Re: [PATCH v2 bpf-next 14/16] libbpf: Generate loader program out of BPF ELF file.

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Mon, Apr 26, 2021 at 03:22:36PM -0700, Andrii Nakryiko wrote:
> On Thu, Apr 22, 2021 at 5:27 PM Alexei Starovoitov
> <alexei.starovoitov@gmail.com> wrote:
> >
> > From: Alexei Starovoitov <ast@kernel.org>
> >
> > The BPF program loading process performed by libbpf is quite complex
> > and consists of the following steps:
> > "open" phase:
> > - parse elf file and remember relocations, sections
> > - collect externs and ksyms including their btf_ids in prog's BTF
> > - patch BTF datasec (since llvm couldn't do it)
> > - init maps (old style map_def, BTF based, global data map, kconfig map)
> > - collect relocations against progs and maps
> > "load" phase:
> > - probe kernel features
> > - load vmlinux BTF
> > - resolve externs (kconfig and ksym)
> > - load program BTF
> > - init struct_ops
> > - create maps
> > - apply CO-RE relocations
> > - patch ld_imm64 insns with src_reg=PSEUDO_MAP, PSEUDO_MAP_VALUE, PSEUDO_BTF_ID
> > - reposition subprograms and adjust call insns
> > - sanitize and load progs
> >
> > During this process libbpf does sys_bpf() calls to load BTF, create maps,
> > populate maps and finally load programs.
> > Instead of actually doing the syscalls generate a trace of what libbpf
> > would have done and represent it as the "loader program".
> > The "loader program" consists of single map with:
> > - union bpf_attr(s)
> > - BTF bytes
> > - map value bytes
> > - insns bytes
> > and single bpf program that passes bpf_attr(s) and data into bpf_sys_bpf() helper.
> > Executing such "loader program" via bpf_prog_test_run() command will
> > replay the sequence of syscalls that libbpf would have done which will result
> > the same maps created and programs loaded as specified in the elf file.
> > The "loader program" removes libelf and majority of libbpf dependency from
> > program loading process.
> >
> > kconfig, typeless ksym, struct_ops and CO-RE are not supported yet.
> >
> > The order of relocate_data and relocate_calls had to change, so that
> > bpf_gen__prog_load() can see all relocations for a given program with
> > correct insn_idx-es.
> >
> > Signed-off-by: Alexei Starovoitov <ast@kernel.org>
> > ---
> >  tools/lib/bpf/Build              |   2 +-
> >  tools/lib/bpf/bpf_gen_internal.h |  40 ++
> >  tools/lib/bpf/gen_loader.c       | 615 +++++++++++++++++++++++++++++++
> >  tools/lib/bpf/libbpf.c           | 204 ++++++++--
> >  tools/lib/bpf/libbpf.h           |  12 +
> >  tools/lib/bpf/libbpf.map         |   1 +
> >  tools/lib/bpf/libbpf_internal.h  |   2 +
> >  tools/lib/bpf/skel_internal.h    | 105 ++++++
> >  8 files changed, 948 insertions(+), 33 deletions(-)
> >  create mode 100644 tools/lib/bpf/bpf_gen_internal.h
> >  create mode 100644 tools/lib/bpf/gen_loader.c
> >  create mode 100644 tools/lib/bpf/skel_internal.h
> >
> > diff --git a/tools/lib/bpf/Build b/tools/lib/bpf/Build
> > index 9b057cc7650a..430f6874fa41 100644
> > --- a/tools/lib/bpf/Build
> > +++ b/tools/lib/bpf/Build
> > @@ -1,3 +1,3 @@
> >  libbpf-y := libbpf.o bpf.o nlattr.o btf.o libbpf_errno.o str_error.o \
> >             netlink.o bpf_prog_linfo.o libbpf_probes.o xsk.o hashmap.o \
> > -           btf_dump.o ringbuf.o strset.o linker.o
> > +           btf_dump.o ringbuf.o strset.o linker.o gen_loader.o
> > diff --git a/tools/lib/bpf/bpf_gen_internal.h b/tools/lib/bpf/bpf_gen_internal.h
> > new file mode 100644
> > index 000000000000..dc3e2cbf9ce3
> > --- /dev/null
> > +++ b/tools/lib/bpf/bpf_gen_internal.h
> > @@ -0,0 +1,40 @@
> > +/* SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause) */
> > +/* Copyright (c) 2021 Facebook */
> > +#ifndef __BPF_GEN_INTERNAL_H
> > +#define __BPF_GEN_INTERNAL_H
> > +
> > +struct relo_desc {
> 
> there is very similarly named reloc_desc struct in libbpf.c, can you
> rename it to something like gen_btf_relo_desc?

