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It is used for > > allowing bpf program to call a limited set of kernel functions > > in a later patch. > > > > When writing bpf prog, the extern kernel function needs > > to be declared under a ELF section (".ksyms") which is > > the same as the current extern kernel variables and that should > > keep its usage consistent without requiring to remember another > > section name. > > > > For example, in a bpf_prog.c: > > > > extern int foo(struct sock *) __attribute__((section(".ksyms"))) > > > > [24] FUNC_PROTO '(anon)' ret_type_id=15 vlen=1 > > '(anon)' type_id=18 > > [25] FUNC 'foo' type_id=24 linkage=extern > > [ ... ] > > [33] DATASEC '.ksyms' size=0 vlen=1 > > type_id=25 offset=0 size=0 > > > > LLVM will put the "func" type into the BTF datasec ".ksyms". > > The current "btf_datasec_check_meta()" assumes everything under > > it is a "var" and ensures it has non-zero size ("!vsi->size" test). > > The non-zero size check is not true for "func". This patch postpones the > > "!vsi-size" test from "btf_datasec_check_meta()" to > > "btf_datasec_resolve()" which has all types collected to decide > > if a vsi is a "var" or a "func" and then enforce the "vsi->size" > > differently. > > > > If the datasec only has "func", its "t->size" could be zero. > > Thus, the current "!t->size" test is no longer valid. The > > invalid "t->size" will still be caught by the later > > "last_vsi_end_off > t->size" check. This patch also takes this > > chance to consolidate other "t->size" tests ("vsi->offset >= t->size" > > "vsi->size > t->size", and "t->size < sum") into the existing > > "last_vsi_end_off > t->size" test. > > > > The LLVM will also put those extern kernel function as an extern > > linkage func in the BTF: > > > > [24] FUNC_PROTO '(anon)' ret_type_id=15 vlen=1 > > '(anon)' type_id=18 > > [25] FUNC 'foo' type_id=24 linkage=extern > > > > This patch allows BTF_FUNC_EXTERN in btf_func_check_meta(). > > Also extern kernel function declaration does not > > necessary have arg name. Another change in btf_func_check() is > > to allow extern function having no arg name. > > > > The btf selftest is adjusted accordingly. New tests are also added. > > > > The required LLVM patch: https://reviews.llvm.org/D93563 > > > > Signed-off-by: Martin KaFai Lau > > --- > > High-level question about EXTERN functions in DATASEC. Does kernel > need to see them under DATASEC? What if libbpf just removed all EXTERN > funcs from under DATASEC and leave them as "free-floating" EXTERN > FUNCs in BTF. > > We need to tag EXTERNs with DATASECs mainly for libbpf to know whether > it's .kconfig or .ksym or other type of externs. Does kernel need to > care? Although the kernel does not need to know, since the a legit llvm generates it, I go with a proper support in the kernel (e.g. bpftool btf dump can better reflect what was there). > > > kernel/bpf/btf.c | 52 ++++--- > > tools/testing/selftests/bpf/prog_tests/btf.c | 154 ++++++++++++++++++- > > 2 files changed, 178 insertions(+), 28 deletions(-) > > > > [...] > > > @@ -3611,9 +3594,28 @@ static int btf_datasec_resolve(struct btf_verifier_env *env, > > u32 var_type_id = vsi->type, type_id, type_size = 0; > > const struct btf_type *var_type = btf_type_by_id(env->btf, > > var_type_id); > > - if (!var_type || !btf_type_is_var(var_type)) { > > + if (!var_type) { > > + btf_verifier_log_vsi(env, v->t, vsi, > > + "type not found"); > > + return -EINVAL; > > + } > > + > > + if (btf_type_is_func(var_type)) { > > + if (vsi->size || vsi->offset) { > > + btf_verifier_log_vsi(env, v->t, vsi, > > + "Invalid size/offset"); > > + return -EINVAL; > > + } > > + continue; > > + } else if (btf_type_is_var(var_type)) { > > + if (!vsi->size) { > > + btf_verifier_log_vsi(env, v->t, vsi, > > + "Invalid size"); > > + return -EINVAL; > > + } > > + } else { > > btf_verifier_log_vsi(env, v->t, vsi, > > - "Not a VAR kind member"); > > + "Neither a VAR nor a FUNC"); > > return -EINVAL; > > can you please structure it as follow (I think it is bit easier to > follow the logic then): > > if (btf_type_is_func()) { > ... > continue; /* no extra checks */ > } > > if (!btf_type_is_var()) { > /* bad, complain, exit */ > return -EINVAL; > } > > /* now we deal with extra checks for variables */ > > That way variable checks are kept all in one place. > > Also a question: is that ok to enable non-extern functions under > DATASEC? Maybe, but that wasn't explicitly mentioned. The patch does not check. We could reject that for now. > > > } > > > > @@ -3849,9 +3851,11 @@ static int btf_func_check(struct btf_verifier_env *env, > > const struct btf_param *args; > > const struct btf *btf; > > u16 nr_args, i; > > + bool is_extern; > > > > btf = env->btf; > > proto_type = btf_type_by_id(btf, t->type); > > + is_extern = btf_type_vlen(t) == BTF_FUNC_EXTERN; > > using btf_type_vlen(t) for getting func linkage is becoming more and > more confusing. Would it be terrible to have btf_func_linkage(t) > helper instead? I have it in my local v2. and also just return when it is extern. > > > > > if (!proto_type || !btf_type_is_func_proto(proto_type)) { > > btf_verifier_log_type(env, t, "Invalid type_id"); > > @@ -3861,7 +3865,7 @@ static int btf_func_check(struct btf_verifier_env *env, > > args = (const struct btf_param *)(proto_type + 1); > > nr_args = btf_type_vlen(proto_type); > > for (i = 0; i < nr_args; i++) { > > - if (!args[i].name_off && args[i].type) { > > + if (!is_extern && !args[i].name_off && args[i].type) { > > btf_verifier_log_type(env, t, "Invalid arg#%u", i + 1); > > return -EINVAL; > > } > > [...]