| Message ID | 20230811201346.3240403-1-davemarchevsky@fb.com (mailing list archive) |
|---|---|
| State | Superseded |
| Delegated to: | BPF |
| Headers | show |
| Series | [bpf-next,1/2] libbpf: Support triple-underscore flavors for kfunc relocation | expand |
On 08/11, 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); > > Signed-off-by: Dave Marchevsky <davemarchevsky@fb.com> > --- > tools/lib/bpf/libbpf.c | 21 ++++++++++++++++++++- > 1 file changed, 20 insertions(+), 1 deletion(-) > > diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c > index 17883f5a44b9..8949d489a35f 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 = malloc(ext_essent_len + 1); Do we care about malloc's potential -ENOMEM here?
On Fri, Aug 11, 2023 at 01:13:45PM -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); > > Signed-off-by: Dave Marchevsky <davemarchevsky@fb.com> > --- > tools/lib/bpf/libbpf.c | 21 ++++++++++++++++++++- > 1 file changed, 20 insertions(+), 1 deletion(-) > > diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c > index 17883f5a44b9..8949d489a35f 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 = malloc(ext_essent_len + 1); > + memcpy(ext->essent_name, ext->name, ext_essent_len); > + ext->essent_name[ext_essent_len] = '\0'; could we use strndup in here? > + } > + > 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; > @@ -7642,6 +7653,9 @@ static int bpf_object__resolve_ksym_func_btf_id(struct bpf_object *obj, > 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); > + > + if (ext->is_weak) > + return 0; > return -EINVAL; > } > > @@ -8370,6 +8384,11 @@ void bpf_object__close(struct bpf_object *obj) > > zfree(&obj->btf_custom_path); > zfree(&obj->kconfig); > + > + for (i = 0; i < obj->nr_extern; i++) > + if (obj->externs[i].essent_name) > + zfree(&obj->externs[i].essent_name); no need to check the pointer, free will take care of that jirka > + > zfree(&obj->externs); > obj->nr_extern = 0; > > -- > 2.34.1 > >
diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c index 17883f5a44b9..8949d489a35f 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 = malloc(ext_essent_len + 1); + memcpy(ext->essent_name, ext->name, ext_essent_len); + ext->essent_name[ext_essent_len] = '\0'; + } + 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; @@ -7642,6 +7653,9 @@ static int bpf_object__resolve_ksym_func_btf_id(struct bpf_object *obj, 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); + + if (ext->is_weak) + return 0; return -EINVAL; } @@ -8370,6 +8384,11 @@ void bpf_object__close(struct bpf_object *obj) zfree(&obj->btf_custom_path); zfree(&obj->kconfig); + + for (i = 0; i < obj->nr_extern; i++) + if (obj->externs[i].essent_name) + zfree(&obj->externs[i].essent_name); + zfree(&obj->externs); obj->nr_extern = 0;
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); Signed-off-by: Dave Marchevsky <davemarchevsky@fb.com> --- tools/lib/bpf/libbpf.c | 21 ++++++++++++++++++++- 1 file changed, 20 insertions(+), 1 deletion(-)