sure.

> > +       const char *name;
> > +       int kind;
> > +       int insn_idx;
> > +};
> > +
> 
> [...]
> 
> > +
> > +static int bpf_gen__realloc_insn_buf(struct bpf_gen *gen, __u32 size)
> > +{
> > +       size_t off = gen->insn_cur - gen->insn_start;
> > +
> > +       if (gen->error)
> > +               return gen->error;
> > +       if (size > INT32_MAX || off + size > INT32_MAX) {
> > +               gen->error = -ERANGE;
> > +               return -ERANGE;
> > +       }
> > +       gen->insn_start = realloc(gen->insn_start, off + size);
> 
> leaking memory here: gen->insn_start will be NULL on failure

ohh. good catch.

> > +       if (!gen->insn_start) {
> > +               gen->error = -ENOMEM;
> > +               return -ENOMEM;
> > +       }
> > +       gen->insn_cur = gen->insn_start + off;
> > +       return 0;
> > +}
> > +
> > +static int bpf_gen__realloc_data_buf(struct bpf_gen *gen, __u32 size)
> > +{
> > +       size_t off = gen->data_cur - gen->data_start;
> > +
> > +       if (gen->error)
> > +               return gen->error;
> > +       if (size > INT32_MAX || off + size > INT32_MAX) {
> > +               gen->error = -ERANGE;
> > +               return -ERANGE;
> > +       }
> > +       gen->data_start = realloc(gen->data_start, off + size);
> 
> same as above
> 
> > +       if (!gen->data_start) {
> > +               gen->error = -ENOMEM;
> > +               return -ENOMEM;
> > +       }
> > +       gen->data_cur = gen->data_start + off;
> > +       return 0;
> > +}
> > +
> 
> [...]
> 
> > +
> > +static void bpf_gen__emit_sys_bpf(struct bpf_gen *gen, int cmd, int attr, int attr_size)
> > +{
> > +       bpf_gen__emit(gen, BPF_MOV64_IMM(BPF_REG_1, cmd));
> > +       bpf_gen__emit2(gen, BPF_LD_IMM64_RAW_FULL(BPF_REG_2, BPF_PSEUDO_MAP_IDX_VALUE, 0, 0, 0, attr));
> 
> is attr an offset into a blob? if yes, attr_off? or attr_base_off,
> anything with _off

yes. it's an offset into a blob, but I don't use _off anywhere
otherwise all variables through out would have to have _off which is too verbose.

> > +       bpf_gen__emit(gen, BPF_MOV64_IMM(BPF_REG_3, attr_size));
> > +       bpf_gen__emit(gen, BPF_EMIT_CALL(BPF_FUNC_sys_bpf));
> > +       /* remember the result in R7 */
> > +       bpf_gen__emit(gen, BPF_MOV64_REG(BPF_REG_7, BPF_REG_0));
> > +}
> > +
> > +static void bpf_gen__emit_check_err(struct bpf_gen *gen)
> > +{
> > +       bpf_gen__emit(gen, BPF_JMP_IMM(BPF_JSGE, BPF_REG_7, 0, 2));
> > +       bpf_gen__emit(gen, BPF_MOV64_REG(BPF_REG_0, BPF_REG_7));
> > +       bpf_gen__emit(gen, BPF_EXIT_INSN());
> > +}
> > +
> > +static void __bpf_gen__debug(struct bpf_gen *gen, int reg1, int reg2, const char *fmt, va_list args)
> 
> Can you please leave a comment on what reg1 and reg2 is, it's not very
> clear and the code clearly assumes that it can't be reg[1-4]. It's
> probably those special R7 and R9 (or -1, of course), but having a
> short comment makes sense to not jump around trying to figure out
> possible inputs.
> 
> Oh, reading further, it can also be R0.

good point. will add a comment.

