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 nested attachment for tracing programs. [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/20231212195413.23942-1-9erthalion6@xxxxxxxxx/ Changes in v9: - Formatting Changes in v8: - Move bookkeping in bpf_tracing_link_release under the tgt_prog condition. - Fix some indentation issues. Changes in v7: - Replace attach_depth with a boolean flag to indicate a program is already tracing an fentry/fexit. 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 + kernel/bpf/syscall.c | 10 +++++++++- kernel/bpf/verifier.c | 39 +++++++++++++++++++++++++-------------- 3 files changed, 35 insertions(+), 15 deletions(-) diff --git a/include/linux/bpf.h b/include/linux/bpf.h index eb447b0a9423..e7393674ab94 100644 --- a/include/linux/bpf.h +++ b/include/linux/bpf.h @@ -1414,6 +1414,7 @@ struct bpf_prog_aux { bool dev_bound; /* Program is bound to the netdev. */ bool offload_requested; /* Program is bound and offloaded to the netdev. */ bool attach_btf_trace; /* true if attaching to BTF-enabled raw tp */ + bool attach_tracing_prog; /* true if tracing another tracing program */ bool func_proto_unreliable; bool sleepable; bool tail_call_reachable; diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c index 5e43ddd1b83f..bcc5d5ab0870 100644 --- a/kernel/bpf/syscall.c +++ b/kernel/bpf/syscall.c @@ -3040,8 +3040,10 @@ static void bpf_tracing_link_release(struct bpf_link *link) bpf_trampoline_put(tr_link->trampoline); /* tgt_prog is NULL if target is a kernel function */ - if (tr_link->tgt_prog) + if (tr_link->tgt_prog) { bpf_prog_put(tr_link->tgt_prog); + link->prog->aux->attach_tracing_prog = false; + } } static void bpf_tracing_link_dealloc(struct bpf_link *link) @@ -3243,6 +3245,12 @@ static int bpf_tracing_prog_attach(struct bpf_prog *prog, goto out_unlock; } + /* Bookkeeping for managing the prog attachment chain */ + if (tgt_prog && + prog->type == BPF_PROG_TYPE_TRACING && + tgt_prog->type == BPF_PROG_TYPE_TRACING) + prog->aux->attach_tracing_prog = true; + link->tgt_prog = tgt_prog; link->trampoline = tr; diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index 8e7b6072e3f4..f8c15ce8fd05 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -20077,6 +20077,7 @@ int bpf_check_attach_target(struct bpf_verifier_log *log, struct bpf_attach_target_info *tgt_info) { bool prog_extension = prog->type == BPF_PROG_TYPE_EXT; + bool prog_tracing = prog->type == BPF_PROG_TYPE_TRACING; const char prefix[] = "btf_trace_"; int ret = 0, subprog = -1, i; const struct btf_type *t; @@ -20147,10 +20148,21 @@ 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 (prog_tracing) { + if (aux->attach_tracing_prog) { + /* + * Target program is an fentry/fexit which is already attached + * to another tracing program. More levels of nesting + * attachment are not allowed. + */ + bpf_log(log, "Cannot nest tracing program attach more than once\n"); + return -EINVAL; + } + } else if (tgt_prog->type == prog->type) { + /* + * 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 +20175,15 @@ 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; -- 2.41.0
