syscall: define Syscall in terms of RawSyscall on linux

This is a re-do of CL 388477, fixing #52472.

It is unsafe to call syscall.RawSyscall from syscall.Syscall with
-coverpkg=all and -race. This is because:

1. Coverage adds a sync/atomic call in RawSyscall to increment the
   coverage counter.
2. Race mode instruments sync/atomic calls with TSAN runtime calls. TSAN
   eventually calls runtime.racecallbackfunc, which expects
   getg().m.p != 0, which is no longer true after entersyscall().

cmd/go actually avoids adding coverage instrumention to package runtime
in race mode entirely to avoid these kinds of problems. Rather than also
excluding all of syscall for this one function, work around by calling
RawSyscall6 instead, which avoids coverage instrumention both by being
written in assembly and in package runtime/*.

For #51087
Fixes #52472

Change-Id: Iaffd27df03753020c4716059a455d6ca7b62f347
Reviewed-on: https://go-review.googlesource.com/c/go/+/401654
Run-TryBot: Michael Pratt <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Auto-Submit: Michael Pratt <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>

Author: prattmic

src/runtime/proc.go

@@ -3658,7 +3658,7 @@ func reentersyscall(pc, sp uintptr) {
 
 // Standard syscall entry used by the go syscall library and normal cgo calls.
 //
-// This is exported via linkname to assembly in the syscall package.
+// This is exported via linkname to assembly in the syscall package and x/sys.
 //
 //go:nosplit
 //go:linkname entersyscall

src/syscall/asm_linux_386.s

@@ -13,32 +13,6 @@
 // instead of the glibc-specific "CALL 0x10(GS)".
 #define INVOKE_SYSCALL	INT	$0x80
 
-// func Syscall(trap uintptr, a1, a2, a3 uintptr) (r1, r2, err uintptr);
-// Trap # in AX, args in BX CX DX SI DI, return in AX
-TEXT ·Syscall(SB),NOSPLIT,$0-28
-	CALL	runtime·entersyscall(SB)
-	MOVL	trap+0(FP), AX	// syscall entry
-	MOVL	a1+4(FP), BX
-	MOVL	a2+8(FP), CX
-	MOVL	a3+12(FP), DX
-	MOVL	$0, SI
-	MOVL	$0, DI
-	INVOKE_SYSCALL
-	CMPL	AX, $0xfffff001
-	JLS	ok
-	MOVL	$-1, r1+16(FP)
-	MOVL	$0, r2+20(FP)
-	NEGL	AX
-	MOVL	AX, err+24(FP)
-	CALL	runtime·exitsyscall(SB)
-	RET
-ok:
-	MOVL	AX, r1+16(FP)
-	MOVL	DX, r2+20(FP)
-	MOVL	$0, err+24(FP)
-	CALL	runtime·exitsyscall(SB)
-	RET
-
 // func Syscall6(trap uintptr, a1, a2, a3, a4, a5, a6 uintptr) (r1, r2, err uintptr);
 TEXT ·Syscall6(SB),NOSPLIT,$0-40
 	CALL	runtime·entersyscall(SB)

src/syscall/asm_linux_amd64.s

@@ -11,33 +11,6 @@
 
 #define SYS_gettimeofday 96
 
-// func Syscall(trap int64, a1, a2, a3 uintptr) (r1, r2, err uintptr);
-// Trap # in AX, args in DI SI DX R10 R8 R9, return in AX DX
-// Note that this differs from "standard" ABI convention, which
-// would pass 4th arg in CX, not R10.
-
-TEXT ·Syscall(SB),NOSPLIT,$0-56
-	CALL	runtime·entersyscall<ABIInternal>(SB)
-	MOVQ	a1+8(FP), DI
-	MOVQ	a2+16(FP), SI
-	MOVQ	a3+24(FP), DX
-	MOVQ	trap+0(FP), AX	// syscall entry
-	SYSCALL
-	CMPQ	AX, $0xfffffffffffff001
-	JLS	ok
-	MOVQ	$-1, r1+32(FP)
-	MOVQ	$0, r2+40(FP)
-	NEGQ	AX
-	MOVQ	AX, err+48(FP)
-	CALL	runtime·exitsyscall<ABIInternal>(SB)
-	RET
-ok:
-	MOVQ	AX, r1+32(FP)
-	MOVQ	DX, r2+40(FP)
-	MOVQ	$0, err+48(FP)
-	CALL	runtime·exitsyscall<ABIInternal>(SB)
-	RET
-
 // func Syscall6(trap, a1, a2, a3, a4, a5, a6 uintptr) (r1, r2, err uintptr)
 TEXT ·Syscall6(SB),NOSPLIT,$0-80
 	CALL	runtime·entersyscall<ABIInternal>(SB)

