Re: [PATCH bpf-next v2 5/7] bpf, libbpf: support global data/bss/rodata sections

[Daniel Borkmann <daniel@iogearbox.net>]

On 03/01/2019 07:53 AM, Andrii Nakryiko wrote:
> On Thu, Feb 28, 2019 at 3:31 PM Daniel Borkmann <daniel@iogearbox.net> wrote:
>>
>> This work adds BPF loader support for global data sections
>> to libbpf. This allows to write BPF programs in more natural
>> C-like way by being able to define global variables and const
>> data.
>>
>> Back at LPC 2018 [0] we presented a first prototype which
>> implemented support for global data sections by extending BPF
>> syscall where union bpf_attr would get additional memory/size
>> pair for each section passed during prog load in order to later
>> add this base address into the ldimm64 instruction along with
>> the user provided offset when accessing a variable. Consensus
>> from LPC was that for proper upstream support, it would be
>> more desirable to use maps instead of bpf_attr extension as
>> this would allow for introspection of these sections as well
>> as potential life updates of their content. This work follows
>> this path by taking the following steps from loader side:
>>
>>  1) In bpf_object__elf_collect() step we pick up ".data",
>>     ".rodata", and ".bss" section information.
>>
>>  2) If present, in bpf_object__init_global_maps() we create
>>     a map that corresponds to each of the present sections.
> 
> Is there any point in having .data and .bss in separate maps? I can
> only see for reasons of inspectiion from bpftool, but other than that
> isn't .bss just an optimization over .data to save space in ELF file,
> but in other regards is just another part of r/w .data section?

Hmm, I actually don't mind too much combining both of them. Had
the same thought with regards to introspection from bpftool which
was why I separated them. But combining the two into a single map
is fine actually, saves a bit of resources in kernel, and offsets
can easily be fixed up from libbpf side. Will do for v3.

>>     Given section size and access properties can differ, a
>>     single entry array map is created with value size that
>>     is corresponding to the ELF section size of .data, .bss
>>     or .rodata. In the latter case, the map is created as
>>     read-only from program side such that verifier rejects
>>     any write attempts into .rodata. In a subsequent step,
>>     for .data and .rodata sections, the section content is
>>     copied into the map through bpf_map_update_elem(). For
>>     .bss this is not necessary since array map is already
>>     zero-initialized by default.
> 
> For .rodata, ideally it would be nice to make it RDONLY from userland
> as well, except for first UPDATE. How hard is it to support that?

Right now the BPF_F_RDONLY, BPF_F_WRONLY semantics to make the
maps read-only or write-only from syscall side are that these
permissions are stored into the struct file front end (file->f_mode)
for the anon inode we use, meaning it's separated from the actual
BPF map, so you can create the map with BPF_F_RDONLY, but root
user can do BPF_MAP_GET_FD_BY_ID without the BPF_F_RDONLY and
again write into it. This design choice would require that we'd
need to add some additional infrastructure on top of this, which
would then need to enforce file->f_mode to read-only after the
first setup. I think there's simple trick we can apply to make
it read-only after setup from syscall side: we'll add a new flag
to the map, and then upon map creation libbpf sets everything
up, holds the id, closes its fd, and refetches the fd by id.
From that point onwards any interface where you would get the
fd from the map in user space will enforce BPF_F_RDONLY behavior
for file->f_mode. Another, less hacky option could be to extend
the struct file ops we currently use for BPF maps and set a
map 'immutable' flag from there which is then enforced once all
pending operations have completed. I can look a bit into this.

