Re: [PATCH 10/19] bpf: Allow to store caller's ip as argument

Andrii Nakryiko
2021-06-08T18:54:09Z

On Sat, Jun 5, 2021 at 4:12 AM Jiri Olsa <jolsa@kernel.org> wrote:
>
> When we will have multiple functions attached to trampoline
> we need to propagate the function's address to the bpf program.
>
> Adding new BPF_TRAMP_F_IP_ARG flag to arch_prepare_bpf_trampoline
> function that will store origin caller's address before function's
> arguments.
>
> Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> ---
> arch/x86/net/bpf_jit_comp.c | 18 ++++++++++++++----
> include/linux/bpf.h | 5 +++++
> 2 files changed, 19 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
> index b77e6bd78354..d2425c18272a 100644
> --- a/arch/x86/net/bpf_jit_comp.c
> +++ b/arch/x86/net/bpf_jit_comp.c
> @@ -1951,7 +1951,7 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
> void *orig_call)
> {
> int ret, i, cnt = 0, nr_args = m->nr_args;
> - int stack_size = nr_args * 8;
> + int stack_size = nr_args * 8, ip_arg = 0;
> struct bpf_tramp_progs *fentry = &tprogs[BPF_TRAMP_FENTRY];
> struct bpf_tramp_progs *fexit = &tprogs[BPF_TRAMP_FEXIT];
> struct bpf_tramp_progs *fmod_ret = &tprogs[BPF_TRAMP_MODIFY_RETURN];
> @@ -1975,6 +1975,9 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
> */
> orig_call += X86_PATCH_SIZE;
>
> + if (flags & BPF_TRAMP_F_IP_ARG)
> + stack_size += 8;
> +

nit: move it a bit up where we adjust stack_size for BPF_TRAMP_F_CALL_ORIG flag?

> prog = image;
>
> EMIT1(0x55); /* push rbp */
> @@ -1982,7 +1985,14 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
> EMIT4(0x48, 0x83, 0xEC, stack_size); /* sub rsp, stack_size */
> EMIT1(0x53); /* push rbx */
>
> - save_regs(m, &prog, nr_args, stack_size);
> + if (flags & BPF_TRAMP_F_IP_ARG) {
> + emit_ldx(&prog, BPF_DW, BPF_REG_0, BPF_REG_FP, 8);
> + EMIT4(0x48, 0x83, 0xe8, X86_PATCH_SIZE); /* sub $X86_PATCH_SIZE,%rax*/
> + emit_stx(&prog, BPF_DW, BPF_REG_FP, BPF_REG_0, -stack_size);
> + ip_arg = 8;
> + }

why not pass flags into save_regs and let it handle this case without
this extra ip_arg adjustment?

> +
> + save_regs(m, &prog, nr_args, stack_size - ip_arg);
>
> if (flags & BPF_TRAMP_F_CALL_ORIG) {
> /* arg1: mov rdi, im */
> @@ -2011,7 +2021,7 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
> }
>
> if (flags & BPF_TRAMP_F_CALL_ORIG) {
> - restore_regs(m, &prog, nr_args, stack_size);
> + restore_regs(m, &prog, nr_args, stack_size - ip_arg);
>

similarly (and symmetrically), pass flags into restore_regs() to
handle that ip_arg transparently?

> if (flags & BPF_TRAMP_F_ORIG_STACK) {
> emit_ldx(&prog, BPF_DW, BPF_REG_0, BPF_REG_FP, 8);
> @@ -2052,7 +2062,7 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
> }
>
> if (flags & BPF_TRAMP_F_RESTORE_REGS)
> - restore_regs(m, &prog, nr_args, stack_size);
> + restore_regs(m, &prog, nr_args, stack_size - ip_arg);
>
> /* This needs to be done regardless. If there were fmod_ret programs,
> * the return value is only updated on the stack and still needs to be
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index 16fc600503fb..6cbf3c81c650 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -559,6 +559,11 @@ struct btf_func_model {
> */
> #define BPF_TRAMP_F_ORIG_STACK BIT(3)
>
> +/* First argument is IP address of the caller. Makes sense for fentry/fexit
> + * programs only.
> + */
> +#define BPF_TRAMP_F_IP_ARG BIT(4)
> +
> /* Each call __bpf_prog_enter + call bpf_func + call __bpf_prog_exit is ~50
> * bytes on x86. Pick a number to fit into BPF_IMAGE_SIZE / 2
> */
> --
> 2.31.1
>