> > +{
> > +       char buf[1024];
> > +       int addr, len, ret;
> > +
> > +       if (!gen->log_level)
> > +               return;
> > +       ret = vsnprintf(buf, sizeof(buf), fmt, args);
> > +       if (ret < 1024 - 7 && reg1 >= 0 && reg2 < 0)
> > +               /* The special case to accommodate common bpf_gen__debug_ret():
> > +                * to avoid specifying BPF_REG_7 and adding " r=%%d" to prints explicitly.
> > +                */
> > +               strcat(buf, " r=%d");
> > +       len = strlen(buf) + 1;
> > +       addr = bpf_gen__add_data(gen, buf, len);
> 
> nit: offset, not address, right?

it's actually an address.
From pov of the program and insn below:

> > +       bpf_gen__emit2(gen, BPF_LD_IMM64_RAW_FULL(BPF_REG_1, BPF_PSEUDO_MAP_IDX_VALUE, 0, 0, 0, addr));

I guess it's kinda both. It's an offset within global data, but no one calls
int var;
&var -> taking an offset here? no. It's taking an address of the glob var.
Our implementation of global data is via single map value element.
So global vars have offsets too.
imo addr is more accurate here and through out this file.

> > +       bpf_gen__emit(gen, BPF_MOV64_IMM(BPF_REG_2, len));
> > +       if (reg1 >= 0)
> > +               bpf_gen__emit(gen, BPF_MOV64_REG(BPF_REG_3, reg1));
> > +       if (reg2 >= 0)
> > +               bpf_gen__emit(gen, BPF_MOV64_REG(BPF_REG_4, reg2));
> > +       bpf_gen__emit(gen, BPF_EMIT_CALL(BPF_FUNC_trace_printk));
> > +}
> > +
> 
> [...]
> 
> > +int bpf_gen__finish(struct bpf_gen *gen)
> > +{
> > +       int i;
> > +
> > +       bpf_gen__emit_sys_close_stack(gen, stack_off(btf_fd));
> > +       for (i = 0; i < gen->nr_progs; i++)
> > +               bpf_gen__move_stack2ctx(gen,
> > +                                       sizeof(struct bpf_loader_ctx) +
> > +                                       sizeof(struct bpf_map_desc) * gen->nr_maps +
> > +                                       sizeof(struct bpf_prog_desc) * i +
> > +                                       offsetof(struct bpf_prog_desc, prog_fd), 4,
> > +                                       stack_off(prog_fd[i]));
> > +       for (i = 0; i < gen->nr_maps; i++)
> > +               bpf_gen__move_stack2ctx(gen,
> > +                                       sizeof(struct bpf_loader_ctx) +
> > +                                       sizeof(struct bpf_map_desc) * i +
> > +                                       offsetof(struct bpf_map_desc, map_fd), 4,
> > +                                       stack_off(map_fd[i]));
> > +       bpf_gen__emit(gen, BPF_MOV64_IMM(BPF_REG_0, 0));
> > +       bpf_gen__emit(gen, BPF_EXIT_INSN());
> > +       pr_debug("bpf_gen__finish %d\n", gen->error);
> 
> maybe prefix all those pr_debug()s with "gen: " to distinguish them
> from the rest of libbpf logging?

sure.

