[PATCH bpf] riscv, bpf: Adapt bpf trampoline to optimized riscv ftrace framework

[Pu Lehui]

From: Pu Lehui <pulehui@xxxxxxxxxx>

Commit 6724a76cff85 ("riscv: ftrace: Reduce the detour code size to
half") optimizes the detour code size of kernel functions to half with
T0 register and the upcoming DYNAMIC_FTRACE_WITH_DIRECT_CALLS of riscv
is based on this optimization, we need to adapt riscv bpf trampoline
based on this. One thing to do is to reduce detour code size of bpf
programs, and the second is to deal with the return address after the
execution of bpf trampoline. Meanwhile, add more comments and rename
some variables to make more sense. The related tests have passed.

This adaptation needs to be merged before the upcoming
DYNAMIC_FTRACE_WITH_DIRECT_CALLS of riscv, otherwise it will crash due
to a mismatch in the return address. So we target this modification to
bpf tree and add fixes tag for locating.

Fixes: 6724a76cff85 ("riscv: ftrace: Reduce the detour code size to half")
Signed-off-by: Pu Lehui <pulehui@xxxxxxxxxx>
---
 arch/riscv/net/bpf_jit_comp64.c | 110 ++++++++++++++------------------
 1 file changed, 47 insertions(+), 63 deletions(-)

diff --git a/arch/riscv/net/bpf_jit_comp64.c b/arch/riscv/net/bpf_jit_comp64.c
index c648864c8cd1..ffc9aa42f918 100644
--- a/arch/riscv/net/bpf_jit_comp64.c
+++ b/arch/riscv/net/bpf_jit_comp64.c
@@ -241,7 +241,7 @@ static void __build_epilogue(bool is_tail_call, struct rv_jit_context *ctx)
 	if (!is_tail_call)
 		emit_mv(RV_REG_A0, RV_REG_A5, ctx);
 	emit_jalr(RV_REG_ZERO, is_tail_call ? RV_REG_T3 : RV_REG_RA,
-		  is_tail_call ? 20 : 0, /* skip reserved nops and TCC init */
+		  is_tail_call ? 12 : 0, /* skip reserved nops and TCC init */
 		  ctx);
 }
 
@@ -618,32 +618,7 @@ static int add_exception_handler(const struct bpf_insn *insn,
 	return 0;
 }
 
-static int gen_call_or_nops(void *target, void *ip, u32 *insns)
-{
-	s64 rvoff;
-	int i, ret;
-	struct rv_jit_context ctx;
-
-	ctx.ninsns = 0;
-	ctx.insns = (u16 *)insns;
-
-	if (!target) {
-		for (i = 0; i < 4; i++)
-			emit(rv_nop(), &ctx);
-		return 0;
-	}
-
-	rvoff = (s64)(target - (ip + 4));
-	emit(rv_sd(RV_REG_SP, -8, RV_REG_RA), &ctx);
-	ret = emit_jump_and_link(RV_REG_RA, rvoff, false, &ctx);
-	if (ret)
-		return ret;
-	emit(rv_ld(RV_REG_RA, -8, RV_REG_SP), &ctx);
-
-	return 0;
-}
-
-static int gen_jump_or_nops(void *target, void *ip, u32 *insns)
+static int gen_jump_or_nops(void *target, void *ip, u32 *insns, bool is_call)
 {
 	s64 rvoff;
 	struct rv_jit_context ctx;
@@ -658,38 +633,38 @@ static int gen_jump_or_nops(void *target, void *ip, u32 *insns)
 	}
 
 	rvoff = (s64)(target - ip);
-	return emit_jump_and_link(RV_REG_ZERO, rvoff, false, &ctx);
+	return emit_jump_and_link(is_call ? RV_REG_T0 : RV_REG_ZERO,
+				  rvoff, false, &ctx);
 }
 
+#define DETOUR_NINSNS	2
+
 int bpf_arch_text_poke(void *ip, enum bpf_text_poke_type poke_type,
 		       void *old_addr, void *new_addr)
 {
-	u32 old_insns[4], new_insns[4];
+	u32 old_insns[DETOUR_NINSNS], new_insns[DETOUR_NINSNS];
 	bool is_call = poke_type == BPF_MOD_CALL;
-	int (*gen_insns)(void *target, void *ip, u32 *insns);
-	int ninsns = is_call ? 4 : 2;
 	int ret;
 
