On 2/28/19 3:18 PM, Daniel Borkmann 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.
> 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.
>
> 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.
>
> 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@xxxxxxxxxxxxx>
> Signed-off-by: Joe Stringer <joe@xxxxxxxxxxx>
> ---
> 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 {
> [...]
> @@ -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;
> + }
> +
> + 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
[...]
> +
> +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;
I want to point to a subtle difference here between handling pure global
variables and static global variables. The "imm" value is only available
for static variables. For example,
-bash-4.4$ cat g.c
static volatile long sg = 2;
static volatile int si = 3;
long g = 4;
int i = 5;
int test() { return sg + si + g + i; }
-bash-4.4$
-bash-4.4$ clang -target bpf -O2 -c g.c
-bash-4.4$ readelf -s g.o
Symbol table '.symtab' contains 8 entries:
Num: Value Size Type Bind Vis Ndx Name
0: 0000000000000000 0 NOTYPE LOCAL DEFAULT UND
1: 0000000000000000 0 FILE LOCAL DEFAULT ABS g.c
2: 0000000000000010 8 OBJECT LOCAL DEFAULT 4 sg
3: 0000000000000018 4 OBJECT LOCAL DEFAULT 4 si
4: 0000000000000000 0 SECTION LOCAL DEFAULT 4
5: 0000000000000000 8 OBJECT GLOBAL DEFAULT 4 g
6: 0000000000000008 4 OBJECT GLOBAL DEFAULT 4 i
7: 0000000000000000 128 FUNC GLOBAL DEFAULT 2 test
-bash-4.4$
-bash-4.4$ llvm-readelf -r g.o
Relocation section '.rel.text' at offset 0x1d8 contains 4 entries:
Offset Info Type Symbol's
Value Symbol's Name
0000000000000000 0000000400000001 R_BPF_64_64
0000000000000000 .data
0000000000000018 0000000400000001 R_BPF_64_64
0000000000000000 .data
0000000000000038 0000000500000001 R_BPF_64_64 0000000000000000 g
0000000000000058 0000000600000001 R_BPF_64_64 0000000000000008 i
-bash-4.4$ llvm-objdump -d g.o
g.o: file format ELF64-BPF
Disassembly of section .text:
0000000000000000 test:
0: 18 01 00 00 10 00 00 00 00 00 00 00 00 00 00 00
r1 = 16 ll
2: 79 11 00 00 00 00 00 00 r1 = *(u64 *)(r1 + 0)
3: 18 02 00 00 18 00 00 00 00 00 00 00 00 00 00 00
r2 = 24 ll
5: 61 22 00 00 00 00 00 00 r2 = *(u32 *)(r2 + 0)
6: 0f 21 00 00 00 00 00 00 r1 += r2
7: 18 02 00 00 00 00 00 00 00 00 00 00 00 00 00 00
r2 = 0 ll
9: 79 22 00 00 00 00 00 00 r2 = *(u64 *)(r2 + 0)
10: 0f 21 00 00 00 00 00 00 r1 += r2
11: 18 02 00 00 00 00 00 00 00 00 00 00 00 00 00 00
r2 = 0 ll
13: 61 20 00 00 00 00 00 00 r0 = *(u32 *)(r2 + 0)
14: 0f 10 00 00 00 00 00 00 r0 += r1
15: 95 00 00 00 00 00 00 00 exit
-bash-4.4$
You can see the above, the non-static global access does not have its
in-section offset encoded in the insn itself. The difference is due to
llvm treating static global and non-static global differently.
To support both cases, during relocation recording stage, you can
also record:
. symbol binding (GELF_ST_BIND(sym.st_info)),
non-static global has binding STB_GLOBAL and static
global has binding STB_LOCAL
. symbol value (sym.st_value)
During the above relocation resolution, if symbol bind is local, do
what you already did here. If symbol bind is global, assign data_off
with symbol value.
This applied to both .data and .rodata sections.
The non initialized
global variable will not be in any allocated section in ELF file,
it is in a COM section which is to be allocated by loader.
So user defines some like
int g;
and later on uses it. Right now, it will not work. The workaround
is "int g = 4", or "static int g". I guess it should be
okay, we should encourage users to use "static" variables instead.
> +
> + 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);
>
