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

[Hari Bathini]

On 13/11/22 6:47 pm, Christophe Leroy wrote:

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

Yeah.

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.

I will see if I can come up with such implementation for the respin.

Signed-off-by: Hari Bathini <hbath...@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;
}

Sure. Will use this.

Thanks
Hari