> > +       if (!gen->error) {
> > +               struct gen_loader_opts *opts = gen->opts;
> > +
> > +               opts->insns = gen->insn_start;
> > +               opts->insns_sz = gen->insn_cur - gen->insn_start;
> > +               opts->data = gen->data_start;
> > +               opts->data_sz = gen->data_cur - gen->data_start;
> > +       }
> > +       return gen->error;
> > +}
> > +
> > +void bpf_gen__free(struct bpf_gen *gen)
> > +{
> > +       if (!gen)
> > +               return;
> > +       free(gen->data_start);
> > +       free(gen->insn_start);
> > +       gen->data_start = NULL;
> > +       gen->insn_start = NULL;
> 
> what's the point of NULL'ing them out if you don't clear gen->data_cur
> and gen->insn_cur?

To spot the bugs quicker if there are issues.

> also should it free(gen) itself?

ohh. bpf_object__close() should probably do it to stay symmetrical with calloc.

> > +}
> > +
> > +void bpf_gen__load_btf(struct bpf_gen *gen, const void *btf_raw_data, __u32 btf_raw_size)
> > +{
> > +       union bpf_attr attr = {};
> 
> here and below: memset(0)?

that's unnecessary. there is no backward/forward compat issue here.
and bpf_attr doesn't have gaps inside.

> > +       int attr_size = offsetofend(union bpf_attr, btf_log_level);
> > +       int btf_data, btf_load_attr;
> > +
> > +       pr_debug("btf_load: size %d\n", btf_raw_size);
> > +       btf_data = bpf_gen__add_data(gen, btf_raw_data, btf_raw_size);
> > +
> 
> [...]
> 
> > +       map_create_attr = bpf_gen__add_data(gen, &attr, attr_size);
> > +       if (attr.btf_value_type_id)
> > +               /* populate union bpf_attr with btf_fd saved in the stack earlier */
> > +               bpf_gen__move_stack2blob(gen, map_create_attr + offsetof(union bpf_attr, btf_fd), 4,
> > +                                        stack_off(btf_fd));
> > +       switch (attr.map_type) {
> > +       case BPF_MAP_TYPE_ARRAY_OF_MAPS:
> > +       case BPF_MAP_TYPE_HASH_OF_MAPS:
> > +               bpf_gen__move_stack2blob(gen, map_create_attr + offsetof(union bpf_attr, inner_map_fd),
> > +                                        4, stack_off(inner_map_fd));
> > +               close_inner_map_fd = true;
> > +               break;
> > +       default:;
> 
> default:
>     break;

why?

> > +       }
> > +       /* emit MAP_CREATE command */
> > +       bpf_gen__emit_sys_bpf(gen, BPF_MAP_CREATE, map_create_attr, attr_size);
> > +       bpf_gen__debug_ret(gen, "map_create %s idx %d type %d value_size %d",
> > +                          attr.map_name, map_idx, map_attr->map_type, attr.value_size);
> > +       bpf_gen__emit_check_err(gen);
> 
> what will happen on error with inner_map_fd and all the other fds
> created by now?

that's a todo item. I'll add a comment that error path is far from perfect.

> > +       /* remember map_fd in the stack, if successful */
> > +       if (map_idx < 0) {
> > +               /* This bpf_gen__map_create() function is called with map_idx >= 0 for all maps
> > +                * that libbpf loading logic tracks.
> > +                * It's called with -1 to create an inner map.
> > +                */
> > +               bpf_gen__emit(gen, BPF_STX_MEM(BPF_W, BPF_REG_10, BPF_REG_7, stack_off(inner_map_fd)));
> > +       } else {
> > +               if (map_idx != gen->nr_maps) {
> 
> why would that happen? defensive programming? and even then `if () {}
> else if () {} else {}` structure is more appropriate

sure. will use that style.