>>  3) In bpf_program__collect_reloc() step, we record the
>>     corresponding map, insn index, and relocation type for
>>     the global data.
>>
>>  4) And last but not least in the actual relocation step in
>>     bpf_program__relocate(), we mark the ldimm64 instruction
>>     with src_reg = BPF_PSEUDO_MAP_VALUE where in the first
>>     imm field the map's file descriptor is stored as similarly
>>     done as in BPF_PSEUDO_MAP_FD, and in the second imm field
>>     (as ldimm64 is 2-insn wide) we store the access offset
>>     into the section.
>>
>>  5) On kernel side, this special marked BPF_PSEUDO_MAP_VALUE
>>     load will then store the actual target address in order
>>     to have a 'map-lookup'-free access. That is, the actual
>>     map value base address + offset. The destination register
>>     in the verifier will then be marked as PTR_TO_MAP_VALUE,
>>     containing the fixed offset as reg->off and backing BPF
>>     map as reg->map_ptr. Meaning, it's treated as any other
>>     normal map value from verification side, only with
>>     efficient, direct value access instead of actual call to
>>     map lookup helper as in the typical case.
>>
>> Simple example dump of program using globals vars in each
>> section:
>>
>>   # readelf -a test_global_data.o
>>   [...]
>>   [ 6] .bss              NOBITS           0000000000000000  00000328
>>        0000000000000010  0000000000000000  WA       0     0     8
>>   [ 7] .data             PROGBITS         0000000000000000  00000328
>>        0000000000000010  0000000000000000  WA       0     0     8
>>   [ 8] .rodata           PROGBITS         0000000000000000  00000338
>>        0000000000000018  0000000000000000   A       0     0     8
>>   [...]
>>     95: 0000000000000000     8 OBJECT  LOCAL  DEFAULT    6 static_bss
>>     96: 0000000000000008     8 OBJECT  LOCAL  DEFAULT    6 static_bss2
>>     97: 0000000000000000     8 OBJECT  LOCAL  DEFAULT    7 static_data
>>     98: 0000000000000008     8 OBJECT  LOCAL  DEFAULT    7 static_data2
>>     99: 0000000000000000     8 OBJECT  LOCAL  DEFAULT    8 static_rodata
>>    100: 0000000000000008     8 OBJECT  LOCAL  DEFAULT    8 static_rodata2
>>    101: 0000000000000010     8 OBJECT  LOCAL  DEFAULT    8 static_rodata3
>>   [...]
>>
>>   # bpftool prog
>>   103: sched_cls  name load_static_dat  tag 37a8b6822fc39a29  gpl
>>        loaded_at 2019-02-28T02:02:35+0000  uid 0
>>        xlated 712B  jited 426B  memlock 4096B  map_ids 63,64,65,66
>>   # bpftool map show id 63
>>   63: array  name .bss  flags 0x0                      <-- .bss area, rw
>>       key 4B  value 16B  max_entries 1  memlock 4096B
>>   # bpftool map show id 64
>>   64: array  name .data  flags 0x0                     <-- .data area, rw
>>       key 4B  value 16B  max_entries 1  memlock 4096B
>>   # bpftool map show id 65
>>   65: array  name .rodata  flags 0x80                  <-- .rodata area, ro
>>       key 4B  value 24B  max_entries 1  memlock 4096B
>>
>>   # bpftool prog dump xlated id 103
>>   int load_static_data(struct __sk_buff * skb):
>>   ; int load_static_data(struct __sk_buff *skb)
>>      0: (b7) r1 = 0
>>   ; key = 0;
>>      1: (63) *(u32 *)(r10 -4) = r1
>>      2: (bf) r6 = r10
>>   ; int load_static_data(struct __sk_buff *skb)
>>      3: (07) r6 += -4
>>   ; bpf_map_update_elem(&result, &key, &static_bss, 0);
>>      4: (18) r1 = map[id:66]
>>      6: (bf) r2 = r6
>>      7: (18) r3 = map[id:63][0]+0         <-- direct static_bss addr in .bss area
>>      9: (b7) r4 = 0
>>     10: (85) call array_map_update_elem#99888
>>     11: (b7) r1 = 1
>>   ; key = 1;
>>     12: (63) *(u32 *)(r10 -4) = r1
>>   ; bpf_map_update_elem(&result, &key, &static_data, 0);