src/syscall/asm_linux_arm.s

@@ -9,36 +9,6 @@
 // System calls for arm, Linux
 //
 
-// func Syscall(syscall uintptr, a1, a2, a3 uintptr) (r1, r2, err uintptr);
-TEXT ·Syscall(SB),NOSPLIT,$0-28
-	BL	runtime·entersyscall(SB)
-	MOVW	trap+0(FP), R7
-	MOVW	a1+4(FP), R0
-	MOVW	a2+8(FP), R1
-	MOVW	a3+12(FP), R2
-	MOVW	$0, R3
-	MOVW	$0, R4
-	MOVW	$0, R5
-	SWI	$0
-	MOVW	$0xfffff001, R1
-	CMP	R1, R0
-	BLS	ok
-	MOVW	$-1, R1
-	MOVW	R1, r1+16(FP)
-	MOVW	$0, R2
-	MOVW	R2, r2+20(FP)
-	RSB	$0, R0, R0
-	MOVW	R0, err+24(FP)
-	BL	runtime·exitsyscall(SB)
-	RET
-ok:
-	MOVW	R0, r1+16(FP)
-	MOVW	$0, R0
-	MOVW	R0, r2+20(FP)
-	MOVW	R0, err+24(FP)
-	BL	runtime·exitsyscall(SB)
-	RET
-
 // func Syscall6(trap uintptr, a1, a2, a3, a4, a5, a6 uintptr) (r1, r2, err uintptr);
 // Actually Syscall5 but the rest of the code expects it to be named Syscall6.
 TEXT ·Syscall6(SB),NOSPLIT,$0-40

src/syscall/asm_linux_arm64.s

@@ -4,33 +4,6 @@
 
 #include "textflag.h"
 
-// func Syscall(trap int64, a1, a2, a3 int64) (r1, r2, err int64);
-TEXT ·Syscall(SB),NOSPLIT,$0-56
-	BL	runtime·entersyscall<ABIInternal>(SB)
-	MOVD	a1+8(FP), R0
-	MOVD	a2+16(FP), R1
-	MOVD	a3+24(FP), R2
-	MOVD	$0, R3
-	MOVD	$0, R4
-	MOVD	$0, R5
-	MOVD	trap+0(FP), R8	// syscall entry
-	SVC
-	CMN	$4095, R0
-	BCC	ok
-	MOVD	$-1, R4
-	MOVD	R4, r1+32(FP)	// r1
-	MOVD	ZR, r2+40(FP)	// r2
-	NEG	R0, R0
-	MOVD	R0, err+48(FP)	// errno
-	BL	runtime·exitsyscall<ABIInternal>(SB)
-	RET
-ok:
-	MOVD	R0, r1+32(FP)	// r1
-	MOVD	R1, r2+40(FP)	// r2
-	MOVD	ZR, err+48(FP)	// errno
-	BL	runtime·exitsyscall<ABIInternal>(SB)
-	RET
-
 TEXT ·Syscall6(SB),NOSPLIT,$0-80
 	BL	runtime·entersyscall<ABIInternal>(SB)
 	MOVD	a1+8(FP), R0

