bpf, x86: allow function arguments up to 12 for TRACING

For now, the BPF program of type BPF_PROG_TYPE_TRACING can only be used
on the kernel functions whose arguments count less than or equal to 6, if
not considering '> 8 bytes' struct argument. This is not friendly at all,
as too many functions have arguments count more than 6.

According to the current kernel version, below is a statistics of the
function arguments count:

argument count | function count
7 | 704
8 | 270
9 | 84
10 | 47
11 | 47
12 | 27
13 | 22
14 | 5
15 | 0
16 | 1

Therefore, let's enhance it by increasing the function arguments count
allowed in arch_prepare_bpf_trampoline(), for now, only x86_64.

For the case that we don't need to call origin function, which means
without BPF_TRAMP_F_CALL_ORIG, we need only copy the function arguments
that stored in the frame of the caller to current frame. The 7th and later
arguments are stored in "$rbp + 0x18", and they will be copied to the
stack area following where register values are saved.

For the case with BPF_TRAMP_F_CALL_ORIG, we need prepare the arguments
in stack before call origin function, which means we need alloc extra
"8 * (arg_count - 6)" memory in the top of the stack. Note, there should
not be any data be pushed to the stack before calling the origin function.
So 'rbx' value will be stored on a stack position higher than where stack
arguments are stored for BPF_TRAMP_F_CALL_ORIG.

According to the research of Yonghong, struct members should be all in
register or all on the stack. Meanwhile, the compiler will pass the
argument on regs if the remaining regs can hold the argument. Therefore,
we need save the arguments in order. Otherwise, disorder of the args can
happen. For example:

struct foo_struct {
long a;
int b;
};
int foo(char, char, char, char, char, struct foo_struct,
char);

the arg1-5,arg7 will be passed by regs, and arg6 will by stack. Therefore,
we should save/restore the arguments in the same order with the
declaration of foo(). And the args used as ctx in stack will be like this:

reg_arg6 -- copy from regs
stack_arg2 -- copy from stack
stack_arg1
reg_arg5 -- copy from regs
reg_arg4
reg_arg3
reg_arg2
reg_arg1

We use EMIT3_off32() or EMIT4() for "lea" and "sub". The range of the
imm in "lea" and "sub" is [-128, 127] if EMIT4() is used. Therefore,
we use EMIT3_off32() instead if the imm out of the range.

It works well for the FENTRY/FEXIT/MODIFY_RETURN.

Signed-off-by: Menglong Dong <[e-mail]>
Acked-by: Yonghong Song <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

arch/x86/net/bpf_jit_comp.c

···
 	return proglen;
 }
 
-static void save_regs(const struct btf_func_model *m, u8 **prog, int nr_regs,
-		      int stack_size)
+static void clean_stack_garbage(const struct btf_func_model *m,
+				u8 **pprog, int nr_stack_slots,
+				int stack_size)
 {
-	int i;
+	int arg_size, off;
+	u8 *prog;
+
+	/* Generally speaking, the compiler will pass the arguments
+	 * on-stack with "push" instruction, which will take 8-byte
+	 * on the stack. In this case, there won't be garbage values
+	 * while we copy the arguments from origin stack frame to current
+	 * in BPF_DW.
+	 *
+	 * However, sometimes the compiler will only allocate 4-byte on
+	 * the stack for the arguments. For now, this case will only
+	 * happen if there is only one argument on-stack and its size
+	 * not more than 4 byte. In this case, there will be garbage
+	 * values on the upper 4-byte where we store the argument on
+	 * current stack frame.
+	 *
+	 * arguments on origin stack:
+	 *
+	 * stack_arg_1(4-byte) xxx(4-byte)
+	 *
+	 * what we copy:
+	 *
+	 * stack_arg_1(8-byte): stack_arg_1(origin) xxx
+	 *
+	 * and the xxx is the garbage values which we should clean here.
+	 */
+	if (nr_stack_slots != 1)
+		return;
+
+	/* the size of the last argument */
+	arg_size = m->arg_size[m->nr_args - 1];
+	if (arg_size <= 4) {
+		off = -(stack_size - 4);
+		prog = *pprog;
+		/* mov DWORD PTR [rbp + off], 0 */
+		if (!is_imm8(off))
+			EMIT2_off32(0xC7, 0x85, off);
+		else
+			EMIT3(0xC7, 0x45, off);
+		EMIT(0, 4);
+		*pprog = prog;
+	}
+}
+
+/* get the count of the regs that are used to pass arguments */
+static int get_nr_used_regs(const struct btf_func_model *m)
+{
+	int i, arg_regs, nr_used_regs = 0;
+
+	for (i = 0; i < min_t(int, m->nr_args, MAX_BPF_FUNC_ARGS); i++) {
+		arg_regs = (m->arg_size[i] + 7) / 8;
+		if (nr_used_regs + arg_regs <= 6)
+			nr_used_regs += arg_regs;
+
+		if (nr_used_regs >= 6)
+			break;
+	}
+
+	return nr_used_regs;
+}
+
+static void save_args(const struct btf_func_model *m, u8 **prog,
+		      int stack_size, bool for_call_origin)
+{
+	int arg_regs, first_off, nr_regs = 0, nr_stack_slots = 0;
+	int i, j;
 