> > +                       gen->error = -EDOM; /* internal bug */
> > +                       return;
> > +               }
> > +               bpf_gen__emit(gen, BPF_STX_MEM(BPF_W, BPF_REG_10, BPF_REG_7, stack_off(map_fd[map_idx])));
> > +               gen->nr_maps++;
> > +       }
> > +       if (close_inner_map_fd)
> > +               bpf_gen__emit_sys_close_stack(gen, stack_off(inner_map_fd));
> > +}
> > +
> 
> [...]
> 
> > +static void bpf_gen__cleanup_relos(struct bpf_gen *gen, int insns)
> > +{
> > +       int i, insn;
> > +
> > +       for (i = 0; i < gen->relo_cnt; i++) {
> > +               if (gen->relos[i].kind != BTF_KIND_VAR)
> > +                       continue;
> > +               /* close fd recorded in insn[insn_idx + 1].imm */
> > +               insn = insns + sizeof(struct bpf_insn) * (gen->relos[i].insn_idx + 1)
> > +                       + offsetof(struct bpf_insn, imm);
> > +               bpf_gen__emit_sys_close_blob(gen, insn);
> 
> wouldn't this close the same FD used across multiple "relos" multiple times?

no. since every relo has its own fd.
Right now the loader gen is simple and doesn't do preprocessing of
all relos for all progs to avoid complicating the code.
It's good enough for now.

> > +       }
> > +       if (gen->relo_cnt) {
> > +               free(gen->relos);
> > +               gen->relo_cnt = 0;
> > +               gen->relos = NULL;
> > +       }
> > +}
> > +
> 
> [...]
> 
> > +       struct bpf_gen *gen_loader;
> > +
> >         /*
> >          * Information when doing elf related work. Only valid if fd
> >          * is valid.
> > @@ -2651,7 +2654,15 @@ static int bpf_object__sanitize_and_load_btf(struct bpf_object *obj)
> >                 bpf_object__sanitize_btf(obj, kern_btf);
> >         }
> >
> > -       err = btf__load(kern_btf);
> > +       if (obj->gen_loader) {
> > +               __u32 raw_size = 0;
> > +               const void *raw_data = btf__get_raw_data(kern_btf, &raw_size);
> 
> this can return NULL on ENOMEM

good point.

> > +
> > +               bpf_gen__load_btf(obj->gen_loader, raw_data, raw_size);
> > +               btf__set_fd(kern_btf, 0);
> 
> why setting fd to 0 (stdin)? does gen depend on this somewhere? The
> problem is that it will eventually be closed on btf__free(), which
> will close stdin, causing a big surprise. What will happen if you
> leave it at -1?

unfortunately there are various piece of the code that check <0
means not init.
I can try to fix them all, but it felt unncessary complex vs screwing up stdin.
It's bpftool's stdin, but you're right that it's not pretty.
I can probably use special very large FD number and check for it.
Still cleaner than fixing all checks.

> 
> > +       } else {
> > +               err = btf__load(kern_btf);
> > +       }
> >         if (sanitize) {
> >                 if (!err) {
> >                         /* move fd to libbpf's BTF */
> > @@ -4262,6 +4273,12 @@ static bool kernel_supports(const struct bpf_object *obj, enum kern_feature_id f
> >         struct kern_feature_desc *feat = &feature_probes[feat_id];
> >         int ret;
> >
> > +       if (obj->gen_loader)
> > +               /* To generate loader program assume the latest kernel
> > +                * to avoid doing extra prog_load, map_create syscalls.
> > +                */
> > +               return true;
> > +
> >         if (READ_ONCE(feat->res) == FEAT_UNKNOWN) {
> >                 ret = feat->probe();
> >                 if (ret > 0) {
> > @@ -4344,6 +4361,13 @@ bpf_object__populate_internal_map(struct bpf_object *obj, struct bpf_map *map)
> >         char *cp, errmsg[STRERR_BUFSIZE];
> >         int err, zero = 0;
> >
> > +       if (obj->gen_loader) {
> > +               bpf_gen__map_update_elem(obj->gen_loader, map - obj->maps,
> 
> it would be great for bpf_gen__map_update_elem to reflect that it's
> not a generic map_update_elem() call, rather special internal map
> update (just use bpf_gen__populate_internal_map?) Whether to freeze or
> not could be just a flag to the same call, they always go together.