-	if (!is_bpf_text_address((unsigned long)ip))
+	if (!is_kernel_text((unsigned long)ip) &&
+	    !is_bpf_text_address((unsigned long)ip))
 		return -ENOTSUPP;
 
-	gen_insns = is_call ? gen_call_or_nops : gen_jump_or_nops;
-
-	ret = gen_insns(old_addr, ip, old_insns);
+	ret = gen_jump_or_nops(old_addr, ip, old_insns, is_call);
 	if (ret)
 		return ret;
 
-	if (memcmp(ip, old_insns, ninsns * 4))
+	if (memcmp(ip, old_insns, DETOUR_NINSNS * 4))
 		return -EFAULT;
 
-	ret = gen_insns(new_addr, ip, new_insns);
+	ret = gen_jump_or_nops(new_addr, ip, new_insns, is_call);
 	if (ret)
 		return ret;
 
 	cpus_read_lock();
 	mutex_lock(&text_mutex);
-	if (memcmp(ip, new_insns, ninsns * 4))
-		ret = patch_text(ip, new_insns, ninsns);
+	if (memcmp(ip, new_insns, DETOUR_NINSNS * 4))
+		ret = patch_text(ip, new_insns, DETOUR_NINSNS);
 	mutex_unlock(&text_mutex);
 	cpus_read_unlock();
 
@@ -717,7 +692,7 @@ static void restore_args(int nregs, int args_off, struct rv_jit_context *ctx)
 }
 
 static int invoke_bpf_prog(struct bpf_tramp_link *l, int args_off, int retval_off,
-			   int run_ctx_off, bool save_ret, struct rv_jit_context *ctx)
+			   int run_ctx_off, bool save_retval, struct rv_jit_context *ctx)
 {
 	int ret, branch_off;
 	struct bpf_prog *p = l->link.prog;
@@ -757,7 +732,7 @@ static int invoke_bpf_prog(struct bpf_tramp_link *l, int args_off, int retval_of
 	if (ret)
 		return ret;
 
-	if (save_ret)
+	if (save_retval)
 		emit_sd(RV_REG_FP, -retval_off, regmap[BPF_REG_0], ctx);
 
 	/* update branch with beqz */
@@ -787,20 +762,19 @@ static int __arch_prepare_bpf_trampoline(struct bpf_tramp_image *im,
 	int i, ret, offset;
 	int *branches_off = NULL;
 	int stack_size = 0, nregs = m->nr_args;
-	int retaddr_off, fp_off, retval_off, args_off;
-	int nregs_off, ip_off, run_ctx_off, sreg_off;
+	int fp_off, retval_off, args_off, nregs_off, ip_off, run_ctx_off, sreg_off;
 	struct bpf_tramp_links *fentry = &tlinks[BPF_TRAMP_FENTRY];
 	struct bpf_tramp_links *fexit = &tlinks[BPF_TRAMP_FEXIT];
 	struct bpf_tramp_links *fmod_ret = &tlinks[BPF_TRAMP_MODIFY_RETURN];
 	void *orig_call = func_addr;
-	bool save_ret;
+	bool save_retval, traced_ret;
 	u32 insn;
 
 	/* Generated trampoline stack layout:
 	 *
 	 * FP - 8	    [ RA of parent func	] return address of parent
 	 *					  function
-	 * FP - retaddr_off [ RA of traced func	] return address of traced
+	 * FP - 16	    [ RA of traced func	] return address of traced
 	 *					  function
 	 * FP - fp_off	    [ FP of parent func ]
 	 *
@@ -833,17 +807,20 @@ static int __arch_prepare_bpf_trampoline(struct bpf_tramp_image *im,
 	if (nregs > 8)
 		return -ENOTSUPP;
 
-	/* room for parent function return address */
+	/* room for return address of parent function */
 	stack_size += 8;
 
-	stack_size += 8;
-	retaddr_off = stack_size;
+	/* whether return to return address of traced function after bpf trampoline */
+	traced_ret = func_addr && !(flags & BPF_TRAMP_F_SKIP_FRAME);
+	/* room for return address of traced function */
+	if (traced_ret)
+		stack_size += 8;
 
 	stack_size += 8;
 	fp_off = stack_size;
 
