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    SubjectRe: [PATCH v2 2/5] seccomp: make underlying bpf ref counted as well
    On 09/11/2015 02:20 AM, Tycho Andersen wrote:
    > In the next patch, we're going to add a way to access the underlying
    > filters via bpf fds. This means that we need to ref-count both the
    > struct seccomp_filter objects and the struct bpf_prog objects separately,
    > in case a process dies but a filter is still referred to by another
    > process.
    >
    > Additionally, we mark classic converted seccomp filters as seccomp eBPF
    > programs, since they are a subset of what is supported in seccomp eBPF.
    >
    > Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
    > CC: Kees Cook <keescook@chromium.org>
    > CC: Will Drewry <wad@chromium.org>
    > CC: Oleg Nesterov <oleg@redhat.com>
    > CC: Andy Lutomirski <luto@amacapital.net>
    > CC: Pavel Emelyanov <xemul@parallels.com>
    > CC: Serge E. Hallyn <serge.hallyn@ubuntu.com>
    > CC: Alexei Starovoitov <ast@kernel.org>
    > CC: Daniel Borkmann <daniel@iogearbox.net>
    > ---
    > kernel/seccomp.c | 4 +++-
    > 1 file changed, 3 insertions(+), 1 deletion(-)
    >
    > diff --git a/kernel/seccomp.c b/kernel/seccomp.c
    > index 245df6b..afaeddf 100644
    > --- a/kernel/seccomp.c
    > +++ b/kernel/seccomp.c
    > @@ -378,6 +378,8 @@ static struct seccomp_filter *seccomp_prepare_filter(struct sock_fprog *fprog)
    > }
    >
    > atomic_set(&sfilter->usage, 1);
    > + atomic_set(&sfilter->prog->aux->refcnt, 1);
    > + sfilter->prog->type = BPF_PROG_TYPE_SECCOMP;

    So, if you do this, then this breaks the assumption of eBPF JITs
    that, currently, all classic converted BPF programs always have a
    prog->type of BPF_PROG_TYPE_UNSPEC (see: bpf_prog_was_classic()).

    Currently, JITs make use of this information to determine whether
    A and X mappings for such programs should or should not be cleared
    in the prologue (s390 currently).

    In the seccomp_prepare_filter() stage, we're already past that, so
    it will not cause an issue, but we certainly would need to be very
    careful in future, if bpf_prog_was_classic() is then used at a later
    stage when we already have a generated bpf_prog somewhere, as then
    this assumption will break.

    > return sfilter;
    > }
    > @@ -470,7 +472,7 @@ void get_seccomp_filter(struct task_struct *tsk)
    > static inline void seccomp_filter_free(struct seccomp_filter *filter)
    > {
    > if (filter) {
    > - bpf_prog_free(filter->prog);
    > + bpf_prog_put(filter->prog);
    > kfree(filter);
    > }
    > }
    >



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