[RFC PATCH 1/3] powerpc/bpf: implement bpf_arch_text_copy

[Christophe Leroy]

Le 10/11/2022 à 19:43, Hari Bathini a écrit :
> bpf_arch_text_copy is used to dump JITed binary to RX page, allowing
> multiple BPF programs to share the same page. Using patch_instruction
> to implement it.

Using patch_instruction() is nice for a quick implementation, but it is 
probably suboptimal. Due to the amount of data to be copied, it is worth 
a dedicated function that maps a RW copy of the page to be updated then 
does the copy at once with memcpy() then unmaps the page.

> 
> Signed-off-by: Hari Bathini <hbathini at linux.ibm.com>
> ---
>   arch/powerpc/net/bpf_jit_comp.c | 39 ++++++++++++++++++++++++++++++++-
>   1 file changed, 38 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/powerpc/net/bpf_jit_comp.c b/arch/powerpc/net/bpf_jit_comp.c
> index 43e634126514..7383e0effad2 100644
> --- a/arch/powerpc/net/bpf_jit_comp.c
> +++ b/arch/powerpc/net/bpf_jit_comp.c
> @@ -13,9 +13,12 @@
>   #include <linux/netdevice.h>
>   #include <linux/filter.h>
>   #include <linux/if_vlan.h>
> -#include <asm/kprobes.h>
> +#include <linux/memory.h>
>   #include <linux/bpf.h>
>   
> +#include <asm/kprobes.h>
> +#include <asm/code-patching.h>
> +
>   #include "bpf_jit.h"
>   
>   static void bpf_jit_fill_ill_insns(void *area, unsigned int size)
> @@ -23,6 +26,35 @@ static void bpf_jit_fill_ill_insns(void *area, unsigned int size)
>   	memset32(area, BREAKPOINT_INSTRUCTION, size / 4);
>   }
>   
> +/*
> + * Patch 'len' bytes of instructions from opcode to addr, one instruction
> + * at a time. Returns addr on success. ERR_PTR(-EINVAL), otherwise.
> + */
> +static void *bpf_patch_instructions(void *addr, void *opcode, size_t len)
> +{
> +	void *ret = ERR_PTR(-EINVAL);
> +	size_t patched = 0;
> +	u32 *inst = opcode;
> +	u32 *start = addr;
> +
> +	if (WARN_ON_ONCE(core_kernel_text((unsigned long)addr)))
> +		return ret;
> +
> +	mutex_lock(&text_mutex);
> +	while (patched < len) {
> +		if (patch_instruction(start++, ppc_inst(*inst)))
> +			goto error;
> +
> +		inst++;
> +		patched += 4;
> +	}
> +
> +	ret = addr;
> +error:
> +	mutex_unlock(&text_mutex);
> +	return ret;
> +}
> +
>   /* Fix updated addresses (for subprog calls, ldimm64, et al) during extra pass */
>   static int bpf_jit_fixup_addresses(struct bpf_prog *fp, u32 *image,
>   				   struct codegen_context *ctx, u32 *addrs)
> @@ -357,3 +389,8 @@ int bpf_add_extable_entry(struct bpf_prog *fp, u32 *image, int pass, struct code
>   	ctx->exentry_idx++;
>   	return 0;
>   }
> +
> +void *bpf_arch_text_copy(void *dst, void *src, size_t len)
> +{
> +	return bpf_patch_instructions(dst, src, len);
> +}

I can't see the added value of having two functions when the first one 
just calls the second one and is the only user of it. Why not have 
implemented bpf_patch_instructions() directly inside bpf_arch_text_copy() ?

By the way, it can be nice to have two functions, but split them 
differently, to avoid the goto: etc ....

I also prefer using for loops instead of while loops.

It could have looked like below (untested):

static void *bpf_patch_instructions(void *addr, void *opcode, size_t len)
{
	u32 *inst = opcode;
	u32 *start = addr;
	u32 *end = addr + len;

	for (inst = opcode, start = addr; start < end; inst++, start++) {
		if (patch_instruction(start, ppc_inst(*inst)))
			return ERR_PTR(-EINVAL);
	}

	return addr;
}

void *bpf_arch_text_copy(void *dst, void *src, size_t len)
{
	if (WARN_ON_ONCE(core_kernel_text((unsigned long)dst)))
		return ret;

	mutex_lock(&text_mutex);

	ret = bpf_patch_instructions(dst, src, len);

	mutex_unlock(&text_mutex);

	return ret;
}