>>     13: (18) r1 = map[id:66]
>>     15: (bf) r2 = r6
>>     16: (18) r3 = map[id:64][0]+0         <-- direct static_data addr in .data area
>>     18: (b7) r4 = 0
>>     19: (85) call array_map_update_elem#99888
>>     20: (b7) r1 = 2
>>   ; key = 2;
>>     21: (63) *(u32 *)(r10 -4) = r1
>>   ; bpf_map_update_elem(&result, &key, &static_rodata, 0);
>>     22: (18) r1 = map[id:66]
>>     24: (bf) r2 = r6
>>     25: (18) r3 = map[id:65][0]+0         <-- direct static_rodata addr in .rodata area
>>     27: (b7) r4 = 0
>>     28: (85) call array_map_update_elem#99888
>>     29: (b7) r1 = 3
>>   ; key = 3;
>>     30: (63) *(u32 *)(r10 -4) = r1
>>   ; bpf_map_update_elem(&result, &key, &static_bss2, 0);
>>     31: (18) r7 = map[id:63][0]+8         <--.
>>     33: (18) r1 = map[id:66]                 |
>>     35: (bf) r2 = r6                         |
>>     36: (18) r3 = map[id:63][0]+8         <-- direct static_bss2 addr in .bss area
>>     38: (b7) r4 = 0
>>     39: (85) call array_map_update_elem#99888
>>   [...]
>>
>> For now .data/.rodata/.bss maps are not exposed via API to the
>> user, but this could be done in a subsequent step.
> 
> See comment about BPF_MAP_TYPE_HEAP/BLOB map in comments to patch #1,
> it would probably make more useful API for .data/.rodata/.bss.
> 
>>
>> Based upon recent fix in LLVM, commit c0db6b6bd444 ("[BPF] Don't
>> fail for static variables").
>>
>> Joint work with Joe Stringer.
>>
>>   [0] LPC 2018, BPF track, "ELF relocation for static data in BPF",
>>       http://vger.kernel.org/lpc-bpf2018.html#session-3
>>
>> Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
>> Signed-off-by: Joe Stringer <joe@wand.net.nz>
>> ---
>>  tools/include/uapi/linux/bpf.h |  10 +-
>>  tools/lib/bpf/libbpf.c         | 259 +++++++++++++++++++++++++++------
>>  2 files changed, 226 insertions(+), 43 deletions(-)
>>
>> diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
>> index 8884072e1a46..04b26f59b413 100644
>> --- a/tools/include/uapi/linux/bpf.h
>> +++ b/tools/include/uapi/linux/bpf.h
>> @@ -287,7 +287,7 @@ enum bpf_attach_type {
>>
>>  #define BPF_OBJ_NAME_LEN 16U
>>
>> -/* Flags for accessing BPF object */
>> +/* Flags for accessing BPF object from syscall side. */
>>  #define BPF_F_RDONLY           (1U << 3)
>>  #define BPF_F_WRONLY           (1U << 4)
>>
>> @@ -297,6 +297,14 @@ enum bpf_attach_type {
>>  /* Zero-initialize hash function seed. This should only be used for testing. */
>>  #define BPF_F_ZERO_SEED                (1U << 6)
>>
>> +/* Flags for accessing BPF object from program side. */
>> +#define BPF_F_RDONLY_PROG      (1U << 7)
>> +#define BPF_F_WRONLY_PROG      (1U << 8)
>> +#define BPF_F_ACCESS_MASK      (BPF_F_RDONLY |         \
>> +                                BPF_F_RDONLY_PROG |    \
>> +                                BPF_F_WRONLY |         \
>> +                                BPF_F_WRONLY_PROG)
>> +
>>  /* flags for BPF_PROG_QUERY */
>>  #define BPF_F_QUERY_EFFECTIVE  (1U << 0)
>>
>> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
>> index 8f8f688f3e9b..969bc3d9f02c 100644
>> --- a/tools/lib/bpf/libbpf.c
>> +++ b/tools/lib/bpf/libbpf.c
>> @@ -139,6 +139,9 @@ struct bpf_program {
>>                 enum {
>>                         RELO_LD64,
>>                         RELO_CALL,
>> +                       RELO_DATA,
>> +                       RELO_RODATA,
>> +                       RELO_BSS,
> 
> All three of those are essentially the same relocations, just applied
> against different ELF sections.
> I think by having just single RELO_GLOBAL_DATA you can actually
> simplify a bunch of code below, please see corresponding comments.

