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

[menglong8 . dong]

From: Menglong Dong <imagedong@xxxxxxxxxxx>

For now, the BPF program of type BPF_PROG_TYPE_TRACING can only be used
on the kernel functions whose arguments count less than 6. This is not
friendly at all, as too many functions have arguments count more than 6.

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 arguments
of arg6-argN are stored in "$rbp + 0x18", we need copy them to
"$rbp - regs_off + (6 * 8)".

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 call the origin function.
Then, we have to store rbx with 'mov' instead of 'push'.

It works well for the FENTRY and FEXIT, I'm not sure if there are other
complicated cases.

Signed-off-by: Menglong Dong <imagedong@xxxxxxxxxxx>
---
 arch/x86/net/bpf_jit_comp.c | 88 ++++++++++++++++++++++++++++++++-----
 1 file changed, 77 insertions(+), 11 deletions(-)

diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
index 1056bbf55b17..a3bc7e86ca19 100644
--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c
@@ -1868,7 +1868,7 @@ static void save_regs(const struct btf_func_model *m, u8 **prog, int nr_regs,
 	 * mov QWORD PTR [rbp-0x10],rdi
 	 * mov QWORD PTR [rbp-0x8],rsi
 	 */
-	for (i = 0, j = 0; i < min(nr_regs, 6); i++) {
+	for (i = 0, j = 0; i < min(nr_regs, 12); i++) {
 		/* The arg_size is at most 16 bytes, enforced by the verifier. */
 		arg_size = m->arg_size[j];
 		if (arg_size > 8) {
@@ -1876,10 +1876,22 @@ static void save_regs(const struct btf_func_model *m, u8 **prog, int nr_regs,
 			next_same_struct = !next_same_struct;
 		}
 
-		emit_stx(prog, bytes_to_bpf_size(arg_size),
-			 BPF_REG_FP,
-			 i == 5 ? X86_REG_R9 : BPF_REG_1 + i,
-			 -(stack_size - i * 8));
+		if (i <= 5) {
+			/* store function arguments in regs */
+			emit_stx(prog, bytes_to_bpf_size(arg_size),
+				 BPF_REG_FP,
+				 i == 5 ? X86_REG_R9 : BPF_REG_1 + i,
+				 -(stack_size - i * 8));
+		} else {
+			/* store function arguments in stack */
+			emit_ldx(prog, bytes_to_bpf_size(arg_size),
+				 BPF_REG_0, BPF_REG_FP,
+				 (i - 6) * 8 + 0x18);
+			emit_stx(prog, bytes_to_bpf_size(arg_size),
+				 BPF_REG_FP,
+				 BPF_REG_0,
+				 -(stack_size - i * 8));
+		}
 
 		j = next_same_struct ? j : j + 1;
 	}
@@ -1913,6 +1925,41 @@ static void restore_regs(const struct btf_func_model *m, u8 **prog, int nr_regs,
 	}
 }
 
+static void prepare_origin_stack(const struct btf_func_model *m, u8 **prog,
+				 int nr_regs, int stack_size)
+{
+	int i, j, arg_size;
+	bool next_same_struct = false;
+
+	if (nr_regs <= 6)
+		return;
+
+	/* Prepare the function arguments in stack before call origin
+	 * function. These arguments must be stored in the top of the
+	 * stack.
+	 */
+	for (i = 0, j = 0; i < min(nr_regs, 12); 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;
+			next_same_struct = !next_same_struct;
+		}
+
+		if (i > 5) {
+			emit_ldx(prog, bytes_to_bpf_size(arg_size),
+				 BPF_REG_0, BPF_REG_FP,
+				 (i - 6) * 8 + 0x18);
+			emit_stx(prog, bytes_to_bpf_size(arg_size),
+				 BPF_REG_FP,
+				 BPF_REG_0,
+				 -(stack_size - (i - 6) * 8));
+		}
+
+		j = next_same_struct ? j : j + 1;
+	}
+}
+
 static int invoke_bpf_prog(const struct btf_func_model *m, u8 **pprog,
 			   struct bpf_tramp_link *l, int stack_size,
 			   int run_ctx_off, bool save_ret)
@@ -2136,7 +2183,7 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
 				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];
@@ -2150,8 +2197,10 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
 		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 12 arguments. 1-6 are passed through
+	 * regs, the remains are through stack.
+	 */
+	if (nr_regs > 12)
 		return -ENOTSUPP;
 
 	/* Generated trampoline stack layout:
@@ -2170,7 +2219,14 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
 	 *
 	 * 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 */
@@ -2190,9 +2246,17 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
 
 	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))
+		stack_size += (nr_regs - 6) * 8;
+
+	arg_stack_off = stack_size;
+
 	if (flags & BPF_TRAMP_F_SKIP_FRAME) {
 		/* skip patched call instruction and point orig_call to actual
 		 * body of the kernel function.
@@ -2212,8 +2276,9 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
 	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 */
+	EMIT3_off32(0x48, 0x81, 0xEC, stack_size); /* sub rsp, 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
@@ -2262,6 +2327,7 @@ 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_regs, regs_off);
+		prepare_origin_stack(m, &prog, nr_regs, arg_stack_off);
 
 		if (flags & BPF_TRAMP_F_ORIG_STACK) {
 			emit_ldx(&prog, BPF_DW, BPF_REG_0, BPF_REG_FP, 8);
@@ -2321,7 +2387,7 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
 	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 */
-- 
2.40.1