Currently, it's not allowed to attach an fentry/fexit prog to another one fentry/fexit. At the same time it's not uncommon to see a tracing program with lots of logic in use, and the attachment limitation prevents usage of fentry/fexit for performance analysis (e.g. with "bpftool prog profile" command) in this case. An example could be falcosecurity libs project that uses tp_btf tracing programs. Following the corresponding discussion [1], the reason for that is to avoid tracing progs call cycles without introducing more complex solutions. But currently it seems impossible to load and attach tracing programs in a way that will form such a cycle. The limitation is coming from the fact that attach_prog_fd is specified at the prog load (thus making it impossible to attach to a program loaded after it in this way), as well as tracing progs not implementing link_detach. Replace "no same type" requirement with verification that no more than one level of attachment nesting is allowed. In this way only one fentry/fexit program could be attached to another fentry/fexit to cover profiling use case, and still no cycle could be formed. To implement, add a new field into bpf_prog_aux to track the depth of attachment. [1]: https://lore.kernel.org/bpf/20191108064039.2041889-16-ast@xxxxxxxxxx/ Signed-off-by: Dmitrii Dolgov <9erthalion6@xxxxxxxxx> --- Previous discussion: https://lore.kernel.org/bpf/20231201154734.8545-1-9erthalion6@xxxxxxxxx/ Changes in v6: - Apply nesting level limitation only to tracing programs, otherwise it's possible to apply it in "fentry->extension" case and break it Changes in v5: - Remove follower_cnt and drop unreachable cycle prevention condition - Allow only one level of attachment nesting - Do not display attach_depth in bpftool, as it doesn't make sense anymore Changes in v3: - Fix incorrect decreasing of attach_depth, setting to 0 instead - Place bookkeeping later, to not miss a cleanup if needed - Display attach_depth in bpftool only if the value is not 0 Changes in v2: - Verify tgt_prog is not null - Replace boolean followed with number of followers, to handle multiple progs attaching/detaching include/linux/bpf.h | 1 + include/uapi/linux/bpf.h | 1 + kernel/bpf/syscall.c | 12 ++++++++++++ kernel/bpf/verifier.c | 33 +++++++++++++++++++-------------- tools/include/uapi/linux/bpf.h | 1 + 5 files changed, 34 insertions(+), 14 deletions(-) diff --git a/include/linux/bpf.h b/include/linux/bpf.h index eb447b0a9423..1588e48fe31c 100644 --- a/include/linux/bpf.h +++ b/include/linux/bpf.h @@ -1400,6 +1400,7 @@ struct bpf_prog_aux { u32 real_func_cnt; /* includes hidden progs, only used for JIT and freeing progs */ u32 func_idx; /* 0 for non-func prog, the index in func array for func prog */ u32 attach_btf_id; /* in-kernel BTF type id to attach to */ + u32 attach_depth; /* for tracing prog, level of attachment nesting */ u32 ctx_arg_info_size; u32 max_rdonly_access; u32 max_rdwr_access; diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h index e88746ba7d21..9cf45ad914f1 100644 --- a/include/uapi/linux/bpf.h +++ b/include/uapi/linux/bpf.h @@ -6468,6 +6468,7 @@ struct bpf_prog_info { __u32 verified_insns; __u32 attach_btf_obj_id; __u32 attach_btf_id; + __u32 attach_depth; } __attribute__((aligned(8))); struct bpf_map_info { diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c index 5e43ddd1b83f..9c56b5970d7e 100644 --- a/kernel/bpf/syscall.c +++ b/kernel/bpf/syscall.c @@ -3039,6 +3039,7 @@ static void bpf_tracing_link_release(struct bpf_link *link) bpf_trampoline_put(tr_link->trampoline); + link->prog->aux->attach_depth = 0; /* tgt_prog is NULL if target is a kernel function */ if (tr_link->tgt_prog) bpf_prog_put(tr_link->tgt_prog); @@ -3243,6 +3244,16 @@ static int bpf_tracing_prog_attach(struct bpf_prog *prog, goto out_unlock; } + if (tgt_prog) { + /* Bookkeeping for managing the prog attachment chain. If it's a + * tracing program, bump the level, otherwise carry it on. + */ + if (prog->type == BPF_PROG_TYPE_TRACING) + prog->aux->attach_depth = tgt_prog->aux->attach_depth + 1; + else + prog->aux->attach_depth = tgt_prog->aux->attach_depth; + } + link->tgt_prog = tgt_prog; link->trampoline = tr; @@ -4510,6 +4521,7 @@ static int bpf_prog_get_info_by_fd(struct file *file, if (prog->aux->btf) info.btf_id = btf_obj_id(prog->aux->btf); info.attach_btf_id = prog->aux->attach_btf_id; + info.attach_depth = prog->aux->attach_depth; if (attach_btf) info.attach_btf_obj_id = btf_obj_id(attach_btf); diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index 8e7b6072e3f4..a3ea2087a6b2 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -20109,6 +20109,11 @@ int bpf_check_attach_target(struct bpf_verifier_log *log, if (tgt_prog) { struct bpf_prog_aux *aux = tgt_prog->aux; + if (aux->attach_depth >= 1) { + bpf_log(log, "Cannot attach with more than one level of nesting\n"); + return -EINVAL; + } + if (bpf_prog_is_dev_bound(prog->aux) && !bpf_prog_dev_bound_match(prog, tgt_prog)) { bpf_log(log, "Target program bound device mismatch"); @@ -20147,10 +20152,11 @@ int bpf_check_attach_target(struct bpf_verifier_log *log, bpf_log(log, "Can attach to only JITed progs\n"); return -EINVAL; } - if (tgt_prog->type == prog->type) { - /* Cannot fentry/fexit another fentry/fexit program. - * Cannot attach program extension to another extension. - * It's ok to attach fentry/fexit to extension program. + if (tgt_prog->type == prog->type && prog_extension) { + /* + * To avoid potential call chain cycles, prevent attaching of a + * program extension to another extension. It's ok to attach + * fentry/fexit to extension program. */ bpf_log(log, "Cannot recursively attach\n"); return -EINVAL; @@ -20163,16 +20169,15 @@ int bpf_check_attach_target(struct bpf_verifier_log *log, * except fentry/fexit. The reason is the following. * The fentry/fexit programs are used for performance * analysis, stats and can be attached to any program - * type except themselves. When extension program is - * replacing XDP function it is necessary to allow - * performance analysis of all functions. Both original - * XDP program and its program extension. Hence - * attaching fentry/fexit to BPF_PROG_TYPE_EXT is - * allowed. If extending of fentry/fexit was allowed it - * would be possible to create long call chain - * fentry->extension->fentry->extension beyond - * reasonable stack size. Hence extending fentry is not - * allowed. + * type. When extension program is replacing XDP function + * it is necessary to allow performance analysis of all + * functions. Both original XDP program and its program + * extension. Hence attaching fentry/fexit to + * BPF_PROG_TYPE_EXT is allowed. If extending of + * fentry/fexit was allowed it would be possible to create + * long call chain fentry->extension->fentry->extension + * beyond reasonable stack size. Hence extending fentry + * is not allowed. */ bpf_log(log, "Cannot extend fentry/fexit\n"); return -EINVAL; diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h index e88746ba7d21..9cf45ad914f1 100644 --- a/tools/include/uapi/linux/bpf.h +++ b/tools/include/uapi/linux/bpf.h @@ -6468,6 +6468,7 @@ struct bpf_prog_info { __u32 verified_insns; __u32 attach_btf_obj_id; __u32 attach_btf_id; + __u32 attach_depth; } __attribute__((aligned(8))); struct bpf_map_info { -- 2.41.0