Ok, sounds like a reasonable simplification, will do all well for v3.

>>                 } type;
>>                 int insn_idx;
>>                 union {
>> @@ -174,7 +177,10 @@ struct bpf_program {
>>  struct bpf_map {
>>         int fd;
>>         char *name;
>> -       size_t offset;
>> +       union {
>> +               __u32 global_type;
> 
> This could be an index into common maps array.
> 
>> +               size_t offset;
>> +       };
>>         int map_ifindex;
>>         int inner_map_fd;
>>         struct bpf_map_def def;
>> @@ -194,6 +200,8 @@ struct bpf_object {
>>         size_t nr_programs;
>>         struct bpf_map *maps;
>>         size_t nr_maps;
>> +       struct bpf_map *maps_global;
>> +       size_t nr_maps_global;
> 
> Global maps could be stored in maps, along other ones, so that we
> don't need to keep track of them separately.
> 
> Another inconvenience of having a separate array of global maps is
> that bpf_map__iter won't iterate them. I don't know if that's
> desirable behavior or not, but it probably would be nice to iterate
> over global ones as well?

My thinking was that these maps are not explicitly user specified,
so libbpf API would expose them through a different interface than
the one we have today in order to not confuse or break application
behavior which would otherwise rely on iterating / processing over
them. Separate API would retain current behavior and definitely
make this unambiguous to apps with regards to what to expect from
each of such API call.

>>         bool loaded;
>>         bool has_pseudo_calls;
>> @@ -209,6 +217,9 @@ struct bpf_object {
>>                 Elf *elf;
>>                 GElf_Ehdr ehdr;
>>                 Elf_Data *symbols;
>> +               Elf_Data *global_data;
>> +               Elf_Data *global_rodata;
>> +               Elf_Data *global_bss;
>>                 size_t strtabidx;
>>                 struct {
>>                         GElf_Shdr shdr;
>> @@ -217,6 +228,9 @@ struct bpf_object {
>>                 int nr_reloc;
>>                 int maps_shndx;
>>                 int text_shndx;
>> +               int data_shndx;
>> +               int rodata_shndx;
>> +               int bss_shndx;
>>         } efile;
>>         /*
>>          * All loaded bpf_object is linked in a list, which is
>> @@ -457,6 +471,9 @@ static struct bpf_object *bpf_object__new(const char *path,
>>         obj->efile.obj_buf = obj_buf;
>>         obj->efile.obj_buf_sz = obj_buf_sz;
>>         obj->efile.maps_shndx = -1;
>> +       obj->efile.data_shndx = -1;
>> +       obj->efile.rodata_shndx = -1;
>> +       obj->efile.bss_shndx = -1;
>>
>>         obj->loaded = false;
>>
>> @@ -475,6 +492,9 @@ static void bpf_object__elf_finish(struct bpf_object *obj)
>>                 obj->efile.elf = NULL;
>>         }
>>         obj->efile.symbols = NULL;
>> +       obj->efile.global_data = NULL;
>> +       obj->efile.global_rodata = NULL;
>> +       obj->efile.global_bss = NULL;
>>
>>         zfree(&obj->efile.reloc);
>>         obj->efile.nr_reloc = 0;
>> @@ -757,6 +777,85 @@ bpf_object__init_maps(struct bpf_object *obj, int flags)
>>         return 0;
>>  }
>>
>> +static int
>> +bpf_object__create_map(struct bpf_object *obj, struct bpf_map *map);
>> +
>> +static int
>> +bpf_object__init_global(struct bpf_object *obj, int i, int type,
>> +                       const char *name, Elf_Data *map_data)
> 
> Instead of deducing flags and looking up for map by index, you can
> just pass struct bpf_map * directly instead of int i and provide
> flags, instead of type.

Yep, agree.

