[PATCH RFC 06/28] ARM: entry: Invoke syscalls using C

[Linus Walleij]

This makes the assembly invoking syscalls switch over to
doing this from C with a small assembly stub to actually
jump into the syscall.

Split the syscall invocation into two paths: one for plain
invocation and one for tracing (also known as "reload"
as it was reloading the registers from regs). We rename
this path with the infix "trace" as that code will be
trace-specific as we move code over to C.

Some registers such as r1 and lr get cobbled during the
C calls and need to be restored when we return.

Right now the part in C doesn't do much more than check the
syscall number to be valid (a test previously done with
a cmp r #NR_syscalls inside the invoke_syscall macro)
but we will gradually factor over more assembly to C that can
then be switched to the generic entry code so the exercise
gets a point.

Tested with a full system boot and by issuing some command
line tools with strace to make sure the tracing path still
works.

Signed-off-by: Linus Walleij <linus.walleij@xxxxxxxxxx>
---
 arch/arm/include/asm/syscall.h |  3 ++
 arch/arm/kernel/Makefile       |  3 +-
 arch/arm/kernel/entry-common.S | 65 ++++++++++++++++++++++++++++++++++++------
 arch/arm/kernel/entry-header.S | 25 ----------------
 arch/arm/kernel/syscall.c      | 26 +++++++++++++++++
 5 files changed, 88 insertions(+), 34 deletions(-)

diff --git a/arch/arm/include/asm/syscall.h b/arch/arm/include/asm/syscall.h
index fe4326d938c1..1d21f26ecf51 100644
--- a/arch/arm/include/asm/syscall.h
+++ b/arch/arm/include/asm/syscall.h
@@ -19,6 +19,9 @@
 
 extern const unsigned long sys_call_table[];
 
+int invoke_syscall(void *table, struct pt_regs *regs, int scno, void *retp);
+int invoke_syscall_trace(void *table, struct pt_regs *regs, int scno, void *retp);
+
 static inline int syscall_get_nr(struct task_struct *task,
 				 struct pt_regs *regs)
 {
diff --git a/arch/arm/kernel/Makefile b/arch/arm/kernel/Makefile
index aaae31b8c4a5..d5a128a4228d 100644
--- a/arch/arm/kernel/Makefile
+++ b/arch/arm/kernel/Makefile
@@ -20,7 +20,8 @@ CFLAGS_REMOVE_return_address.o = -pg
 obj-y		:= elf.o entry-common.o irq.o opcodes.o \
 		   process.o ptrace.o reboot.o io.o \
 		   setup.o signal.o sigreturn_codes.o \
-		   stacktrace.o sys_arm.o time.o traps.o
+		   stacktrace.o sys_arm.o time.o traps.o \
+		   syscall.o
 
 KASAN_SANITIZE_stacktrace.o := n
 KASAN_SANITIZE_traps.o := n
diff --git a/arch/arm/kernel/entry-common.S b/arch/arm/kernel/entry-common.S
index da5c2d4b62e5..57aa1084a047 100644
--- a/arch/arm/kernel/entry-common.S
+++ b/arch/arm/kernel/entry-common.S
@@ -254,21 +254,23 @@ ENTRY(vector_swi)
 #else
 	str	scno, [tsk, #TI_ABI_SYSCALL]
 #endif
-	/*
-	 * Reload the registers that may have been corrupted on entry to
-	 * the syscall assembly (by tracing or context tracking.)
-	 */
- TRACE(	ldmia	sp, {r0 - r3}		)
-
+	mov	r1, sp				@ put regs into r1
 	ldr	r10, [tsk, #TI_FLAGS]		@ check for syscall tracing
 	stmdb	sp!, {r4, r5}			@ push fifth and sixth args
 
 	tst	r10, #_TIF_SYSCALL_WORK		@ are we tracing syscalls?
 	bne	__sys_trace
 
-	invoke_syscall tbl, scno, r10, __ret_fast_syscall
+	mov	r0, tbl
+	/* r1 already contains regs */
+	mov	r2, scno			@ syscall number from r7
+	badr	r3, __ret_fast_syscall
+	bl 	invoke_syscall
 
+	/* Restore regs into r1 and lr after C call */
+	badr	lr, __ret_fast_syscall
 	add	r1, sp, #S_OFF
+
 2:	cmp	scno, #(__ARM_NR_BASE - __NR_SYSCALL_BASE)
 	eor	r0, scno, #__NR_SYSCALL_BASE	@ put OS number back
 	bcs	arm_syscall
@@ -301,7 +303,16 @@ __sys_trace:
 	add	r0, sp, #S_OFF
 	bl	syscall_trace_enter
 	mov	scno, r0
-	invoke_syscall tbl, scno, r10, __sys_trace_return, reload=1
+	mov	r2, r0				@ scno into r2
+	add	r1, sp, #S_R0 + S_OFF		@ pointer to regs
+	mov	r0, tbl
+	badr	r3, __sys_trace_return
+	bl	invoke_syscall_trace
+
+	/* Restore regs into r1 and lr after C call */
+	add	r1, sp, #S_R0 + S_OFF		@ pointer to regs
+	badr	lr, __sys_trace_return
+
 	cmp	scno, #-1			@ skip the syscall?
 	bne	2b
 	add	sp, sp, #S_OFF			@ restore stack
@@ -415,6 +426,44 @@ sys_mmap2:
 		b	sys_mmap_pgoff
 ENDPROC(sys_mmap2)
 
