Daniel Borkmann <daniel@xxxxxxxxxxxxx> writes: > On Mon, Oct 07, 2019 at 07:20:37PM +0200, Toke Høiland-Jørgensen wrote: >> From: Toke Høiland-Jørgensen <toke@xxxxxxxxxx> >> >> This adds support for setting and deleting bpf chain call programs through >> a couple of new commands in the bpf() syscall. The CHAIN_ADD and CHAIN_DEL >> commands take two eBPF program fds and a return code, and install the >> 'next' program to be chain called after the 'prev' program if that program >> returns 'retcode'. A retcode of -1 means "wildcard", so that the program >> will be executed regardless of the previous program's return code. >> >> >> The syscall command names are based on Alexei's prog_chain example[0], >> which Alan helpfully rebased on current bpf-next. However, the logic and >> program storage is obviously adapted to the execution logic in the previous >> commit. >> >> [0] https://git.kernel.org/pub/scm/linux/kernel/git/ast/bpf.git/commit/?h=prog_chain&id=f54f45d00f91e083f6aec2abe35b6f0be52ae85b&context=15 >> >> Signed-off-by: Alan Maguire <alan.maguire@xxxxxxxxxx> >> Signed-off-by: Toke Høiland-Jørgensen <toke@xxxxxxxxxx> >> --- >> include/uapi/linux/bpf.h | 10 ++++++ >> kernel/bpf/syscall.c | 78 ++++++++++++++++++++++++++++++++++++++++++++++ >> 2 files changed, 88 insertions(+) >> >> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h >> index 1ce80a227be3..b03c23963af8 100644 >> --- a/include/uapi/linux/bpf.h >> +++ b/include/uapi/linux/bpf.h >> @@ -107,6 +107,9 @@ enum bpf_cmd { >> BPF_MAP_LOOKUP_AND_DELETE_ELEM, >> BPF_MAP_FREEZE, >> BPF_BTF_GET_NEXT_ID, >> + BPF_PROG_CHAIN_ADD, >> + BPF_PROG_CHAIN_DEL, >> + BPF_PROG_CHAIN_GET, >> }; >> >> enum bpf_map_type { >> @@ -516,6 +519,13 @@ union bpf_attr { >> __u64 probe_offset; /* output: probe_offset */ >> __u64 probe_addr; /* output: probe_addr */ >> } task_fd_query; >> + >> + struct { /* anonymous struct used by BPF_PROG_CHAIN_* commands */ >> + __u32 prev_prog_fd; >> + __u32 next_prog_fd; >> + __u32 retcode; >> + __u32 next_prog_id; /* output: prog_id */ >> + }; >> } __attribute__((aligned(8))); >> >> /* The description below is an attempt at providing documentation to eBPF >> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c >> index b8a203a05881..be8112e08a88 100644 >> --- a/kernel/bpf/syscall.c >> +++ b/kernel/bpf/syscall.c >> @@ -2113,6 +2113,79 @@ static int bpf_prog_test_run(const union bpf_attr *attr, >> return ret; >> } >> >> +#define BPF_PROG_CHAIN_LAST_FIELD next_prog_id >> + >> +static int bpf_prog_chain(int cmd, const union bpf_attr *attr, >> + union bpf_attr __user *uattr) >> +{ >> + struct bpf_prog *prog, *next_prog, *old_prog; >> + struct bpf_prog **array; >> + int ret = -EOPNOTSUPP; >> + u32 index, prog_id; >> + >> + if (CHECK_ATTR(BPF_PROG_CHAIN)) >> + return -EINVAL; >> + >> + /* Index 0 is wildcard, encoded as ~0 by userspace */ >> + if (attr->retcode == ((u32) ~0)) >> + index = 0; >> + else >> + index = attr->retcode + 1; >> + >> + if (index >= BPF_NUM_CHAIN_SLOTS) >> + return -E2BIG; >> + >> + prog = bpf_prog_get(attr->prev_prog_fd); >> + if (IS_ERR(prog)) >> + return PTR_ERR(prog); >> + >> + /* If the chain_calls bit is not set, that's because the chain call flag >> + * was not set on program load, and so we can't support chain calls. >> + */ >> + if (!prog->chain_calls) >> + goto out; >> + >> + array = prog->aux->chain_progs; >> + >> + switch (cmd) { >> + case BPF_PROG_CHAIN_ADD: >> + next_prog = bpf_prog_get(attr->next_prog_fd); >> + if (IS_ERR(next_prog)) { >> + ret = PTR_ERR(next_prog); >> + break; >> + } >> + old_prog = xchg(array + index, next_prog); >> + if (old_prog) >> + bpf_prog_put(old_prog); >> + ret = 0; >> + break; > > How are circular dependencies resolved here? Seems the situation is > not prevented, so progs unloaded via XDP won't get the __bpf_prog_free() > call where they then drop the references of all the other progs in the > chain. Yeah, that's true. My plan was to just walk the "call graph" on insert and reject any circular inserts. Just haven't gotten around to adding that yet; will fix that in the next version. -Toke
