bpf: Reserve space for BPF trampoline in BPF programs

BPF trampoline can be made to work with existing 5 bytes of BPF program
prologue, but let's add 5 bytes of NOPs to the beginning of every JITed BPF
program to make BPF trampoline job easier. They can be removed in the future.
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20191114185720.1641606-14-ast@kernel.org

arch/x86/net/bpf_jit_comp.c

@@ -206,7 +206,7 @@ struct jit_context {
 /* number of bytes emit_call() needs to generate call instruction */
 #define X86_CALL_SIZE		5
 
-#define PROLOGUE_SIZE		20
+#define PROLOGUE_SIZE		25
 
 /*
  * Emit x86-64 prologue code for BPF program and check its size.
@@ -215,8 +215,13 @@ struct jit_context {
 static void emit_prologue(u8 **pprog, u32 stack_depth, bool ebpf_from_cbpf)
 {
 	u8 *prog = *pprog;
-	int cnt = 0;
+	int cnt = X86_CALL_SIZE;
 
+	/* BPF trampoline can be made to work without these nops,
+	 * but let's waste 5 bytes for now and optimize later
+	 */
+	memcpy(prog, ideal_nops[NOP_ATOMIC5], cnt);
+	prog += cnt;
 	EMIT1(0x55);             /* push rbp */
 	EMIT3(0x48, 0x89, 0xE5); /* mov rbp, rsp */
 	/* sub rsp, rounded_stack_depth */