>> +{
>> +       struct bpf_map *map = &obj->maps_global[i];
>> +       struct bpf_map_def *def = &map->def;
>> +       char *cp, errmsg[STRERR_BUFSIZE];
>> +       int err, slot0 = 0;
>> +
>> +       def->type = BPF_MAP_TYPE_ARRAY;
>> +       def->key_size = sizeof(int);
>> +       def->value_size = map_data->d_size;
>> +       def->max_entries = 1;
>> +       def->map_flags = type == RELO_RODATA ? BPF_F_RDONLY_PROG : 0;
>> +
>> +       map->name = strdup(name);
>> +       map->global_type = type;
>> +       map->fd = bpf_object__create_map(obj, map);
>> +       if (map->fd < 0) {
>> +               err = map->fd;
>> +               cp = libbpf_strerror_r(errno, errmsg, sizeof(errmsg));
>> +               pr_warning("failed to create map (name: '%s'): %s\n",
>> +                          map->name, cp);
>> +               goto destroy;
>> +       }
>> +
>> +       pr_debug("create map %s: fd=%d\n", map->name, map->fd);
>> +
>> +       if (type != RELO_BSS) {
>> +               err = bpf_map_update_elem(map->fd, &slot0, map_data->d_buf, 0);
>> +               if (err < 0) {
>> +                       cp = libbpf_strerror_r(errno, errmsg, sizeof(errmsg));
>> +                       pr_warning("failed to update map (name: '%s'): %s\n",
>> +                                  map->name, cp);
>> +                       goto destroy;
>> +               }
>> +
>> +               pr_debug("updated map %s with elf data: fd=%d\n", map->name,
>> +                        map->fd);
>> +       }
>> +       return 0;
>> +destroy:
>> +       for (i = 0; i < obj->nr_maps_global; i++)
>> +               zclose(obj->maps_global[i].fd);
>> +       return err;
>> +}
>> +
>> +static int
>> +bpf_object__init_global_maps(struct bpf_object *obj)
>> +{
>> +       int nr_maps_global = (obj->efile.data_shndx >= 0) +
>> +                            (obj->efile.rodata_shndx >= 0) +
>> +                            (obj->efile.bss_shndx >= 0), i, err = 0;
> 
> This looks like a good candidate for separate static function? It can
> also be reused below to check if there is any global map present.

Sounds good.

>> +
>> +       obj->maps_global = calloc(nr_maps_global, sizeof(obj->maps_global[0]));
>> +       if (!obj->maps_global) {
> 
> If nr_maps_global is 0, calloc might or might not return NULL, so this
> check might erroneously return error.

Good point, just read it up as well from man page, will fix.

>> +               pr_warning("alloc maps for object failed\n");
>> +               return -ENOMEM;
>> +       }
>> +
>> +       obj->nr_maps_global = nr_maps_global;
>> +       for (i = 0; i < obj->nr_maps_global; i++)
>> +               obj->maps[i].fd = -1;
>> +       i = 0;
>> +       if (obj->efile.bss_shndx >= 0)
>> +               err = bpf_object__init_global(obj, i++, RELO_BSS, ".bss",
>> +                                             obj->efile.global_bss);
>> +       if (obj->efile.data_shndx >= 0 && !err)
>> +               err = bpf_object__init_global(obj, i++, RELO_DATA, ".data",
>> +                                             obj->efile.global_data);
>> +       if (obj->efile.rodata_shndx >= 0 && !err)
>> +               err = bpf_object__init_global(obj, i++, RELO_RODATA, ".rodata",
>> +                                             obj->efile.global_rodata);
> 
> Here we know exactly what type of map we are creating, so we can just
> directly pass all the required structs/flags/data.
> 
> Also, to speed up and simplify relocation processing below, I think
> it's better to store map indexes for each of available .bss, .data and
> .rodata maps, eliminating another need for having three different
> types of data relocations.

Yep, I'll clean this up.

>> +       return err;
>> +}
>> +
>>  static bool section_have_execinstr(struct bpf_object *obj, int idx)
>>  {
>>         Elf_Scn *scn;
>> @@ -865,6 +964,12 @@ static int bpf_object__elf_collect(struct bpf_object *obj, int flags)
>>                                         pr_warning("failed to alloc program %s (%s): %s",
>>                                                    name, obj->path, cp);
>>                                 }
>> +                       } else if (strcmp(name, ".data") == 0) {
>> +                               obj->efile.global_data = data;
>> +                               obj->efile.data_shndx = idx;
>> +                       } else if (strcmp(name, ".rodata") == 0) {
>> +                               obj->efile.global_rodata = data;
>> +                               obj->efile.rodata_shndx = idx;
>>                         }
> 
> Previously if we encountered unknown PROGBITS section, we'd emit debug
> message about skipping section, should we add that message here?

Sounds reasonable, I'll add a similar 'skip section' debug output there.

