On Thu, Aug 17, 2023 at 03:53:52PM -0700, Dave Marchevsky wrote: > The function signature of kfuncs can change at any time due to their > intentional lack of stability guarantees. As kfuncs become more widely > used, BPF program writers will need facilities to support calling > different versions of a kfunc from a single BPF object. Consider this > simplified example based on a real scenario we ran into at Meta: > > /* initial kfunc signature */ > int some_kfunc(void *ptr) > > /* Oops, we need to add some flag to modify behavior. No problem, > change the kfunc. flags = 0 retains original behavior */ > int some_kfunc(void *ptr, long flags) > > If the initial version of the kfunc is deployed on some portion of the > fleet and the new version on the rest, a fleetwide service that uses > some_kfunc will currently need to load different BPF programs depending > on which some_kfunc is available. > > Luckily CO-RE provides a facility to solve a very similar problem, > struct definition changes, by allowing program writers to declare > my_struct___old and my_struct___new, with ___suffix being considered a > 'flavor' of the non-suffixed name and being ignored by > bpf_core_type_exists and similar calls. > > This patch extends the 'flavor' facility to the kfunc extern > relocation process. BPF program writers can now declare > > extern int some_kfunc___old(void *ptr) > extern int some_kfunc___new(void *ptr, int flags) > > then test which version of the kfunc exists with bpf_ksym_exists. > Relocation and verifier's dead code elimination will work in concert as > expected, allowing this pattern: > > if (bpf_ksym_exists(some_kfunc___old)) > some_kfunc___old(ptr); > else > some_kfunc___new(ptr, 0); > > Changelog: > > v1 -> v2: https://lore.kernel.org/bpf/20230811201346.3240403-1-davemarchevsky@xxxxxx/ > * No need to check obj->externs[i].essent_name before zfree (Jiri) > * Use strndup instead of replicating same functionality (Jiri) > * Properly handle memory allocation falure (Stanislav) > > v2 -> v3: https://lore.kernel.org/bpf/20230816165813.3718580-1-davemarchevsky@xxxxxx/ > * Move if (ext->is_weak) test above pr_warn to match existing similar behavior > (David Vernet) > > Signed-off-by: Dave Marchevsky <davemarchevsky@xxxxxx> LGTM, thanks for working on this. Acked-by: David Vernet <void@xxxxxxxxxxxxx> > --- > tools/lib/bpf/libbpf.c | 20 +++++++++++++++++++- > 1 file changed, 19 insertions(+), 1 deletion(-) > > diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c > index b14a4376a86e..2178b28878e2 100644 > --- a/tools/lib/bpf/libbpf.c > +++ b/tools/lib/bpf/libbpf.c > @@ -550,6 +550,7 @@ struct extern_desc { > int btf_id; > int sec_btf_id; > const char *name; > + char *essent_name; > bool is_set; > bool is_weak; > union { > @@ -3770,6 +3771,7 @@ static int bpf_object__collect_externs(struct bpf_object *obj) > struct extern_desc *ext; > int i, n, off, dummy_var_btf_id; > const char *ext_name, *sec_name; > + size_t ext_essent_len; > Elf_Scn *scn; > Elf64_Shdr *sh; > > @@ -3819,6 +3821,14 @@ static int bpf_object__collect_externs(struct bpf_object *obj) > ext->sym_idx = i; > ext->is_weak = ELF64_ST_BIND(sym->st_info) == STB_WEAK; > > + ext_essent_len = bpf_core_essential_name_len(ext->name); > + ext->essent_name = NULL; > + if (ext_essent_len != strlen(ext->name)) { > + ext->essent_name = strndup(ext->name, ext_essent_len); > + if (!ext->essent_name) > + return -ENOMEM; > + } > + > ext->sec_btf_id = find_extern_sec_btf_id(obj->btf, ext->btf_id); > if (ext->sec_btf_id <= 0) { > pr_warn("failed to find BTF for extern '%s' [%d] section: %d\n", > @@ -7624,7 +7634,8 @@ static int bpf_object__resolve_ksym_func_btf_id(struct bpf_object *obj, > > local_func_proto_id = ext->ksym.type_id; > > - kfunc_id = find_ksym_btf_id(obj, ext->name, BTF_KIND_FUNC, &kern_btf, &mod_btf); > + kfunc_id = find_ksym_btf_id(obj, ext->essent_name ?: ext->name, BTF_KIND_FUNC, &kern_btf, > + &mod_btf); > if (kfunc_id < 0) { > if (kfunc_id == -ESRCH && ext->is_weak) > return 0; > @@ -7639,6 +7650,9 @@ static int bpf_object__resolve_ksym_func_btf_id(struct bpf_object *obj, > ret = bpf_core_types_are_compat(obj->btf, local_func_proto_id, > kern_btf, kfunc_proto_id); > if (ret <= 0) { > + if (ext->is_weak) > + return 0; > + > pr_warn("extern (func ksym) '%s': func_proto [%d] incompatible with %s [%d]\n", > ext->name, local_func_proto_id, > mod_btf ? mod_btf->name : "vmlinux", kfunc_proto_id); > @@ -8370,6 +8384,10 @@ void bpf_object__close(struct bpf_object *obj) > > zfree(&obj->btf_custom_path); > zfree(&obj->kconfig); > + > + for (i = 0; i < obj->nr_extern; i++) > + zfree(&obj->externs[i].essent_name); > + > zfree(&obj->externs); > obj->nr_extern = 0; > > -- > 2.34.1 > >
