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libbpf: Implement enum value-based CO-RE relocations

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Implement two relocations of a new enumerator value-based CO-RE relocation
kind: ENUMVAL_EXISTS and ENUMVAL_VALUE.

First, ENUMVAL_EXISTS, allows to detect the presence of a named enumerator
value in the target (kernel) BTF. This is useful to do BPF helper/map/program
type support detection from BPF program side. bpf_core_enum_value_exists()
macro helper is provided to simplify built-in usage.

Second, ENUMVAL_VALUE, allows to capture enumerator integer value and relocate
it according to the target BTF, if it changes. This is useful to have
a guarantee against intentional or accidental re-ordering/re-numbering of some
of the internal (non-UAPI) enumerations, where kernel developers don't care
about UAPI backwards compatiblity concerns. bpf_core_enum_value() allows to
capture this succinctly and use correct enum values in code.

LLVM uses ldimm64 instruction to capture enumerator value-based relocations,
so add support for ldimm64 instruction patching as well.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20200819194519.3375898-5-andriin@fb.com
This commit is contained in:
Andrii Nakryiko 2020-08-19 12:45:18 -07:00 committed by Alexei Starovoitov
parent 4836bf5e2e
commit eacaaed784
3 changed files with 170 additions and 5 deletions
repo.diff.show_diff_stats Download patch file Download diff file Expand all files Collapse all files

28
tools/lib/bpf/bpf_core_read.h
View file

@ -31,6 +31,12 @@ enum bpf_type_info_kind {
BPF_TYPE_SIZE = 1, /* type size in target kernel */
};
/* second argument to __builtin_preserve_enum_value() built-in */
enum bpf_enum_value_kind {
BPF_ENUMVAL_EXISTS = 0, /* enum value existence in kernel */
BPF_ENUMVAL_VALUE = 1, /* enum value value relocation */
};
#define __CORE_RELO(src, field, info) \
__builtin_preserve_field_info((src)->field, BPF_FIELD_##info)
@ -150,6 +156,28 @@ enum bpf_type_info_kind {
#define bpf_core_type_size(type) \
__builtin_preserve_type_info(*(typeof(type) *)0, BPF_TYPE_SIZE)
/*
* Convenience macro to check that provided enumerator value is defined in
* a target kernel.
* Returns:
* 1, if specified enum type and its enumerator value are present in target
* kernel's BTF;
* 0, if no matching enum and/or enum value within that enum is found.
*/
#define bpf_core_enum_value_exists(enum_type, enum_value) \
__builtin_preserve_enum_value(*(typeof(enum_type) *)enum_value, BPF_ENUMVAL_EXISTS)
/*
* Convenience macro to get the integer value of an enumerator value in
* a target kernel.
* Returns:
* 64-bit value, if specified enum type and its enumerator value are
* present in target kernel's BTF;
* 0, if no matching enum and/or enum value within that enum is found.
*/
#define bpf_core_enum_value(enum_type, enum_value) \
__builtin_preserve_enum_value(*(typeof(enum_type) *)enum_value, BPF_ENUMVAL_VALUE)
/*
* bpf_core_read() abstracts away bpf_probe_read_kernel() call and captures
* offset relocation for source address using __builtin_preserve_access_index()

