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Enable BTF for BPF map on 4.19+ kernel

Browse source 
Flag: EXEMPT mainline
Test: Boot on 4.14 and BTF is disabled
Test: Boot on 4.19 and BTF is enabled
Bug: 429278733
Change-Id: Icdb85891c3d70f1f1949c5b6f5f82ed6bb862873
This commit is contained in:
Motomu Utsumi 2025-06-04 09:15:36 +09:00
commit 4a3b537580
1 changed files with 67 additions and 3 deletions
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70
bpf/loader/NetBpfLoad.cpp
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@ -721,6 +721,67 @@ static int setBtfVarOffset(ifstream &elfFile, struct btf *btf,
return 0;
}
#define BTF_INFO_ENC(kind, kind_flag, vlen) \
((!!(kind_flag) << 31) | ((kind) << 24) | ((vlen) & BTF_MAX_VLEN))
#define BTF_INT_ENC(encoding, bits_offset, nr_bits) \
((encoding) << 24 | (bits_offset) << 16 | (nr_bits))
static int sanitizeBtf(struct btf *btf) {
for (unsigned int i = 1; i < btf__type_cnt(btf); ++i) {
struct btf_type *bt = (struct btf_type *)btf__type_by_id(btf, i);
// Replace BTF_KIND_VAR (5.2+) with BTF_KIND_INT (4.18+)
if (btf_is_var(bt)) {
bt->info = BTF_INFO_ENC(BTF_KIND_INT, 0, 0);
// using size = 1 is the safest choice, 4 will be too
// big and cause kernel BTF validation failure if
// original variable took less than 4 bytes
bt->size = 1;
*(int *)(bt + 1) = BTF_INT_ENC(0, 0, 8);
continue;
}
// Replace BTF_KIND_FUNC_PROTO (5.0+) with BTF_KIND_ENUM (4.18+)
if (btf_is_func_proto(bt)) {
int vlen = btf_vlen(bt);
bt->info = BTF_INFO_ENC(BTF_KIND_ENUM, 0, vlen);
bt->size = sizeof(__u32); // kernel enforced
continue;
}
// Replace BTF_KIND_FUNC (5.0+) with BTF_KIND_TYPEDEF (4.18+)
if (btf_is_func(bt)) {
bt->info = BTF_INFO_ENC(BTF_KIND_TYPEDEF, 0, 0);
continue;
}
// Replace BTF_KIND_DATASEC (5.2+) with BTF_KIND_STRUCT (4.18+)
if (btf_is_datasec(bt)) {
const struct btf_var_secinfo *v = btf_var_secinfos(bt);
struct btf_member *m = btf_members(bt);
char *name;
name = (char *)btf__name_by_offset(btf, bt->name_off);
while (*name) {
if (*name == '.' || *name == '?') *name = '_';
name++;
}
int vlen = btf_vlen(bt);
bt->info = BTF_INFO_ENC(BTF_KIND_STRUCT, 0, vlen);
for (int j = 0; j < vlen; j++, v++, m++) {
// order of field assignments is important
m->offset = v->offset * 8;
m->type = v->type;
// preserve variable name as member name
const struct btf_type *vt = btf__type_by_id(btf, v->type);
m->name_off = vt->name_off;
}
}
}
return 0;
}
static int loadBtf(ifstream &elfFile, struct btf *btf) {
int ret;
for (unsigned int i = 1; i < btf__type_cnt(btf); ++i) {
@ -732,6 +793,11 @@ static int loadBtf(ifstream &elfFile, struct btf *btf) {
if (ret) return ret;
}
if (!isAtLeastKernelVersion(5, 10, 0)) {
// Likely unnecessary on kernel 5.4 but untested.
sanitizeBtf(btf);
}
ret = btf__load_into_kernel(btf);
if (ret) {
if (errno != EINVAL) {
@ -855,10 +921,8 @@ static int createMaps(const char* elfPath, ifstream& elfFile, vector<unique_fd>&
struct btf *btf = NULL;
auto btfGuard = base::make_scope_guard([&btf] { if (btf) btf__free(btf); });
if (isAtLeastKernelVersion(5, 10, 0)) {
// Untested on Linux Kernel 5.4, but likely compatible.
if (isAtLeastKernelVersion(4, 19, 0)) {
// On Linux Kernels older than 4.18 BPF_BTF_LOAD command doesn't exist.
// On Linux Kernels older than 5.2 BTF_KIND_VAR and BTF_KIND_DATASEC don't exist.
ret = readSectionByName(".BTF", elfFile, btfData);
if (ret) {
ALOGE("Failed to read .BTF section, ret:%d", ret);