-	save_ret = flags & (BPF_TRAMP_F_CALL_ORIG | BPF_TRAMP_F_RET_FENTRY_RET);
-	if (save_ret) {
+	save_retval = flags & (BPF_TRAMP_F_CALL_ORIG | BPF_TRAMP_F_RET_FENTRY_RET);
+	if (save_retval) {
 		stack_size += 8;
 		retval_off = stack_size;
 	}
@@ -869,7 +846,11 @@ static int __arch_prepare_bpf_trampoline(struct bpf_tramp_image *im,
 
 	emit_addi(RV_REG_SP, RV_REG_SP, -stack_size, ctx);
 
-	emit_sd(RV_REG_SP, stack_size - retaddr_off, RV_REG_RA, ctx);
+	/* store return address of parent function */
+	emit_sd(RV_REG_SP, stack_size - 8, RV_REG_RA, ctx);
+	/* store return address of traced function */
+	if (traced_ret)
+		emit_sd(RV_REG_SP, stack_size - 16, RV_REG_T0, ctx);
 	emit_sd(RV_REG_SP, stack_size - fp_off, RV_REG_FP, ctx);
 
 	emit_addi(RV_REG_FP, RV_REG_SP, stack_size, ctx);
@@ -890,7 +871,7 @@ static int __arch_prepare_bpf_trampoline(struct bpf_tramp_image *im,
 
 	/* skip to actual body of traced function */
 	if (flags & BPF_TRAMP_F_SKIP_FRAME)
-		orig_call += 16;
+		orig_call += 8;
 
 	if (flags & BPF_TRAMP_F_CALL_ORIG) {
 		emit_imm(RV_REG_A0, (const s64)im, ctx);
@@ -962,22 +943,25 @@ static int __arch_prepare_bpf_trampoline(struct bpf_tramp_image *im,
 	if (flags & BPF_TRAMP_F_RESTORE_REGS)
 		restore_args(nregs, args_off, ctx);
 
-	if (save_ret)
+	if (save_retval)
 		emit_ld(RV_REG_A0, -retval_off, RV_REG_FP, ctx);
 
 	emit_ld(RV_REG_S1, -sreg_off, RV_REG_FP, ctx);
 
-	if (flags & BPF_TRAMP_F_SKIP_FRAME)
-		/* return address of parent function */
+	if (traced_ret) {
+		/* restore return address of parent function */
 		emit_ld(RV_REG_RA, stack_size - 8, RV_REG_SP, ctx);
-	else
-		/* return address of traced function */
-		emit_ld(RV_REG_RA, stack_size - retaddr_off, RV_REG_SP, ctx);
+		/* restore return address of traced function */
+		emit_ld(RV_REG_T0, stack_size - 16, RV_REG_SP, ctx);
+	} else {
+		/* restore return address of parent function */
+		emit_ld(RV_REG_T0, stack_size - 8, RV_REG_SP, ctx);
+	}
 
 	emit_ld(RV_REG_FP, stack_size - fp_off, RV_REG_SP, ctx);
 	emit_addi(RV_REG_SP, RV_REG_SP, stack_size, ctx);
 
-	emit_jalr(RV_REG_ZERO, RV_REG_RA, 0, ctx);
+	emit_jalr(RV_REG_ZERO, RV_REG_T0, 0, ctx);
 
 	ret = ctx->ninsns;
 out:
@@ -1664,7 +1648,7 @@ int bpf_jit_emit_insn(const struct bpf_insn *insn, struct rv_jit_context *ctx,
 
 void bpf_jit_build_prologue(struct rv_jit_context *ctx)
 {
-	int i, stack_adjust = 0, store_offset, bpf_stack_adjust;
+	int stack_adjust = 0, store_offset, bpf_stack_adjust;
 
 	bpf_stack_adjust = round_up(ctx->prog->aux->stack_depth, 16);
 	if (bpf_stack_adjust)
@@ -1691,9 +1675,9 @@ void bpf_jit_build_prologue(struct rv_jit_context *ctx)
 
 	store_offset = stack_adjust - 8;
 
-	/* reserve 4 nop insns */
-	for (i = 0; i < 4; i++)
-		emit(rv_nop(), ctx);
+	/* 2 nops reserved for auipc+jalr pair */
+	emit(rv_nop(), ctx);
+	emit(rv_nop(), ctx);
 
 	/* First instruction is always setting the tail-call-counter
 	 * (TCC) register. This instruction is skipped for tail calls.
-- 
2.25.1