[PATCH bpf-next] bpftool: add support for split BTF to gen min_core_btf

[PATCH bpf-next] bpftool: add support for split BTF to gen min_core_btf

[Bryce Kahle]

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@datadoghq.com>
---
 .../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 5006e724d1bc..9c357d339000 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 ee3ce2b8000d..634c809a5173 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);
-- 
2.39.2

Re: [PATCH bpf-next] bpftool: add support for split BTF to gen min_core_btf

[Yonghong Song]

On 1/19/24 1:18 PM, Bryce Kahle wrote:
> 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@datadoghq.com>
> ---
>   .../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 5006e724d1bc..9c357d339000 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 ee3ce2b8000d..634c809a5173 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;

Looks like you have formating issues here. The proper identation is gone.

>    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));

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