145
tools/lib/bpf/libbpf.c
View file

@ -4115,6 +4115,8 @@ static const char *core_relo_kind_str(enum bpf_core_relo_kind kind)
case BPF_TYPE_ID_TARGET: return "target_type_id";
case BPF_TYPE_EXISTS: return "type_exists";
case BPF_TYPE_SIZE: return "type_size";
case BPF_ENUMVAL_EXISTS: return "enumval_exists";
case BPF_ENUMVAL_VALUE: return "enumval_value";
default: return "unknown";
}
}
@ -4147,6 +4149,17 @@ static bool core_relo_is_type_based(enum bpf_core_relo_kind kind)
}
}
static bool core_relo_is_enumval_based(enum bpf_core_relo_kind kind)
{
switch (kind) {
case BPF_ENUMVAL_EXISTS:
case BPF_ENUMVAL_VALUE:
return true;
default:
return false;
}
}
/*
* Turn bpf_core_relo into a low- and high-level spec representation,
* validating correctness along the way, as well as calculating resulting
@ -4180,6 +4193,9 @@ static bool core_relo_is_type_based(enum bpf_core_relo_kind kind)
* Type-based relocations (TYPE_EXISTS/TYPE_SIZE,
* TYPE_ID_LOCAL/TYPE_ID_TARGET) don't capture any field information. Their
* spec and raw_spec are kept empty.
*
* Enum value-based relocations (ENUMVAL_EXISTS/ENUMVAL_VALUE) use access
* string to specify enumerator's value index that need to be relocated.
*/
static int bpf_core_parse_spec(const struct btf *btf,
__u32 type_id,
@ -4224,16 +4240,25 @@ static int bpf_core_parse_spec(const struct btf *btf,
if (spec->raw_len == 0)
return -EINVAL;
/* first spec value is always reloc type array index */
t = skip_mods_and_typedefs(btf, type_id, &id);
if (!t)
return -EINVAL;
access_idx = spec->raw_spec[0];
spec->spec[0].type_id = id;
spec->spec[0].idx = access_idx;
acc = &spec->spec[0];
acc->type_id = id;
acc->idx = access_idx;
spec->len++;
if (core_relo_is_enumval_based(relo_kind)) {
if (!btf_is_enum(t) || spec->raw_len > 1 || access_idx >= btf_vlen(t))
return -EINVAL;
/* record enumerator name in a first accessor */
acc->name = btf__name_by_offset(btf, btf_enum(t)[access_idx].name_off);
return 0;
}
if (!core_relo_is_field_based(relo_kind))
return -EINVAL;
@ -4676,6 +4701,39 @@ static int bpf_core_spec_match(struct bpf_core_spec *local_spec,
local_acc = &local_spec->spec[0];
targ_acc = &targ_spec->spec[0];
if (core_relo_is_enumval_based(local_spec->relo_kind)) {
size_t local_essent_len, targ_essent_len;
const struct btf_enum *e;
const char *targ_name;
/* has to resolve to an enum */
targ_type = skip_mods_and_typedefs(targ_spec->btf, targ_id, &targ_id);
if (!btf_is_enum(targ_type))
return 0;
local_essent_len = bpf_core_essential_name_len(local_acc->name);
for (i = 0, e = btf_enum(targ_type); i < btf_vlen(targ_type); i++, e++) {
targ_name = btf__name_by_offset(targ_spec->btf, e->name_off);
targ_essent_len = bpf_core_essential_name_len(targ_name);
if (targ_essent_len != local_essent_len)
continue;
if (strncmp(local_acc->name, targ_name, local_essent_len) == 0) {
targ_acc->type_id = targ_id;
targ_acc->idx = i;
targ_acc->name = targ_name;
targ_spec->len++;
targ_spec->raw_spec[targ_spec->raw_len] = targ_acc->idx;
targ_spec->raw_len++;
return 1;
}
}
return 0;
}
if (!core_relo_is_field_based(local_spec->relo_kind))
return -EINVAL;
for (i = 0; i < local_spec->len; i++, local_acc++, targ_acc++) {
targ_type = skip_mods_and_typedefs(targ_spec->btf, targ_id,
&targ_id);
@ -4880,6 +4938,31 @@ static int bpf_core_calc_type_relo(const struct bpf_core_relo *relo,
return 0;
}
static int bpf_core_calc_enumval_relo(const struct bpf_core_relo *relo,
const struct bpf_core_spec *spec,
__u32 *val)
{
const struct btf_type *t;
const struct btf_enum *e;
switch (relo->kind) {
case BPF_ENUMVAL_EXISTS:
*val = spec ? 1 : 0;
break;
case BPF_ENUMVAL_VALUE:
if (!spec)
return -EUCLEAN; /* request instruction poisoning */
t = btf__type_by_id(spec->btf, spec->spec[0].type_id);
e = btf_enum(t) + spec->spec[0].idx;
*val = e->val;
break;
default:
return -EOPNOTSUPP;
}
return 0;
}
struct bpf_core_relo_res
{
/* expected value in the instruction, unless validate == false */
@ -4918,6 +5001,9 @@ static int bpf_core_calc_relo(const struct bpf_program *prog,
} else if (core_relo_is_type_based(relo->kind)) {
err = bpf_core_calc_type_relo(relo, local_spec, &res->orig_val);
err = err ?: bpf_core_calc_type_relo(relo, targ_spec, &res->new_val);
} else if (core_relo_is_enumval_based(relo->kind)) {
