Commit 08691752 authored by Daniel Borkmann's avatar Daniel Borkmann Committed by Alexei Starovoitov
Browse files

bpf, x64: save 5 bytes in prologue when ebpf insns came from cbpf 



While it's rather cumbersome to reduce prologue for cBPF->eBPF
migrations wrt spill/fill for r15 which is callee saved register
due to bpf_error path in bpf_jit.S that is both used by migrations
as well as native eBPF, we can still trivially save 5 bytes in
prologue for the former since tail calls can never be used there.
cBPF->eBPF migrations also have their own custom prologue in BPF
asm that xors A and X reg anyway, so it's fine we skip this here.

Signed-off-by: default avatarDaniel Borkmann <daniel@iogearbox.net>
Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
parent 4c38e2f3

arch/x86/net/bpf_jit_comp.c

+16 −12
Original line number Diff line number Diff line
@@ -216,7 +216,7 @@ struct jit_context { 
/* emit x64 prologue code for BPF program and check it's size.

 * bpf_tail_call helper will skip it while jumping into another program

 */

static void emit_prologue(u8 **pprog, u32 stack_depth)

static void emit_prologue(u8 **pprog, u32 stack_depth, bool ebpf_from_cbpf)

{

	u8 *prog = *pprog;

	int cnt = 0;
@@ -251,10 +251,11 @@ static void emit_prologue(u8 **pprog, u32 stack_depth) 
	/* mov qword ptr [rbp+24],r15 */

	EMIT4(0x4C, 0x89, 0x7D, 24);



	/* Clear the tail call counter (tail_call_cnt): for eBPF tail calls

	 * we need to reset the counter to 0. It's done in two instructions,

	 * resetting rax register to 0 (xor on eax gets 0 extended), and

	 * moving it to the counter location.

	if (!ebpf_from_cbpf) {

		/* Clear the tail call counter (tail_call_cnt): for eBPF tail

		 * calls we need to reset the counter to 0. It's done in two

		 * instructions, resetting rax register to 0, and moving it

		 * to the counter location.

		 */



		/* xor eax, eax */
@@ -263,6 +264,8 @@ static void emit_prologue(u8 **pprog, u32 stack_depth) 
		EMIT4(0x48, 0x89, 0x45, 32);



		BUILD_BUG_ON(cnt != PROLOGUE_SIZE);

	}



	*pprog = prog;

}
@@ -453,7 +456,8 @@ static int do_jit(struct bpf_prog *bpf_prog, int *addrs, u8 *image, 
	int proglen = 0;

	u8 *prog = temp;



	emit_prologue(&prog, bpf_prog->aux->stack_depth);

	emit_prologue(&prog, bpf_prog->aux->stack_depth,

		      bpf_prog_was_classic(bpf_prog));



	if (seen_ld_abs)

		emit_load_skb_data_hlen(&prog);

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