftool-gen.rst @@ -16,7 +16,7 @@ SYNOPSIS **bpftool** [*OPTIONS*] **gen** *COMMAND* - *OPTIONS* := { |COMMON_OPTIONS| | { **-L** | **--use-loader** } } + *OPTIONS* := { |COMMON_OPTIONS| | { **-L** | **--use-loader** } | { **-B** | **--base-btf** } } *COMMAND* := { **object** | **skeleton** | **help** } @@ -208,6 +208,20 @@ OPTIONS not use the majority of the libbpf infrastructure, and does not need libelf. + -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. + + When the main BTF object to process (for example, the + module BTF to dump) is passed as a *FILE*, bpftool attempts + 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. + EXAMPLES ======== **$ cat example1.bpf.c** @@ -444,3 +458,7 @@ 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-example.btf gen min_core_btf 5.4.0-module.btf 5.4.0-module-smaller.btf one.bpf.o** diff --git a/src/gen.c b/src/gen.c index ee3ce2b8..c03e6701 100644 --- a/src/gen.c +++ b/src/gen.c @@ -1631,6 +1631,7 @@ static int do_help(int argc, char **argv) "\n" " " HELP_SPEC_OPTIONS " |\n" " {-L|--use-loader} }\n" + " {-B|--base-btf} }\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);