>>                 } else if (sh.sh_type == SHT_REL) {
>>                         void *reloc = obj->efile.reloc;
>> @@ -892,6 +997,9 @@ static int bpf_object__elf_collect(struct bpf_object *obj, int flags)
>>                                 obj->efile.reloc[n].shdr = sh;
>>                                 obj->efile.reloc[n].data = data;
>>                         }
>> +               } else if (sh.sh_type == SHT_NOBITS && strcmp(name, ".bss") == 0) {
>> +                       obj->efile.global_bss = data;
>> +                       obj->efile.bss_shndx = idx;
>>                 } else {
>>                         pr_debug("skip section(%d) %s\n", idx, name);
>>                 }
>> @@ -923,6 +1031,14 @@ static int bpf_object__elf_collect(struct bpf_object *obj, int flags)
>>                 if (err)
>>                         goto out;
>>         }
>> +       if (obj->efile.data_shndx >= 0 ||
>> +           obj->efile.rodata_shndx >= 0 ||
>> +           obj->efile.bss_shndx >= 0) {
>> +               err = bpf_object__init_global_maps(obj);
>> +               if (err)
>> +                       goto out;
>> +       }
>> +
>>         err = bpf_object__init_prog_names(obj);
>>  out:
>>         return err;
>> @@ -961,6 +1077,11 @@ bpf_program__collect_reloc(struct bpf_program *prog, GElf_Shdr *shdr,
>>         Elf_Data *symbols = obj->efile.symbols;
>>         int text_shndx = obj->efile.text_shndx;
>>         int maps_shndx = obj->efile.maps_shndx;
>> +       int data_shndx = obj->efile.data_shndx;
>> +       int rodata_shndx = obj->efile.rodata_shndx;
>> +       int bss_shndx = obj->efile.bss_shndx;
>> +       struct bpf_map *maps_global = obj->maps_global;
>> +       size_t nr_maps_global = obj->nr_maps_global;
>>         struct bpf_map *maps = obj->maps;
>>         size_t nr_maps = obj->nr_maps;
>>         int i, nrels;
>> @@ -999,8 +1120,10 @@ bpf_program__collect_reloc(struct bpf_program *prog, GElf_Shdr *shdr,
>>                          (long long) (rel.r_info >> 32),
>>                          (long long) sym.st_value, sym.st_name);
>>
>> -               if (sym.st_shndx != maps_shndx && sym.st_shndx != text_shndx) {
>> -                       pr_warning("Program '%s' contains non-map related relo data pointing to section %u\n",
>> +               if (sym.st_shndx != maps_shndx && sym.st_shndx != text_shndx &&
>> +                   sym.st_shndx != data_shndx && sym.st_shndx != rodata_shndx &&
>> +                   sym.st_shndx != bss_shndx) {
>> +                       pr_warning("Program '%s' contains unrecognized relo data pointing to section %u\n",
>>                                    prog->section_name, sym.st_shndx);
>>                         return -LIBBPF_ERRNO__RELOC;
>>                 }
>> @@ -1045,6 +1168,30 @@ bpf_program__collect_reloc(struct bpf_program *prog, GElf_Shdr *shdr,
>>                         prog->reloc_desc[i].type = RELO_LD64;
>>                         prog->reloc_desc[i].insn_idx = insn_idx;
>>                         prog->reloc_desc[i].map_idx = map_idx;
>> +               } else if (sym.st_shndx == data_shndx ||
>> +                          sym.st_shndx == rodata_shndx ||
>> +                          sym.st_shndx == bss_shndx) {
>> +                       int type = (sym.st_shndx == data_shndx)   ? RELO_DATA :
>> +                                  (sym.st_shndx == rodata_shndx) ? RELO_RODATA :
>> +                                                                   RELO_BSS;
>> +
>> +                       for (map_idx = 0; map_idx < nr_maps_global; map_idx++) {
>> +                               if (maps_global[map_idx].global_type == type) {
>> +                                       pr_debug("relocation: find map %zd (%s) for insn %u\n",
>> +                                                map_idx, maps_global[map_idx].name, insn_idx);
>> +                                       break;
>> +                               }
>> +                       }
>> +
>> +                       if (map_idx >= nr_maps_global) {
>> +                               pr_warning("bpf relocation: map_idx %d large than %d\n",