 	/* 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; i < min(nr_regs, 6); i++)
-		emit_stx(prog, BPF_DW, BPF_REG_FP,
-			 i == 5 ? X86_REG_R9 : BPF_REG_1 + i,
-			 -(stack_size - i * 8));
+	for (i = 0; i < min_t(int, m->nr_args, MAX_BPF_FUNC_ARGS); i++) {
+		arg_regs = (m->arg_size[i] + 7) / 8;
+
+		/* According to the research of Yonghong, struct members
+		 * should be all in register or all on the stack.
+		 * Meanwhile, the compiler will pass the argument on regs
+		 * if the remaining regs can hold the argument.
+		 *
+		 * Disorder of the args can happen. For example:
+		 *
+		 * struct foo_struct {
+		 *     long a;
+		 *     int b;
+		 * };
+		 * int foo(char, char, char, char, char, struct foo_struct,
+		 *         char);
+		 *
+		 * the arg1-5,arg7 will be passed by regs, and arg6 will
+		 * by stack.
+		 */
+		if (nr_regs + arg_regs > 6) {
+			/* copy function arguments from origin stack frame
+			 * into current stack frame.
+			 *
+			 * The starting address of the arguments on-stack
+			 * is:
+			 *   rbp + 8(push rbp) +
+			 *   8(return addr of origin call) +
+			 *   8(return addr of the caller)
+			 * which means: rbp + 24
+			 */
+			for (j = 0; j < arg_regs; j++) {
+				emit_ldx(prog, BPF_DW, BPF_REG_0, BPF_REG_FP,
+					 nr_stack_slots * 8 + 0x18);
+				emit_stx(prog, BPF_DW, BPF_REG_FP, BPF_REG_0,
+					 -stack_size);
+
+				if (!nr_stack_slots)
+					first_off = stack_size;
+				stack_size -= 8;
+				nr_stack_slots++;
+			}
+		} else {
+			/* Only copy the arguments on-stack to current
+			 * 'stack_size' and ignore the regs, used to
+			 * prepare the arguments on-stack for orign call.
+			 */
+			if (for_call_origin) {
+				nr_regs += arg_regs;
+				continue;
+			}
+
+			/* copy the arguments from regs into stack */
+			for (j = 0; j < arg_regs; j++) {
+				emit_stx(prog, BPF_DW, BPF_REG_FP,
+					 nr_regs == 5 ? X86_REG_R9 : BPF_REG_1 + nr_regs,
+					 -stack_size);
+				stack_size -= 8;
+				nr_regs++;
+			}
+		}
+	}
+
+	clean_stack_garbage(m, prog, nr_stack_slots, first_off);
 }
 
-static void restore_regs(const struct btf_func_model *m, u8 **prog, int nr_regs,
+static void restore_regs(const struct btf_func_model *m, u8 **prog,
 			 int stack_size)
 {
-	int i;
+	int i, j, arg_regs, nr_regs = 0;
 
