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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: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
This commit is contained in:
Daniel Borkmann 2018-02-24 01:08:02 +01:00 committed by Alexei Starovoitov
parent 4c38e2f386
commit 0869175220
1 changed files with 16 additions and 12 deletions
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28
arch/x86/net/bpf_jit_comp.c
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@ -216,7 +216,7 @@ struct jit_context {
/* emit x64 prologue code for BPF program and check it's size. /* emit x64 prologue code for BPF program and check it's size.
* bpf_tail_call helper will skip it while jumping into another program * 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; u8 *prog = *pprog;
int cnt = 0; int cnt = 0;
@ -251,18 +251,21 @@ static void emit_prologue(u8 **pprog, u32 stack_depth)
/* mov qword ptr [rbp+24],r15 */ /* mov qword ptr [rbp+24],r15 */
EMIT4(0x4C, 0x89, 0x7D, 24); EMIT4(0x4C, 0x89, 0x7D, 24);
/* Clear the tail call counter (tail_call_cnt): for eBPF tail calls if (!ebpf_from_cbpf) {
* we need to reset the counter to 0. It's done in two instructions, /* Clear the tail call counter (tail_call_cnt): for eBPF tail
* resetting rax register to 0 (xor on eax gets 0 extended), and * calls we need to reset the counter to 0. It's done in two
* moving it to the counter location. * instructions, resetting rax register to 0, and moving it
*/ * to the counter location.
*/
/* xor eax, eax */ /* xor eax, eax */
EMIT2(0x31, 0xc0); EMIT2(0x31, 0xc0);
/* mov qword ptr [rbp+32], rax */ /* mov qword ptr [rbp+32], rax */
EMIT4(0x48, 0x89, 0x45, 32); EMIT4(0x48, 0x89, 0x45, 32);
BUILD_BUG_ON(cnt != PROLOGUE_SIZE);
}
BUILD_BUG_ON(cnt != PROLOGUE_SIZE);
*pprog = prog; *pprog = prog;
} }
@ -453,7 +456,8 @@ static int do_jit(struct bpf_prog *bpf_prog, int *addrs, u8 *image,
int proglen = 0; int proglen = 0;
u8 *prog = temp; 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) if (seen_ld_abs)
emit_load_skb_data_hlen(&prog); emit_load_skb_data_hlen(&prog);