src/syscall/asm_linux_mips64x.s

@@ -10,31 +10,6 @@
 // System calls for mips64, Linux
 //
 
-// func Syscall(trap int64, a1, a2, a3 int64) (r1, r2, err int64);
-TEXT ·Syscall(SB),NOSPLIT,$0-56
-	JAL	runtime·entersyscall(SB)
-	MOVV	a1+8(FP), R4
-	MOVV	a2+16(FP), R5
-	MOVV	a3+24(FP), R6
-	MOVV	R0, R7
-	MOVV	R0, R8
-	MOVV	R0, R9
-	MOVV	trap+0(FP), R2	// syscall entry
-	SYSCALL
-	BEQ	R7, ok
-	MOVV	$-1, R1
-	MOVV	R1, r1+32(FP)	// r1
-	MOVV	R0, r2+40(FP)	// r2
-	MOVV	R2, err+48(FP)	// errno
-	JAL	runtime·exitsyscall(SB)
-	RET
-ok:
-	MOVV	R2, r1+32(FP)	// r1
-	MOVV	R3, r2+40(FP)	// r2
-	MOVV	R0, err+48(FP)	// errno
-	JAL	runtime·exitsyscall(SB)
-	RET
-
 TEXT ·Syscall6(SB),NOSPLIT,$0-80
 	JAL	runtime·entersyscall(SB)
 	MOVV	a1+8(FP), R4

src/syscall/asm_linux_mipsx.s

@@ -11,29 +11,6 @@
 // System calls for mips, Linux
 //
 
-// func Syscall(trap uintptr, a1, a2, a3 uintptr) (r1, r2, err uintptr);
-TEXT ·Syscall(SB),NOSPLIT,$0-28
-	JAL	runtime·entersyscall(SB)
-	MOVW	a1+4(FP), R4
-	MOVW	a2+8(FP), R5
-	MOVW	a3+12(FP), R6
-	MOVW	R0, R7
-	MOVW	trap+0(FP), R2	// syscall entry
-	SYSCALL
-	BEQ	R7, ok
-	MOVW	$-1, R1
-	MOVW	R1, r1+16(FP)	// r1
-	MOVW	R0, r2+20(FP)	// r2
-	MOVW	R2, err+24(FP)	// errno
-	JAL	runtime·exitsyscall(SB)
-	RET
-ok:
-	MOVW	R2, r1+16(FP)	// r1
-	MOVW	R3, r2+20(FP)	// r2
-	MOVW	R0, err+24(FP)	// errno
-	JAL	runtime·exitsyscall(SB)
-	RET
-
 // func Syscall6(trap trap, a1, a2, a3, a4, a5, a6 uintptr) (r1, r2, err uintptr);
 // 5th and 6th arg go at sp+16, sp+20.
 // Note that frame size of 20 means that 24 bytes gets reserved on stack.

src/syscall/asm_linux_ppc64x.s

@@ -10,31 +10,6 @@
 // System calls for ppc64, Linux
 //
 
-// func Syscall(trap int64, a1, a2, a3 int64) (r1, r2, err int64);
-TEXT ·Syscall(SB),NOSPLIT,$0-56
-	BL	runtime·entersyscall<ABIInternal>(SB)
-	MOVD	a1+8(FP), R3
-	MOVD	a2+16(FP), R4
-	MOVD	a3+24(FP), R5
-	MOVD	R0, R6
-	MOVD	R0, R7
-	MOVD	R0, R8
-	MOVD	trap+0(FP), R9	// syscall entry
-	SYSCALL R9
-	BVC	ok
-	MOVD	$-1, R4
-	MOVD	R4, r1+32(FP)	// r1
-	MOVD	R0, r2+40(FP)	// r2
-	MOVD	R3, err+48(FP)	// errno
-	BL	runtime·exitsyscall<ABIInternal>(SB)
-	RET
-ok:
-	MOVD	R3, r1+32(FP)	// r1
-	MOVD	R0, r2+40(FP)	// r2
-	MOVD	R0, err+48(FP)	// errno
-	BL	runtime·exitsyscall<ABIInternal>(SB)
-	RET
-
 TEXT ·Syscall6(SB),NOSPLIT,$0-80
 	BL	runtime·entersyscall<ABIInternal>(SB)
 	MOVD	a1+8(FP), R3