 	/* 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]
+	 *
+	 * The logic here is similar to what we do in save_args()
 	 */
-	for (i = 0; i < min(nr_regs, 6); i++)
-		emit_ldx(prog, BPF_DW,
-			 i == 5 ? X86_REG_R9 : BPF_REG_1 + i,
-			 BPF_REG_FP,
-			 -(stack_size - i * 8));
+	for (i = 0; i < min_t(int, m->nr_args, MAX_BPF_FUNC_ARGS); i++) {
+		arg_regs = (m->arg_size[i] + 7) / 8;
+		if (nr_regs + arg_regs <= 6) {
+			for (j = 0; j < arg_regs; j++) {
+				emit_ldx(prog, BPF_DW,
+					 nr_regs == 5 ? X86_REG_R9 : BPF_REG_1 + nr_regs,
+					 BPF_REG_FP,
+					 -stack_size);
+				stack_size -= 8;
+				nr_regs++;
+			}
+		} else {
+			stack_size -= 8 * arg_regs;
+		}
+
+		if (nr_regs >= 6)
+			break;
+	}
 }
 
 static int invoke_bpf_prog(const struct btf_func_model *m, u8 **pprog,
···
 	/* arg1: mov rdi, progs[i] */
 	emit_mov_imm64(&prog, BPF_REG_1, (long) p >> 32, (u32) (long) p);
 	/* arg2: lea rsi, [rbp - ctx_cookie_off] */
-	EMIT4(0x48, 0x8D, 0x75, -run_ctx_off);
+	if (!is_imm8(-run_ctx_off))
+		EMIT3_off32(0x48, 0x8D, 0xB5, -run_ctx_off);
+	else
+		EMIT4(0x48, 0x8D, 0x75, -run_ctx_off);
 
 	if (emit_rsb_call(&prog, bpf_trampoline_enter(p), prog))
 		return -EINVAL;
···
 	emit_nops(&prog, 2);
 
 	/* arg1: lea rdi, [rbp - stack_size] */
-	EMIT4(0x48, 0x8D, 0x7D, -stack_size);
+	if (!is_imm8(-stack_size))
+		EMIT3_off32(0x48, 0x8D, 0xBD, -stack_size);
+	else
+		EMIT4(0x48, 0x8D, 0x7D, -stack_size);
 	/* arg2: progs[i]->insnsi for interpreter */
 	if (!p->jited)
 		emit_mov_imm64(&prog, BPF_REG_2,
···
 	/* arg2: mov rsi, rbx <- start time in nsec */
 	emit_mov_reg(&prog, true, BPF_REG_2, BPF_REG_6);
 	/* arg3: lea rdx, [rbp - run_ctx_off] */
-	EMIT4(0x48, 0x8D, 0x55, -run_ctx_off);
+	if (!is_imm8(-run_ctx_off))
+		EMIT3_off32(0x48, 0x8D, 0x95, -run_ctx_off);
+	else
+		EMIT4(0x48, 0x8D, 0x55, -run_ctx_off);
 	if (emit_rsb_call(&prog, bpf_trampoline_exit(p), prog))
 		return -EINVAL;
 
···
 				void *func_addr)
 {
 	int i, ret, nr_regs = m->nr_args, stack_size = 0;
-	int regs_off, nregs_off, ip_off, run_ctx_off;
+	int regs_off, nregs_off, ip_off, run_ctx_off, arg_stack_off, rbx_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];
···
 		if (m->arg_flags[i] & BTF_FMODEL_STRUCT_ARG)
 			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)
+	/* x86-64 supports up to MAX_BPF_FUNC_ARGS arguments. 1-6
+	 * are passed through regs, the remains are through stack.
+	 */
+	if (nr_regs > MAX_BPF_FUNC_ARGS)
 		return -ENOTSUPP;
 
