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libbpf: Add support for field existance CO-RE relocation 

Add support for BPF_FRK_EXISTS relocation kind to detect existence of
captured field in a destination BTF, allowing conditional logic to
handle incompatible differences between kernels.

Also introduce opt-in relaxed CO-RE relocation handling option, which
makes libbpf emit warning for failed relocations, but proceed with other
relocations. Instruction, for which relocation failed, is patched with
(u32)-1 value.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20191015182849.3922287-4-andriin@fb.com
This commit is contained in:
Andrii Nakryiko 2019-10-15 11:28:47 -07:00 committed by Alexei Starovoitov
parent 291ee02b5e
commit 62561eb442
2 changed files with 61 additions and 17 deletions
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74
tools/lib/bpf/libbpf.c
View File

@ -249,6 +249,7 @@ struct bpf_object {
bool loaded;
bool has_pseudo_calls;
bool relaxed_core_relocs;
/*
* Information when doing elf related work. Only valid if fd
@ -2771,26 +2772,54 @@ static int bpf_core_spec_match(struct bpf_core_spec *local_spec,
/*
* Patch relocatable BPF instruction.
* Expected insn->imm value is provided for validation, as well as the new
* relocated value.
*
* Patched value is determined by relocation kind and target specification.
* For field existence relocation target spec will be NULL if field is not
* found.
* Expected insn->imm value is determined using relocation kind and local
* spec, and is checked before patching instruction. If actual insn->imm value
* is wrong, bail out with error.
*
* 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) = <imm> (indirect memory assignment with immediate operand).
*
* If actual insn->imm value is wrong, bail out.
*/
static int bpf_core_reloc_insn(struct bpf_program *prog, int insn_off,
__u32 orig_off, __u32 new_off)
static int bpf_core_reloc_insn(struct bpf_program *prog,
const struct bpf_field_reloc *relo,
const struct bpf_core_spec *local_spec,
const struct bpf_core_spec *targ_spec)
{
__u32 orig_val, new_val;
struct bpf_insn *insn;
int insn_idx;
__u8 class;
if (insn_off % sizeof(struct bpf_insn))
if (relo->insn_off % sizeof(struct bpf_insn))
return -EINVAL;
insn_idx = insn_off / sizeof(struct bpf_insn);
insn_idx = relo->insn_off / sizeof(struct bpf_insn);
switch (relo->kind) {
case BPF_FIELD_BYTE_OFFSET:
orig_val = local_spec->offset;
if (targ_spec) {
new_val = targ_spec->offset;
} else {
pr_warning("prog '%s': patching insn #%d w/ failed reloc, imm %d -> %d\n",
bpf_program__title(prog, false), insn_idx,
orig_val, -1);
new_val = (__u32)-1;
}
break;
case BPF_FIELD_EXISTS:
orig_val = 1; /* can't generate EXISTS relo w/o local field */
new_val = targ_spec ? 1 : 0;
break;
default:
pr_warning("prog '%s': unknown relo %d at insn #%d'\n",
bpf_program__title(prog, false),
relo->kind, insn_idx);
return -EINVAL;
}
insn = &prog->insns[insn_idx];
class = BPF_CLASS(insn->code);
@ -2798,12 +2827,12 @@ static int bpf_core_reloc_insn(struct bpf_program *prog, int insn_off,
if (class == BPF_ALU || class == BPF_ALU64) {
if (BPF_SRC(insn->code) != BPF_K)
return -EINVAL;
if (insn->imm != orig_off)
if (insn->imm != orig_val)
return -EINVAL;
insn->imm = new_off;
insn->imm = new_val;
pr_debug("prog '%s': patched insn #%d (ALU/ALU64) imm %d -> %d\n",
bpf_program__title(prog, false),
insn_idx, orig_off, new_off);
insn_idx, orig_val, new_val);
} else {
pr_warning("prog '%s': trying to relocate unrecognized insn #%d, code:%x, src:%x, dst:%x, off:%x, imm:%x\n",
bpf_program__title(prog, false),
@ -2811,6 +2840,7 @@ static int bpf_core_reloc_insn(struct bpf_program *prog, int insn_off,
insn->off, insn->imm);
return -EINVAL;
}
return 0;
}
@ -3087,15 +3117,26 @@ static int bpf_core_reloc_field(struct bpf_program *prog,
cand_ids->data[j++] = cand_spec.spec[0].type_id;
}
cand_ids->len = j;
if (cand_ids->len == 0) {
/*
* For BPF_FIELD_EXISTS relo or when relaxed CO-RE reloc mode is
* requested, it's expected that we might not find any candidates.
* In this case, if field wasn't found in any candidate, the list of
* candidates shouldn't change at all, we'll just handle relocating
* appropriately, depending on relo's kind.
*/
if (j > 0)
cand_ids->len = j;
if (j == 0 && !prog->obj->relaxed_core_relocs &&
relo->kind != BPF_FIELD_EXISTS) {
pr_warning("prog '%s': relo #%d: no matching targets found for [%d] %s + %s\n",
prog_name, relo_idx, local_id, local_name, spec_str);
return -ESRCH;
}
err = bpf_core_reloc_insn(prog, relo->insn_off,
local_spec.offset, targ_spec.offset);
/* bpf_core_reloc_insn should know how to handle missing targ_spec */
err = bpf_core_reloc_insn(prog, relo, &local_spec,
j ? &targ_spec : NULL);
if (err) {
pr_warning("prog '%s': relo #%d: failed to patch insn at offset %d: %d\n",
prog_name, relo_idx, relo->insn_off, err);
@ -3587,6 +3628,7 @@ __bpf_object__open(const char *path, const void *obj_buf, size_t obj_buf_sz,
if (IS_ERR(obj))
return obj;
obj->relaxed_core_relocs = OPTS_GET(opts, relaxed_core_relocs, false);
relaxed_maps = OPTS_GET(opts, relaxed_maps, false);
CHECK_ERR(bpf_object__elf_init(obj), err, out);

4
tools/lib/bpf/libbpf.h
View File

@ -96,8 +96,10 @@ struct bpf_object_open_opts {
const char *object_name;
/* parse map definitions non-strictly, allowing extra attributes/data */
bool relaxed_maps;
/* process CO-RE relocations non-strictly, allowing them to fail */
bool relaxed_core_relocs;
};
#define bpf_object_open_opts__last_field relaxed_maps
#define bpf_object_open_opts__last_field relaxed_core_relocs
LIBBPF_API struct bpf_object *bpf_object__open(const char *path);
LIBBPF_API struct bpf_object *