>> +                                          (int)map_idx, (int)nr_maps_global - 1);
>> +                               return -LIBBPF_ERRNO__RELOC;
>> +                       }
> 
> We don't need to handle all of this if we just remember global map
> indicies during creation, instead of calculating type, we can just
> pick correct index (and check it exists). And type can be just generic
> RELO_DATA.
> 
>> +
>> +                       prog->reloc_desc[i].type = type;
>> +                       prog->reloc_desc[i].insn_idx = insn_idx;
>> +                       prog->reloc_desc[i].map_idx = map_idx;
>>                 }
>>         }
>>         return 0;
>> @@ -1176,15 +1323,58 @@ bpf_object__probe_caps(struct bpf_object *obj)
>>  }
>>
>>  static int
>> -bpf_object__create_maps(struct bpf_object *obj)
>> +bpf_object__create_map(struct bpf_object *obj, struct bpf_map *map)
>>  {
>>         struct bpf_create_map_attr create_attr = {};
>> +       struct bpf_map_def *def = &map->def;
>> +       char *cp, errmsg[STRERR_BUFSIZE];
>> +       int fd;
>> +
>> +       if (obj->caps.name)
>> +               create_attr.name = map->name;
>> +       create_attr.map_ifindex = map->map_ifindex;
>> +       create_attr.map_type = def->type;
>> +       create_attr.map_flags = def->map_flags;
>> +       create_attr.key_size = def->key_size;
>> +       create_attr.value_size = def->value_size;
>> +       create_attr.max_entries = def->max_entries;
>> +       create_attr.btf_fd = 0;
>> +       create_attr.btf_key_type_id = 0;
>> +       create_attr.btf_value_type_id = 0;
>> +       if (bpf_map_type__is_map_in_map(def->type) &&
>> +           map->inner_map_fd >= 0)
>> +               create_attr.inner_map_fd = map->inner_map_fd;
>> +       if (obj->btf && !bpf_map_find_btf_info(map, obj->btf)) {
>> +               create_attr.btf_fd = btf__fd(obj->btf);
>> +               create_attr.btf_key_type_id = map->btf_key_type_id;
>> +               create_attr.btf_value_type_id = map->btf_value_type_id;
>> +       }
>> +
>> +       fd = bpf_create_map_xattr(&create_attr);
>> +       if (fd < 0 && create_attr.btf_key_type_id) {
>> +               cp = libbpf_strerror_r(errno, errmsg, sizeof(errmsg));
>> +               pr_warning("Error in bpf_create_map_xattr(%s):%s(%d). Retrying without BTF.\n",
>> +                          map->name, cp, errno);
>> +
>> +               create_attr.btf_fd = 0;
>> +               create_attr.btf_key_type_id = 0;
>> +               create_attr.btf_value_type_id = 0;
>> +               map->btf_key_type_id = 0;
>> +               map->btf_value_type_id = 0;
>> +               fd = bpf_create_map_xattr(&create_attr);
>> +       }
>> +
>> +       return fd;
>> +}
>> +
>> +static int
>> +bpf_object__create_maps(struct bpf_object *obj)
>> +{
>>         unsigned int i;
>>         int err;
>>
>>         for (i = 0; i < obj->nr_maps; i++) {
>>                 struct bpf_map *map = &obj->maps[i];
>> -               struct bpf_map_def *def = &map->def;
>>                 char *cp, errmsg[STRERR_BUFSIZE];
>>                 int *pfd = &map->fd;
>>
>> @@ -1193,41 +1383,7 @@ bpf_object__create_maps(struct bpf_object *obj)
>>                                  map->name, map->fd);
>>                         continue;
>>                 }
>> -
>> -               if (obj->caps.name)
>> -                       create_attr.name = map->name;
>> -               create_attr.map_ifindex = map->map_ifindex;
>> -               create_attr.map_type = def->type;
>> -               create_attr.map_flags = def->map_flags;
>> -               create_attr.key_size = def->key_size;
>> -               create_attr.value_size = def->value_size;
>> -               create_attr.max_entries = def->max_entries;
>> -               create_attr.btf_fd = 0;
>> -               create_attr.btf_key_type_id = 0;
>> -               create_attr.btf_value_type_id = 0;
>> -               if (bpf_map_type__is_map_in_map(def->type) &&
>> -                   map->inner_map_fd >= 0)
>> -                       create_attr.inner_map_fd = map->inner_map_fd;
>> -