 	/* Generated trampoline stack layout:
···
 	 *
 	 * RBP - ip_off    [ traced function ]  BPF_TRAMP_F_IP_ARG flag
 	 *
+	 * RBP - rbx_off   [ rbx value       ]  always
+	 *
 	 * RBP - run_ctx_off [ bpf_tramp_run_ctx ]
+	 *
+	 *                     [ stack_argN ]  BPF_TRAMP_F_CALL_ORIG
+	 *                     [ ...        ]
+	 *                     [ stack_arg2 ]
+	 * RBP - arg_stack_off [ stack_arg1 ]
 	 */
 
 	/* room for return value of orig_call or fentry prog */
···
 
 	ip_off = stack_size;
 
+	stack_size += 8;
+	rbx_off = stack_size;
+
 	stack_size += (sizeof(struct bpf_tramp_run_ctx) + 7) & ~0x7;
 	run_ctx_off = stack_size;
+
+	if (nr_regs > 6 && (flags & BPF_TRAMP_F_CALL_ORIG)) {
+		/* the space that used to pass arguments on-stack */
+		stack_size += (nr_regs - get_nr_used_regs(m)) * 8;
+		/* make sure the stack pointer is 16-byte aligned if we
+		 * need pass arguments on stack, which means
+		 *  [stack_size + 8(rbp) + 8(rip) + 8(origin rip)]
+		 * should be 16-byte aligned. Following code depend on
+		 * that stack_size is already 8-byte aligned.
+		 */
+		stack_size += (stack_size % 16) ? 0 : 8;
+	}
+
+	arg_stack_off = stack_size;
 
 	if (flags & BPF_TRAMP_F_SKIP_FRAME) {
 		/* skip patched call instruction and point orig_call to actual
···
 	x86_call_depth_emit_accounting(&prog, NULL);
 	EMIT1(0x55);		 /* push rbp */
 	EMIT3(0x48, 0x89, 0xE5); /* mov rbp, rsp */
-	EMIT4(0x48, 0x83, 0xEC, stack_size); /* sub rsp, stack_size */
-	EMIT1(0x53);		 /* push rbx */
+	if (!is_imm8(stack_size))
+		/* sub rsp, stack_size */
+		EMIT3_off32(0x48, 0x81, 0xEC, stack_size);
+	else
+		/* sub rsp, stack_size */
+		EMIT4(0x48, 0x83, 0xEC, stack_size);
+	/* mov QWORD PTR [rbp - rbx_off], rbx */
+	emit_stx(&prog, BPF_DW, BPF_REG_FP, BPF_REG_6, -rbx_off);
 
 	/* Store number of argument registers of the traced function:
 	 *   mov rax, nr_regs
···
 		emit_stx(&prog, BPF_DW, BPF_REG_FP, BPF_REG_0, -ip_off);
 	}
 
-	save_regs(m, &prog, nr_regs, regs_off);
+	save_args(m, &prog, regs_off, false);
 
 	if (flags & BPF_TRAMP_F_CALL_ORIG) {
 		/* arg1: mov rdi, im */
···
 	}
 
 	if (flags & BPF_TRAMP_F_CALL_ORIG) {
-		restore_regs(m, &prog, nr_regs, regs_off);
+		restore_regs(m, &prog, regs_off);
+		save_args(m, &prog, arg_stack_off, true);
 
 		if (flags & BPF_TRAMP_F_ORIG_STACK) {
 			emit_ldx(&prog, BPF_DW, BPF_REG_0, BPF_REG_FP, 8);
···
 		}
 
 	if (flags & BPF_TRAMP_F_RESTORE_REGS)
-		restore_regs(m, &prog, nr_regs, regs_off);
+		restore_regs(m, &prog, 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
···
 	if (save_ret)
 		emit_ldx(&prog, BPF_DW, BPF_REG_0, BPF_REG_FP, -8);
 
-	EMIT1(0x5B); /* pop rbx */
+	emit_ldx(&prog, BPF_DW, BPF_REG_6, BPF_REG_FP, -rbx_off);
 	EMIT1(0xC9); /* leave */
 	if (flags & BPF_TRAMP_F_SKIP_FRAME)
 		/* skip our return address and return to parent */