bpf: Add bpf_arch_text_poke() helper

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@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Song Liu <songliubraving@fb.com>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20191114185720.1641606-4-ast@kernel.org

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)
 {
@@ -486,6 +488,55 @@ 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_bp(ip, new_insn, X86_CALL_SIZE, NULL);
+		break;
+	case BPF_MOD_CALL_TO_CALL:
+		if (memcmp(ip, old_insn, X86_CALL_SIZE))
+			goto out;
+		text_poke_bp(ip, new_insn, X86_CALL_SIZE, NULL);
+		break;
+	case BPF_MOD_CALL_TO_NOP:
+		if (memcmp(ip, old_insn, X86_CALL_SIZE))
+			goto out;
+		text_poke_bp(ip, ideal_nops[NOP_ATOMIC5], X86_CALL_SIZE, NULL);
+		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)

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 */

kernel/bpf/core.c

@@ -2144,6 +2144,12 @@ 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);