Re: [RFC patch 09/19] bpf: Use BPF_PROG_RUN_PIN_ON_CPU() at simple call sites.

On Wed, Feb 19, 2020 at 1:01 AM Thomas Gleixner [off-list ref] wrote:

Vinicius Costa Gomes [off-list ref] writes:

Cc+: seccomp folks

quoted

Thomas Gleixner [off-list ref] writes:

quoted

From: David Miller <davem@davemloft.net>

Leaving content for reference

quoted

quoted

All of these cases are strictly of the form:

     preempt_disable();
     BPF_PROG_RUN(...);
     preempt_enable();

Replace this with BPF_PROG_RUN_PIN_ON_CPU() which wraps BPF_PROG_RUN()
with:

     migrate_disable();
     BPF_PROG_RUN(...);
     migrate_enable();

On non RT enabled kernels this maps to preempt_disable/enable() and on RT
enabled kernels this solely prevents migration, which is sufficient as
there is no requirement to prevent reentrancy to any BPF program from a
preempting task. The only requirement is that the program stays on the same
CPU.

Therefore, this is a trivially correct transformation.

[ tglx: Converted to BPF_PROG_RUN_PIN_ON_CPU() ]

Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Thomas Gleixner <redacted>

---
 include/linux/filter.h    |    4 +---
 kernel/seccomp.c          |    4 +---
 net/core/flow_dissector.c |    4 +---
 net/core/skmsg.c          |    8 ++------
 net/kcm/kcmsock.c         |    4 +---
 5 files changed, 6 insertions(+), 18 deletions(-)

--- a/include/linux/filter.h
+++ b/include/linux/filter.h

@@ -713,9 +713,7 @@ static inline u32 bpf_prog_run_clear_cb(
     if (unlikely(prog->cb_access))
             memset(cb_data, 0, BPF_SKB_CB_LEN);

-    preempt_disable();
-    res = BPF_PROG_RUN(prog, skb);
-    preempt_enable();
+    res = BPF_PROG_RUN_PIN_ON_CPU(prog, skb);
     return res;
 }

--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c

@@ -268,16 +268,14 @@ static u32 seccomp_run_filters(const str
      * All filters in the list are evaluated and the lowest BPF return
      * value always takes priority (ignoring the DATA).
      */
-    preempt_disable();
     for (; f; f = f->prev) {
-            u32 cur_ret = BPF_PROG_RUN(f->prog, sd);
+            u32 cur_ret = BPF_PROG_RUN_PIN_ON_CPU(f->prog, sd);

More a question really, isn't the behavior changing here? i.e. shouldn't
migrate_disable()/migrate_enable() be moved to outside the loop? Or is
running seccomp filters on different cpus not a problem?

In my understanding this is a list of filters and they are independent
of each other.

Yes. It's fine to be preempted between filters.