On 03-Mär 14:37, Andrii Nakryiko wrote:
> 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
Fixed. Thanks!
>
> > {
> > // 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
Good catch, sloppy work here by me.
>
> > + 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?
yeah, agreed, updated to doing a cleanup at the end.
- KP
>
> > 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);
> > + }
> > +
>
> [...]