>> -               if (obj->btf && !bpf_map_find_btf_info(map, obj->btf)) {
>> -                       create_attr.btf_fd = btf__fd(obj->btf);
>> -                       create_attr.btf_key_type_id = map->btf_key_type_id;
>> -                       create_attr.btf_value_type_id = map->btf_value_type_id;
>> -               }
>> -
>> -               *pfd = bpf_create_map_xattr(&create_attr);
>> -               if (*pfd < 0 && create_attr.btf_key_type_id) {
>> -                       cp = libbpf_strerror_r(errno, errmsg, sizeof(errmsg));
>> -                       pr_warning("Error in bpf_create_map_xattr(%s):%s(%d). Retrying without BTF.\n",
>> -                                  map->name, cp, errno);
>> -                       create_attr.btf_fd = 0;
>> -                       create_attr.btf_key_type_id = 0;
>> -                       create_attr.btf_value_type_id = 0;
>> -                       map->btf_key_type_id = 0;
>> -                       map->btf_value_type_id = 0;
>> -                       *pfd = bpf_create_map_xattr(&create_attr);
>> -               }
>> -
>> +               *pfd = bpf_object__create_map(obj, map);
>>                 if (*pfd < 0) {
>>                         size_t j;
>>
>> @@ -1412,6 +1568,24 @@ bpf_program__relocate(struct bpf_program *prog, struct bpf_object *obj)
>>                                                       &prog->reloc_desc[i]);
>>                         if (err)
>>                                 return err;
>> +               } else if (prog->reloc_desc[i].type == RELO_DATA ||
>> +                          prog->reloc_desc[i].type == RELO_RODATA ||
>> +                          prog->reloc_desc[i].type == RELO_BSS) {
>> +                       struct bpf_insn *insns = prog->insns;
>> +                       int insn_idx, map_idx, data_off;
>> +
>> +                       insn_idx = prog->reloc_desc[i].insn_idx;
>> +                       map_idx  = prog->reloc_desc[i].map_idx;
>> +                       data_off = insns[insn_idx].imm;
>> +
>> +                       if (insn_idx + 1 >= (int)prog->insns_cnt) {
>> +                               pr_warning("relocation out of range: '%s'\n",
>> +                                          prog->section_name);
>> +                               return -LIBBPF_ERRNO__RELOC;
>> +                       }
>> +                       insns[insn_idx].src_reg = BPF_PSEUDO_MAP_VALUE;
>> +                       insns[insn_idx].imm = obj->maps_global[map_idx].fd;
>> +                       insns[insn_idx + 1].imm = data_off;
>>                 }
>>         }
>>
>> @@ -1717,6 +1891,7 @@ __bpf_object__open(const char *path, void *obj_buf, size_t obj_buf_sz,
>>
>>         CHECK_ERR(bpf_object__elf_init(obj), err, out);
>>         CHECK_ERR(bpf_object__check_endianness(obj), err, out);
>> +       CHECK_ERR(bpf_object__probe_caps(obj), err, out);
>>         CHECK_ERR(bpf_object__elf_collect(obj, flags), err, out);
>>         CHECK_ERR(bpf_object__collect_reloc(obj), err, out);
>>         CHECK_ERR(bpf_object__validate(obj, needs_kver), err, out);
>> @@ -1789,7 +1964,8 @@ int bpf_object__unload(struct bpf_object *obj)
>>
>>         for (i = 0; i < obj->nr_maps; i++)
>>                 zclose(obj->maps[i].fd);
>> -
>> +       for (i = 0; i < obj->nr_maps_global; i++)
>> +               zclose(obj->maps_global[i].fd);
>>         for (i = 0; i < obj->nr_programs; i++)
>>                 bpf_program__unload(&obj->programs[i]);
>>
>> @@ -1810,7 +1986,6 @@ int bpf_object__load(struct bpf_object *obj)
>>
>>         obj->loaded = true;
>>
>> -       CHECK_ERR(bpf_object__probe_caps(obj), err, out);
>>         CHECK_ERR(bpf_object__create_maps(obj), err, out);
>>         CHECK_ERR(bpf_object__relocate(obj), err, out);
>>         CHECK_ERR(bpf_object__load_progs(obj), err, out);
>> --
>> 2.17.1
>>
> 
> I'm sorry if I seem a bit too obsessed with those three new relocation
> types. I just believe that having one generic and storing global maps
> along with other maps is cleaner and more uniform.

No worries, thanks for all your feedback and review!

Thanks,
Daniel