It's actually generic map_update_elem. I haven't used it in inner map init yet.
Still on my todo list.

> > +                                        map->mmaped, map->def.value_size);
> > +               if (map_type == LIBBPF_MAP_RODATA || map_type == LIBBPF_MAP_KCONFIG)
> > +                       bpf_gen__map_freeze(obj->gen_loader, map - obj->maps);
> > +               return 0;
> > +       }
> >         err = bpf_map_update_elem(map->fd, &zero, map->mmaped, 0);
> >         if (err) {
> >                 err = -errno;
> > @@ -4369,7 +4393,7 @@ bpf_object__populate_internal_map(struct bpf_object *obj, struct bpf_map *map)
> >
> >  static void bpf_map__destroy(struct bpf_map *map);
> >
> > -static int bpf_object__create_map(struct bpf_object *obj, struct bpf_map *map)
> > +static int bpf_object__create_map(struct bpf_object *obj, struct bpf_map *map, bool is_inner)
> >  {
> >         struct bpf_create_map_attr create_attr;
> >         struct bpf_map_def *def = &map->def;
> > @@ -4415,9 +4439,9 @@ static int bpf_object__create_map(struct bpf_object *obj, struct bpf_map *map)
> >
> >         if (bpf_map_type__is_map_in_map(def->type)) {
> >                 if (map->inner_map) {
> > -                       int err;
> > +                       int err = 0;
> 
> no need to initialize to zero, you are assigning it right below
> 
> >
> > -                       err = bpf_object__create_map(obj, map->inner_map);
> > +                       err = bpf_object__create_map(obj, map->inner_map, true);
> >                         if (err) {
> >                                 pr_warn("map '%s': failed to create inner map: %d\n",
> >                                         map->name, err);
> 
> [...]
> 
> > @@ -4469,7 +4498,12 @@ static int init_map_slots(struct bpf_map *map)
> >
> >                 targ_map = map->init_slots[i];
> >                 fd = bpf_map__fd(targ_map);
> > -               err = bpf_map_update_elem(map->fd, &i, &fd, 0);
> > +               if (obj->gen_loader) {
> > +                       printf("// TODO map_update_elem: idx %ld key %d value==map_idx %ld\n",
> > +                              map - obj->maps, i, targ_map - obj->maps);
> 
> return error for now?
> 
> > +               } else {
> > +                       err = bpf_map_update_elem(map->fd, &i, &fd, 0);
> > +               }
> >                 if (err) {
> >                         err = -errno;
> >                         pr_warn("map '%s': failed to initialize slot [%d] to map '%s' fd=%d: %d\n",
> 
> [...]
> 
> > @@ -6082,6 +6119,11 @@ static int bpf_core_apply_relo(struct bpf_program *prog,
> >         if (str_is_empty(spec_str))
> >                 return -EINVAL;
> >
> > +       if (prog->obj->gen_loader) {
> > +               printf("// TODO core_relo: prog %ld insn[%d] %s %s kind %d\n",
> > +                      prog - prog->obj->programs, relo->insn_off / 8,
> > +                      local_name, spec_str, relo->kind);
> 
> same, return error? Drop printf, maybe leave pr_debug()?

sure. pr_debug with error sounds fine.

