netdev - Re: [PATCH v3 bpf-next 02/18] bpf: Add bpf_arch_text

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Date:   Fri, 8 Nov 2019 09:23:57 +0100
From:   Björn Töpel <bjorn.topel@...il.com>
To:     Alexei Starovoitov <ast@...nel.org>
Cc:     David Miller <davem@...emloft.net>,
        Daniel Borkmann <daniel@...earbox.net>, x86@...nel.org,
        Netdev <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Kernel Team <kernel-team@...com>
Subject: Re: [PATCH v3 bpf-next 02/18] bpf: Add bpf_arch_text_poke() helper

On Fri, 8 Nov 2019 at 07:41, Alexei Starovoitov <ast@...nel.org> wrote:
>
> Add bpf_arch_text_poke() helper that is used by BPF trampoline logic to patch
> nops/calls in kernel text into calls into BPF trampoline and to patch
> calls/nops inside BPF programs too.
>
> Signed-off-by: Alexei Starovoitov <ast@...nel.org>
> ---
>  arch/x86/net/bpf_jit_comp.c | 51 +++++++++++++++++++++++++++++++++++++
>  include/linux/bpf.h         |  8 ++++++
>  kernel/bpf/core.c           |  6 +++++
>  3 files changed, 65 insertions(+)
>
> diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
> index 0399b1f83c23..bb8467fd6715 100644
> --- a/arch/x86/net/bpf_jit_comp.c
> +++ b/arch/x86/net/bpf_jit_comp.c
> @@ -9,9 +9,11 @@
>  #include <linux/filter.h>
>  #include <linux/if_vlan.h>
>  #include <linux/bpf.h>
> +#include <linux/memory.h>
>  #include <asm/extable.h>
>  #include <asm/set_memory.h>
>  #include <asm/nospec-branch.h>
> +#include <asm/text-patching.h>
>
>  static u8 *emit_code(u8 *ptr, u32 bytes, unsigned int len)
>  {
> @@ -487,6 +489,55 @@ static int emit_call(u8 **pprog, void *func, void *ip)
>         return 0;
>  }
>
> +int bpf_arch_text_poke(void *ip, enum bpf_text_poke_type t,
> +                      void *old_addr, void *new_addr)
> +{
> +       u8 old_insn[X86_CALL_SIZE] = {};
> +       u8 new_insn[X86_CALL_SIZE] = {};
> +       u8 *prog;
> +       int ret;
> +
> +       if (!is_kernel_text((long)ip))
> +               /* BPF trampoline in modules is not supported */
> +               return -EINVAL;
> +
> +       if (old_addr) {
> +               prog = old_insn;
> +               ret = emit_call(&prog, old_addr, (void *)ip);
> +               if (ret)
> +                       return ret;
> +       }
> +       if (new_addr) {
> +               prog = new_insn;
> +               ret = emit_call(&prog, new_addr, (void *)ip);
> +               if (ret)
> +                       return ret;
> +       }
> +       ret = -EBUSY;
> +       mutex_lock(&text_mutex);
> +       switch (t) {
> +       case BPF_MOD_NOP_TO_CALL:
> +               if (memcmp(ip, ideal_nops[NOP_ATOMIC5], X86_CALL_SIZE))
> +                       goto out;
> +               text_poke(ip, new_insn, X86_CALL_SIZE);

I'm probably missing something, but why isn't text_poke_bp() needed here?

> +               break;
> +       case BPF_MOD_CALL_TO_CALL:
> +               if (memcmp(ip, old_insn, X86_CALL_SIZE))
> +                       goto out;
> +               text_poke(ip, new_insn, X86_CALL_SIZE);
> +               break;
> +       case BPF_MOD_CALL_TO_NOP:
> +               if (memcmp(ip, old_insn, X86_CALL_SIZE))
> +                       goto out;
> +               text_poke(ip, ideal_nops[NOP_ATOMIC5], X86_CALL_SIZE);
> +               break;
> +       }
> +       ret = 0;
> +out:
> +       mutex_unlock(&text_mutex);
> +       return ret;
> +}
> +
>  static bool ex_handler_bpf(const struct exception_table_entry *x,
>                            struct pt_regs *regs, int trapnr,
>                            unsigned long error_code, unsigned long fault_addr)
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index 7c7f518811a6..8b90db25348a 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -1157,4 +1157,12 @@ static inline u32 bpf_xdp_sock_convert_ctx_access(enum bpf_access_type type,
>  }
>  #endif /* CONFIG_INET */
>
> +enum bpf_text_poke_type {
> +       BPF_MOD_NOP_TO_CALL,
> +       BPF_MOD_CALL_TO_CALL,
> +       BPF_MOD_CALL_TO_NOP,
> +};
> +int bpf_arch_text_poke(void *ip, enum bpf_text_poke_type t,
> +                      void *addr1, void *addr2);
> +
>  #endif /* _LINUX_BPF_H */
> diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
> index c1fde0303280..c4bcec1014a9 100644
> --- a/kernel/bpf/core.c
> +++ b/kernel/bpf/core.c
> @@ -2140,6 +2140,12 @@ int __weak skb_copy_bits(const struct sk_buff *skb, int offset, void *to,
>         return -EFAULT;
>  }
>
> +int __weak bpf_arch_text_poke(void *ip, enum bpf_text_poke_type t,
> +                             void *addr1, void *addr2)
> +{
> +       return -ENOTSUPP;
> +}
> +
>  DEFINE_STATIC_KEY_FALSE(bpf_stats_enabled_key);
>  EXPORT_SYMBOL(bpf_stats_enabled_key);
>
> --
> 2.23.0
>