err = bpf_core_calc_enumval_relo(relo, local_spec, &res->orig_val);
err = err ?: bpf_core_calc_enumval_relo(relo, targ_spec, &res->new_val);
}
if (err == -EUCLEAN) {
@ -4954,6 +5040,11 @@ static void bpf_core_poison_insn(struct bpf_program *prog, int relo_idx,
insn->imm = 195896080; /* => 0xbad2310 => "bad relo" */
}
static bool is_ldimm64(struct bpf_insn *insn)
{
return insn->code == (BPF_LD | BPF_IMM | BPF_DW);
}
/*
* Patch relocatable BPF instruction.
*
@ -4966,6 +5057,7 @@ static void bpf_core_poison_insn(struct bpf_program *prog, int relo_idx,
* Currently three kinds of BPF instructions are supported:
* 1. rX = <imm> (assignment with immediate operand);
* 2. rX += <imm> (arithmetic operations with immediate operand);
* 3. rX = <imm64> (load with 64-bit immediate value).
*/
static int bpf_core_patch_insn(struct bpf_program *prog,
const struct bpf_core_relo *relo,
@ -4984,6 +5076,11 @@ static int bpf_core_patch_insn(struct bpf_program *prog,
class = BPF_CLASS(insn->code);
if (res->poison) {
/* poison second part of ldimm64 to avoid confusing error from
* verifier about "unknown opcode 00"
*/
if (is_ldimm64(insn))
bpf_core_poison_insn(prog, relo_idx, insn_idx + 1, insn + 1);
bpf_core_poison_insn(prog, relo_idx, insn_idx, insn);
return 0;
}
@ -5012,7 +5109,7 @@ static int bpf_core_patch_insn(struct bpf_program *prog,
case BPF_ST:
case BPF_STX:
if (res->validate && insn->off != orig_val) {
pr_warn("prog '%s': relo #%d: unexpected insn #%d (LD/LDX/ST/STX) value: got %u, exp %u -> %u\n",
pr_warn("prog '%s': relo #%d: unexpected insn #%d (LDX/ST/STX) value: got %u, exp %u -> %u\n",
bpf_program__title(prog, false), relo_idx,
insn_idx, insn->off, orig_val, new_val);
return -EINVAL;
@ -5029,8 +5126,36 @@ static int bpf_core_patch_insn(struct bpf_program *prog,
bpf_program__title(prog, false), relo_idx, insn_idx,
orig_val, new_val);
break;
case BPF_LD: {
__u64 imm;
if (!is_ldimm64(insn) ||
insn[0].src_reg != 0 || insn[0].off != 0 ||
insn_idx + 1 >= prog->insns_cnt ||
insn[1].code != 0 || insn[1].dst_reg != 0 ||
insn[1].src_reg != 0 || insn[1].off != 0) {
pr_warn("prog '%s': relo #%d: insn #%d (LDIMM64) has unexpected form\n",
bpf_program__title(prog, false), relo_idx, insn_idx);
return -EINVAL;
}
imm = insn[0].imm + ((__u64)insn[1].imm << 32);
if (res->validate && imm != orig_val) {
pr_warn("prog '%s': relo #%d: unexpected insn #%d (LDIMM64) value: got %llu, exp %u -> %u\n",
bpf_program__title(prog, false), relo_idx,
insn_idx, imm, orig_val, new_val);
return -EINVAL;
}
insn[0].imm = new_val;
insn[1].imm = 0; /* currently only 32-bit values are supported */
pr_debug("prog '%s': relo #%d: patched insn #%d (LDIMM64) imm64 %llu -> %u\n",
bpf_program__title(prog, false), relo_idx, insn_idx,
imm, new_val);
break;
}
default:
pr_warn("prog '%s': relo #%d: trying to relocate unrecognized insn #%d, code:%x, src:%x, dst:%x, off:%x, imm:%x\n",
pr_warn("prog '%s': relo #%d: trying to relocate unrecognized insn #%d, code:0x%x, src:0x%x, dst:0x%x, off:0x%x, imm:0x%x\n",
bpf_program__title(prog, false), relo_idx,
insn_idx, insn->code, insn->src_reg, insn->dst_reg,
insn->off, insn->imm);
@ -5047,6 +5172,7 @@ static int bpf_core_patch_insn(struct bpf_program *prog,
static void bpf_core_dump_spec(int level, const struct bpf_core_spec *spec)
{
const struct btf_type *t;
const struct btf_enum *e;
const char *s;
__u32 type_id;
int i;
@ -5060,6 +5186,15 @@ static void bpf_core_dump_spec(int level, const struct bpf_core_spec *spec)
if (core_relo_is_type_based(spec->relo_kind))
return;
if (core_relo_is_enumval_based(spec->relo_kind)) {
t = skip_mods_and_typedefs(spec->btf, type_id, NULL);
e = btf_enum(t) + spec->raw_spec[0];
s = btf__name_by_offset(spec->btf, e->name_off);
libbpf_print(level, "::%s = %u", s, e->val);
return;
}
if (core_relo_is_field_based(spec->relo_kind)) {
for (i = 0; i < spec->len; i++) {
if (spec->spec[i].name)

2
tools/lib/bpf/libbpf_internal.h
View file

@ -242,6 +242,8 @@ enum bpf_core_relo_kind {
BPF_TYPE_ID_TARGET = 7, /* type ID in target kernel */
BPF_TYPE_EXISTS = 8, /* type existence in target kernel */
BPF_TYPE_SIZE = 9, /* type size in bytes */
BPF_ENUMVAL_EXISTS = 10, /* enum value existence in target kernel */
BPF_ENUMVAL_VALUE = 11, /* enum value integer value */
};
/* The minimum bpf_core_relo checked by the loader