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

[Alexei Starovoitov]

On Thu, Feb 20, 2020 at 06:52:45PM +0100, KP Singh wrote:
> From: KP Singh <kpsingh@xxxxxxxxxx>
> 
> 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@xxxxxxxxxxxx/T/#m068becce588a0cdf01913f368a97aea4c62d8266
> 
> 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.

> +
> +/* 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.

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