Re: [PATCH bpf-next v4 3/8] bpf: lsm: provide attachment points for BPF LSM programs

On 20-Feb 18:25, Alexei Starovoitov wrote:

On Thu, Feb 20, 2020 at 06:52:45PM +0100, KP Singh wrote:

quoted

From: KP Singh <redacted>

The BPF LSM programs are implemented as fexit trampolines to avoid the
overhead of retpolines. These programs cannot be attached to security_*
wrappers as there are quite a few security_* functions that do more than
just calling the LSM callbacks.

This was discussed on the lists in:

  https://lore.kernel.org/bpf/20200123152440.28956-1-kpsingh@chromium.org/T/#m068becce588a0cdf01913f368a97aea4c62d8266 (local)

Adding a NOP callback after all the static LSM callbacks are called has
the following benefits:

- The BPF programs run at the right stage of the security_* wrappers.
- They run after all the static LSM hooks allowed the operation,
  therefore cannot allow an action that was already denied.

There are some hooks which do not call call_int_hooks or
call_void_hooks. It's not possible to call the bpf_lsm_* functions
without checking if there is BPF LSM program attached to these hooks.
This is added further in a subsequent patch. For now, these hooks are
marked as NO_BPF (i.e. attachment of BPF programs is not possible).

the commit log doesn't match the code.

Fixed. Thanks!

quoted

+
+/* For every LSM hook  that allows attachment of BPF programs, declare a NOP
+ * function where a BPF program can be attached as an fexit trampoline.
+ */
+#define LSM_HOOK(RET, NAME, ...) LSM_HOOK_##RET(NAME, __VA_ARGS__)
+#define LSM_HOOK_int(NAME, ...) noinline int bpf_lsm_##NAME(__VA_ARGS__)  \

Did you check generated asm?
I think I saw cases when gcc ignored 'noinline' when function is defined in the
same file and still performed inlining while keeping the function body.
To be safe I think __weak is necessary. That will guarantee noinline.

Sure, will change it to __weak.

And please reduce your cc next time. It's way too long.

Will do.

- KP