> > +       }
> >         err = bpf_core_parse_spec(local_btf, local_id, spec_str, relo->kind, &local_spec);
> >         if (err) {
> >                 pr_warn("prog '%s': relo #%d: parsing [%d] %s %s + %s failed: %d\n",
> > @@ -6821,6 +6863,19 @@ bpf_object__relocate_calls(struct bpf_object *obj, struct bpf_program *prog)
> >
> >         return 0;
> >  }
> 
> empty line here
> 
> > +static void
> > +bpf_object__free_relocs(struct bpf_object *obj)
> > +{
> > +       struct bpf_program *prog;
> > +       int i;
> > +
> > +       /* free up relocation descriptors */
> > +       for (i = 0; i < obj->nr_programs; i++) {
> > +               prog = &obj->programs[i];
> > +               zfree(&prog->reloc_desc);
> > +               prog->nr_reloc = 0;
> > +       }
> > +}
> >
> 
> [...]
> 
> > +static int bpf_program__record_externs(struct bpf_program *prog)
> > +{
> > +       struct bpf_object *obj = prog->obj;
> > +       int i;
> > +
> > +       for (i = 0; i < prog->nr_reloc; i++) {
> > +               struct reloc_desc *relo = &prog->reloc_desc[i];
> > +               struct extern_desc *ext = &obj->externs[relo->sym_off];
> > +
> > +               switch (relo->type) {
> > +               case RELO_EXTERN_VAR:
> > +                       if (ext->type != EXT_KSYM)
> > +                               continue;
> > +                       if (!ext->ksym.type_id) /* typeless ksym */
> > +                               continue;
> 
> this shouldn't be silently ignored, if it's not supported, it should
> return error

agree

> > +                       bpf_gen__record_extern(obj->gen_loader, ext->name, BTF_KIND_VAR,
> > +                                              relo->insn_idx);
> > +                       break;
> > +               case RELO_EXTERN_FUNC:
> > +                       bpf_gen__record_extern(obj->gen_loader, ext->name, BTF_KIND_FUNC,
> > +                                              relo->insn_idx);
> > +                       break;
> > +               default:
> > +                       continue;
> > +               }
> > +       }
> > +       return 0;
> > +}
> > +
> 
> [...]
> 
> > @@ -7868,6 +7970,9 @@ int bpf_object__load_xattr(struct bpf_object_load_attr *attr)
> >         err = err ? : bpf_object__relocate(obj, attr->target_btf_path);
> >         err = err ? : bpf_object__load_progs(obj, attr->log_level);
> >
> > +       if (obj->gen_loader && !err)
> > +               err = bpf_gen__finish(obj->gen_loader);
> > +
> >         /* clean up module BTFs */
> >         for (i = 0; i < obj->btf_module_cnt; i++) {
> >                 close(obj->btf_modules[i].fd);
> > @@ -8493,6 +8598,7 @@ void bpf_object__close(struct bpf_object *obj)
> >         if (obj->clear_priv)
> >                 obj->clear_priv(obj, obj->priv);
> 
> bpf_object__close() will close all those FD=0 in maps/progs, that's not good
> 
> >
> > +       bpf_gen__free(obj->gen_loader);
> >         bpf_object__elf_finish(obj);
> >         bpf_object__unload(obj);
> >         btf__free(obj->btf);
> 
> [...]
> 
> > @@ -9387,7 +9521,13 @@ static int libbpf_find_attach_btf_id(struct bpf_program *prog, int *btf_obj_fd,
> >         }
> >
> >         /* kernel/module BTF ID */
> > -       err = find_kernel_btf_id(prog->obj, attach_name, attach_type, btf_obj_fd, btf_type_id);
> > +       if (prog->obj->gen_loader) {
> > +               bpf_gen__record_attach_target(prog->obj->gen_loader, attach_name, attach_type);
> > +               *btf_obj_fd = 0;
> 
> this will leak kernel module BTF FDs

I don't follow.
When gen_loader is happening the find_kernel_btf_id() is not called and modules BTFs are not loaded.

> > +               *btf_type_id = 1;
> > +       } else {
> > +               err = find_kernel_btf_id(prog->obj, attach_name, attach_type, btf_obj_fd, btf_type_id);
> > +       }
> >         if (err) {
> >                 pr_warn("failed to find kernel BTF type ID of '%s': %d\n", attach_name, err);
> >                 return err;
> 
> [...]
> 
> > +out:
> > +       close(map_fd);
> > +       close(prog_fd);
> 
> this does close(-1), check >= 0

Right. I felt there is no risk in doing that. I guess extra check is fine too.