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

[Mark Rutland]

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@xxxxxxxxxx>
> Acked-by: Song Liu <songliubraving@xxxxxx>
> ---
>  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
>