On Tue, Mar 3, 2020 at 6:12 AM KP Singh <kpsingh@xxxxxxxxxxxx> wrote:
>
> From: KP Singh <kpsingh@xxxxxxxxxx>
>
> When multiple programs are attached, each program receives the return
> value from the previous program on the stack and the last program
> provides the return value to the attached function.
>
> The fmod_ret bpf programs are run after the fentry programs and before
> the fexit programs. The original function is only called if all the
> fmod_ret programs return 0 to avoid any unintended side-effects. The
> success value, i.e. 0 is not currently configurable but can be made so
> where user-space can specify it at load time.
>
> For example:
>
> int func_to_be_attached(int a, int b)
> { <--- do_fentry
>
> do_fmod_ret:
> <update ret by calling fmod_ret>
> if (ret != 0)
> goto do_fexit;
>
> original_function:
>
> <side_effects_happen_here>
>
> } <--- do_fexit
>
> The fmod_ret program attached to this function can be defined as:
>
> SEC("fmod_ret/func_to_be_attached")
> BPF_PROG(func_name, int a, int b, int ret)
same as on cover letter, return type is missing
> {
> // This will skip the original function logic.
> return 1;
> }
>
> The first fmod_ret program is passed 0 in its return argument.
>
> Signed-off-by: KP Singh <kpsingh@xxxxxxxxxx>
> ---
> arch/x86/net/bpf_jit_comp.c | 96 ++++++++++++++++++++++++++++++++--
> include/linux/bpf.h | 1 +
> include/uapi/linux/bpf.h | 1 +
> kernel/bpf/btf.c | 3 +-
> kernel/bpf/syscall.c | 1 +
> kernel/bpf/trampoline.c | 5 +-
> kernel/bpf/verifier.c | 1 +
> tools/include/uapi/linux/bpf.h | 1 +
> 8 files changed, 103 insertions(+), 6 deletions(-)
>
[...]
>
> + if (fmod_ret->nr_progs) {
> + branches = kcalloc(fmod_ret->nr_progs, sizeof(u8 *),
> + GFP_KERNEL);
> + if (!branches)
> + return -ENOMEM;
> + if (invoke_bpf_mod_ret(m, &prog, fmod_ret, stack_size,
> + branches))
branches leaks here
> + return -EINVAL;
> + }
> +
> if (flags & BPF_TRAMP_F_CALL_ORIG) {
> - if (fentry->nr_progs)
> + if (fentry->nr_progs || fmod_ret->nr_progs)
> restore_regs(m, &prog, nr_args, stack_size);
>
> /* call original function */
> @@ -1573,6 +1649,14 @@ int arch_prepare_bpf_trampoline(void *image, void *image_end,
there is early return one line above here, you need to free branches
in that case to not leak memory
So I guess it's better to do goto cleanup approach at this point?
> emit_stx(&prog, BPF_DW, BPF_REG_FP, BPF_REG_0, -8);
> }
>
> + if (fmod_ret->nr_progs) {
> + align16_branch_target(&prog);
> + for (i = 0; i < fmod_ret->nr_progs; i++)
> + emit_cond_near_jump(&branches[i], prog, branches[i],
> + X86_JNE);
> + kfree(branches);
> + }
> +
[...]
