bpf, x86: Simplify the parsing logic of structure parameters

Extra_nregs of structure parameters and nr_args can be
added directly at the beginning, and using a flip flag
to identifiy structure parameters. Meantime, renaming
some variables to make them more sense.
Signed-off-by: Pu Lehui <pulehui@huawei.com>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/r/20230105035026.3091988-1-pulehui@huaweicloud.comSigned-off-by: Martin KaFai Lau <martin.lau@kernel.org>

arch/x86/net/bpf_jit_comp.c

@@ -1857,62 +1857,59 @@ st:			if (is_imm8(insn->off))
 	return proglen;
 }
 
-static void save_regs(const struct btf_func_model *m, u8 **prog, int nr_args,
+static void save_regs(const struct btf_func_model *m, u8 **prog, int nr_regs,
 		      int stack_size)
 {
-	int i, j, arg_size, nr_regs;
+	int i, j, arg_size;
+	bool next_same_struct = false;
+
 	/* Store function arguments to stack.
 	 * For a function that accepts two pointers the sequence will be:
 	 * mov QWORD PTR [rbp-0x10],rdi
 	 * mov QWORD PTR [rbp-0x8],rsi
 	 */
-	for (i = 0, j = 0; i < min(nr_args, 6); i++) {
-		if (m->arg_flags[i] & BTF_FMODEL_STRUCT_ARG) {
-			nr_regs = (m->arg_size[i] + 7) / 8;
+	for (i = 0, j = 0; i < min(nr_regs, 6); i++) {
+		/* The arg_size is at most 16 bytes, enforced by the verifier. */
+		arg_size = m->arg_size[j];
+		if (arg_size > 8) {
 			arg_size = 8;
-		} else {
-			nr_regs = 1;
-			arg_size = m->arg_size[i];
+			next_same_struct = !next_same_struct;
 		}
 
-		while (nr_regs) {
 		emit_stx(prog, bytes_to_bpf_size(arg_size),
 			 BPF_REG_FP,
-				 j == 5 ? X86_REG_R9 : BPF_REG_1 + j,
-				 -(stack_size - j * 8));
-			nr_regs--;
-			j++;
-		}
+			 i == 5 ? X86_REG_R9 : BPF_REG_1 + i,
+			 -(stack_size - i * 8));
+
+		j = next_same_struct ? j : j + 1;
 	}
 }
 
-static void restore_regs(const struct btf_func_model *m, u8 **prog, int nr_args,
+static void restore_regs(const struct btf_func_model *m, u8 **prog, int nr_regs,
 			 int stack_size)
 {
-	int i, j, arg_size, nr_regs;
+	int i, j, arg_size;
+	bool next_same_struct = false;
 
 	/* Restore function arguments from stack.
 	 * For a function that accepts two pointers the sequence will be:
 	 * EMIT4(0x48, 0x8B, 0x7D, 0xF0); mov rdi,QWORD PTR [rbp-0x10]
 	 * EMIT4(0x48, 0x8B, 0x75, 0xF8); mov rsi,QWORD PTR [rbp-0x8]
 	 */
-	for (i = 0, j = 0; i < min(nr_args, 6); i++) {
-		if (m->arg_flags[i] & BTF_FMODEL_STRUCT_ARG) {
-			nr_regs = (m->arg_size[i] + 7) / 8;
+	for (i = 0, j = 0; i < min(nr_regs, 6); i++) {
+		/* The arg_size is at most 16 bytes, enforced by the verifier. */
+		arg_size = m->arg_size[j];
+		if (arg_size > 8) {
 			arg_size = 8;
-		} else {
-			nr_regs = 1;
-			arg_size = m->arg_size[i];
+			next_same_struct = !next_same_struct;
 		}
 
-		while (nr_regs) {
 		emit_ldx(prog, bytes_to_bpf_size(arg_size),
-				 j == 5 ? X86_REG_R9 : BPF_REG_1 + j,
+			 i == 5 ? X86_REG_R9 : BPF_REG_1 + i,
 			 BPF_REG_FP,
-				 -(stack_size - j * 8));
-			nr_regs--;
-			j++;
-		}
+			 -(stack_size - i * 8));
+
+		j = next_same_struct ? j : j + 1;
 	}
 }
 
