Re: [PATCH bpf-next v3 4/7] bpf, arm64: Impelment bpf_arch_text_poke() for arm64

On Sun, Apr 24, 2022 at 11:40:25AM -0400, Xu Kuohai wrote:

Impelment bpf_arch_text_poke() for arm64, so bpf trampoline code can use it to replace nop with jump, or replace jump with nop.

Signed-off-by: Xu Kuohai xukuohai@huawei.com Acked-by: Song Liu songliubraving@fb.com

---

arch/arm64/net/bpf_jit_comp.c | 63 +++++++++++++++++++++++++++++++++++ 1 file changed, 63 insertions(+)

diff --git a/arch/arm64/net/bpf_jit_comp.c b/arch/arm64/net/bpf_jit_comp.c index 8ab4035dea27..3f9bdfec54c4 100644 --- a/arch/arm64/net/bpf_jit_comp.c +++ b/arch/arm64/net/bpf_jit_comp.c @@ -9,6 +9,7 @@ #include <linux/bitfield.h> #include <linux/bpf.h> +#include <linux/memory.h> #include <linux/filter.h> #include <linux/printk.h> #include <linux/slab.h> @@ -18,6 +19,7 @@ #include <asm/cacheflush.h> #include <asm/debug-monitors.h> #include <asm/insn.h> +#include <asm/patching.h> #include <asm/set_memory.h> #include "bpf_jit.h" @@ -1529,3 +1531,64 @@ void bpf_jit_free_exec(void *addr) { return vfree(addr); }

+static int gen_branch_or_nop(enum aarch64_insn_branch_type type, void *ip,

• 	     void *addr, u32 *insn)
  

+{

• if (!addr)

• *insn = aarch64_insn_gen_nop();
  

• else

• *insn = aarch64_insn_gen_branch_imm((unsigned long)ip,
  

• 				    (unsigned long)addr,
  

• 				    type);
  

• return *insn != AARCH64_BREAK_FAULT ? 0 : -EFAULT;

+}

+int bpf_arch_text_poke(void *ip, enum bpf_text_poke_type poke_type,

•        void *old_addr, void *new_addr)
  

+{

• int ret;
• u32 old_insn;
• u32 new_insn;
• u32 replaced;
• enum aarch64_insn_branch_type branch_type;
• if (!is_bpf_text_address((long)ip))

• /* Only poking bpf text is supported. Since kernel function
  

•  * entry is set up by ftrace, we reply on ftrace to poke kernel
  

•  * functions. For kernel funcitons, bpf_arch_text_poke() is only
  

•  * called after a failed poke with ftrace. In this case, there
  

•  * is probably something wrong with fentry, so there is nothing
  

•  * we can do here. See register_fentry, unregister_fentry and
  

•  * modify_fentry for details.
  

•  */
  

• return -EINVAL;
  

If you rely on ftrace to poke functions, why do you need to patch text at all? Why does the rest of this function exist?

I really don't like having another piece of code outside of ftrace patching the ftrace patch-site; this needs a much better explanation.

• if (poke_type == BPF_MOD_CALL)

• branch_type = AARCH64_INSN_BRANCH_LINK;
  

• else

• branch_type = AARCH64_INSN_BRANCH_NOLINK;
  

• if (gen_branch_or_nop(branch_type, ip, old_addr, &old_insn) < 0)

• return -EFAULT;
  

• if (gen_branch_or_nop(branch_type, ip, new_addr, &new_insn) < 0)

• return -EFAULT;
  

• mutex_lock(&text_mutex);
• if (aarch64_insn_read(ip, &replaced)) {

• ret = -EFAULT;
  

• goto out;
  

• }
• if (replaced != old_insn) {

• ret = -EFAULT;
  

• goto out;
  

• }
• ret = aarch64_insn_patch_text_nosync((void *)ip, new_insn);

... and where does the actual synchronization come from in this case?

Thanks, Mark.

+out:

• mutex_unlock(&text_mutex);
• return ret;

+}

2.30.2