+/*
+ * This call wants:
+ * r0: syscall table
+ * r1: regs
+ * r2: syscall number
+ * r3: pointer to return function
+ */
+SYM_TYPED_FUNC_START(invoke_syscall_asm)
+#ifdef CONFIG_CPU_SPECTRE
+	csdb
+#endif
+	mov	tbl, r0
+	mov	scno, r2
+	mov	lr, r3				@ return address
+	ldmia 	r1, {r0 - r3}			@ reload r0-r3
+	/* Arguments 5 and 6 are (hopefully) on the stack */
+	ldr	pc, [tbl, scno, lsl #2]		@ call sys_* routine
+SYM_FUNC_END(invoke_syscall_asm)
+
+/*
+ * This call wants:
+ * r0: syscall table
+ * r1: regs
+ * r2: syscall number
+ * r3: pointer to return function
+ */
+SYM_TYPED_FUNC_START(invoke_syscall_trace_asm)
+#ifdef CONFIG_CPU_SPECTRE
+	csdb
+#endif
+	mov	tbl, r0
+	mov	scno, r2
+	mov	lr, r3				@ return address
+	ldmia	r1, {r0 - r6}			@ reload r0-r6
+	stmia	sp, {r4, r5}			@ update stack arguments
+	ldr	pc, [tbl, scno, lsl #2]		@ call sys_* routine
+SYM_FUNC_END(invoke_syscall_trace_asm)
+
 #ifdef CONFIG_OABI_COMPAT
 
 /*
diff --git a/arch/arm/kernel/entry-header.S b/arch/arm/kernel/entry-header.S
index 99411fa91350..52b4fa97226d 100644
--- a/arch/arm/kernel/entry-header.S
+++ b/arch/arm/kernel/entry-header.S
@@ -389,31 +389,6 @@ ALT_UP_B(.L1_\@)
 #endif
 	.endm
 
-	.macro	invoke_syscall, table, nr, tmp, ret, reload=0
-#ifdef CONFIG_CPU_SPECTRE
-	mov	\tmp, \nr
-	cmp	\tmp, #NR_syscalls		@ check upper syscall limit
-	movcs	\tmp, #0
-	csdb
-	badr	lr, \ret			@ return address
-	.if	\reload
-	add	r1, sp, #S_R0 + S_OFF		@ pointer to regs
-	ldmiacc	r1, {r0 - r6}			@ reload r0-r6
-	stmiacc	sp, {r4, r5}			@ update stack arguments
-	.endif
-	ldrcc	pc, [\table, \tmp, lsl #2]	@ call sys_* routine
-#else
-	cmp	\nr, #NR_syscalls		@ check upper syscall limit
-	badr	lr, \ret			@ return address
-	.if	\reload
-	add	r1, sp, #S_R0 + S_OFF		@ pointer to regs
-	ldmiacc	r1, {r0 - r6}			@ reload r0-r6
-	stmiacc	sp, {r4, r5}			@ update stack arguments
-	.endif
-	ldrcc	pc, [\table, \nr, lsl #2]	@ call sys_* routine
-#endif
-	.endm
-
 /*
  * These are the registers used in the syscall handler, and allow us to
  * have in theory up to 7 arguments to a function - r0 to r6.
diff --git a/arch/arm/kernel/syscall.c b/arch/arm/kernel/syscall.c
new file mode 100644
index 000000000000..be193266299f
--- /dev/null
+++ b/arch/arm/kernel/syscall.c
@@ -0,0 +1,26 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/syscalls.h>
+#include <asm/syscall.h>
+
+int invoke_syscall_asm(void *table, struct pt_regs *regs, int scno, void *retp);
+
+__visible int invoke_syscall(void *table, struct pt_regs *regs, int scno, void *retp)
+{
+	if (scno < NR_syscalls)
+		/* Doing this with return makes sure the stack gets pop:ed */
+		return invoke_syscall_asm(table, regs, scno, retp);
+
+	return 0;
+}
+
+int invoke_syscall_trace_asm(void *table, struct pt_regs *regs, int scno, void *retp);
+
+__visible int invoke_syscall_trace(void *table, struct pt_regs *regs, int scno, void *retp)
+{
+	if (scno < NR_syscalls)
+		/* Doing this with return makes sure the stack gets pop:ed */
+		return invoke_syscall_trace_asm(table, regs, scno, retp);
+
+	return 0;
+}

-- 
2.46.2