@@ -2138,8 +2135,8 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
 				struct bpf_tramp_links *tlinks,
 				void *func_addr)
 {
-	int ret, i, nr_args = m->nr_args, extra_nregs = 0;
-	int regs_off, ip_off, args_off, stack_size = nr_args * 8, run_ctx_off;
+	int i, ret, nr_regs = m->nr_args, stack_size = 0;
+	int regs_off, nregs_off, ip_off, run_ctx_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];
@@ -2148,17 +2145,14 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
 	u8 *prog;
 	bool save_ret;
 
-	/* x86-64 supports up to 6 arguments. 7+ can be added in the future */
-	if (nr_args > 6)
-		return -ENOTSUPP;
-
-	for (i = 0; i < MAX_BPF_FUNC_ARGS; i++) {
+	/* extra registers for struct arguments */
+	for (i = 0; i < m->nr_args; i++)
 		if (m->arg_flags[i] & BTF_FMODEL_STRUCT_ARG)
-			extra_nregs += (m->arg_size[i] + 7) / 8 - 1;
-	}
-	if (nr_args + extra_nregs > 6)
+			nr_regs += (m->arg_size[i] + 7) / 8 - 1;
+
+	/* x86-64 supports up to 6 arguments. 7+ can be added in the future */
+	if (nr_regs > 6)
 		return -ENOTSUPP;
-	stack_size += extra_nregs * 8;
 
 	/* Generated trampoline stack layout:
 	 *
@@ -2172,7 +2166,7 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
 	 *                 [ ...             ]
 	 * RBP - regs_off  [ reg_arg1        ]  program's ctx pointer
 	 *
-	 * RBP - args_off  [ arg regs count  ]  always
+	 * RBP - nregs_off [ regs count	     ]  always
 	 *
 	 * RBP - ip_off    [ traced function ]  BPF_TRAMP_F_IP_ARG flag
 	 *
@@ -2184,11 +2178,12 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
 	if (save_ret)
 		stack_size += 8;
 
+	stack_size += nr_regs * 8;
 	regs_off = stack_size;
 
-	/* args count  */
+	/* regs count  */
 	stack_size += 8;
-	args_off = stack_size;
+	nregs_off = stack_size;
 
 	if (flags & BPF_TRAMP_F_IP_ARG)
 		stack_size += 8; /* room for IP address argument */
@@ -2221,11 +2216,11 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
 	EMIT1(0x53);		 /* push rbx */
 
 	/* Store number of argument registers of the traced function:
-	 *   mov rax, nr_args + extra_nregs
-	 *   mov QWORD PTR [rbp - args_off], rax
+	 *   mov rax, nr_regs
+	 *   mov QWORD PTR [rbp - nregs_off], rax
 	 */
-	emit_mov_imm64(&prog, BPF_REG_0, 0, (u32) nr_args + extra_nregs);
-	emit_stx(&prog, BPF_DW, BPF_REG_FP, BPF_REG_0, -args_off);
+	emit_mov_imm64(&prog, BPF_REG_0, 0, (u32) nr_regs);
+	emit_stx(&prog, BPF_DW, BPF_REG_FP, BPF_REG_0, -nregs_off);
 
 	if (flags & BPF_TRAMP_F_IP_ARG) {
 		/* Store IP address of the traced function:
@@ -2236,7 +2231,7 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
 		emit_stx(&prog, BPF_DW, BPF_REG_FP, BPF_REG_0, -ip_off);
 	}
 
-	save_regs(m, &prog, nr_args, regs_off);
+	save_regs(m, &prog, nr_regs, regs_off);
 
 	if (flags & BPF_TRAMP_F_CALL_ORIG) {
 		/* arg1: mov rdi, im */
@@ -2266,7 +2261,7 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
 	}
 
 	if (flags & BPF_TRAMP_F_CALL_ORIG) {
-		restore_regs(m, &prog, nr_args, regs_off);
+		restore_regs(m, &prog, nr_regs, regs_off);
 
 		if (flags & BPF_TRAMP_F_ORIG_STACK) {
 			emit_ldx(&prog, BPF_DW, BPF_REG_0, BPF_REG_FP, 8);
@@ -2307,7 +2302,7 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
 		}
 
 	if (flags & BPF_TRAMP_F_RESTORE_REGS)
-		restore_regs(m, &prog, nr_args, regs_off);
+		restore_regs(m, &prog, nr_regs, regs_off);
 
 	/* This needs to be done regardless. If there were fmod_ret programs,
 	 * the return value is only updated on the stack and still needs to be