src/syscall/asm_linux_riscv64.s

@@ -8,30 +8,6 @@
 // System calls for riscv64, Linux
 //
 
-// func Syscall(trap int64, a1, a2, a3 int64) (r1, r2, err int64)
-TEXT ·Syscall(SB),NOSPLIT,$0-56
-	CALL	runtime·entersyscall(SB)
-	MOV	a1+8(FP), A0
-	MOV	a2+16(FP), A1
-	MOV	a3+24(FP), A2
-	MOV	trap+0(FP), A7	// syscall entry
-	ECALL
-	MOV	$-4096, T0
-	BLTU	T0, A0, err
-	MOV	A0, r1+32(FP)	// r1
-	MOV	A1, r2+40(FP)	// r2
-	MOV	ZERO, err+48(FP)	// errno
-	CALL	runtime·exitsyscall(SB)
-	RET
-err:
-	MOV	$-1, T0
-	MOV	T0, r1+32(FP)	// r1
-	MOV	ZERO, r2+40(FP)	// r2
-	SUB	A0, ZERO, A0
-	MOV	A0, err+48(FP)	// errno
-	CALL	runtime·exitsyscall(SB)
-	RET
-
 // func Syscall6(trap, a1, a2, a3, a4, a5, a6 uintptr) (r1, r2, err uintptr)
 TEXT ·Syscall6(SB),NOSPLIT,$0-80
 	CALL	runtime·entersyscall(SB)

src/syscall/asm_linux_s390x.s

@@ -8,32 +8,6 @@
 // System calls for s390x, Linux
 //
 
-// func Syscall(trap int64, a1, a2, a3 int64) (r1, r2, err int64)
-TEXT ·Syscall(SB),NOSPLIT,$0-56
-	BL	runtime·entersyscall(SB)
-	MOVD	a1+8(FP), R2
-	MOVD	a2+16(FP), R3
-	MOVD	a3+24(FP), R4
-	MOVD	$0, R5
-	MOVD	$0, R6
-	MOVD	$0, R7
-	MOVD	trap+0(FP), R1	// syscall entry
-	SYSCALL
-	MOVD	$0xfffffffffffff001, R8
-	CMPUBLT	R2, R8, ok
-	MOVD	$-1, r1+32(FP)
-	MOVD	$0, r2+40(FP)
-	NEG	R2, R2
-	MOVD	R2, err+48(FP)	// errno
-	BL	runtime·exitsyscall(SB)
-	RET
-ok:
-	MOVD	R2, r1+32(FP)
-	MOVD	R3, r2+40(FP)
-	MOVD	$0, err+48(FP)	// errno
-	BL	runtime·exitsyscall(SB)
-	RET
-
 // func Syscall6(trap, a1, a2, a3, a4, a5, a6 uintptr) (r1, r2, err uintptr)
 TEXT ·Syscall6(SB),NOSPLIT,$0-80
 	BL	runtime·entersyscall(SB)

src/syscall/syscall_linux.go

@@ -16,7 +16,6 @@ import (
 	"unsafe"
 )
 
-func Syscall(trap, a1, a2, a3 uintptr) (r1, r2 uintptr, err Errno)
 func Syscall6(trap, a1, a2, a3, a4, a5, a6 uintptr) (r1, r2 uintptr, err Errno)
 
 // N.B. RawSyscall6 is provided via linkname by runtime/internal/syscall.
@@ -26,6 +25,18 @@ func Syscall6(trap, a1, a2, a3, a4, a5, a6 uintptr) (r1, r2 uintptr, err Errno)
 
 func RawSyscall6(trap, a1, a2, a3, a4, a5, a6 uintptr) (r1, r2 uintptr, err Errno)
 
+// Pull in entersyscall/exitsyscall for Syscall/Syscall6.
+//
+// Note that this can't be a push linkname because the runtime already has a
+// nameless linkname to export to assembly here and in x/sys. Additionally,
+// entersyscall fetches the caller PC and SP and thus can't have a wrapper
+// inbetween.
+
+//go:linkname runtime_entersyscall runtime.entersyscall
+func runtime_entersyscall()
+//go:linkname runtime_exitsyscall runtime.exitsyscall
+func runtime_exitsyscall()
+
 // N.B. For the Syscall functions below:
 //
 // //go:uintptrkeepalive because the uintptr argument may be converted pointers
@@ -47,6 +58,28 @@ func RawSyscall(trap, a1, a2, a3 uintptr) (r1, r2 uintptr, err Errno) {
 	return RawSyscall6(trap, a1, a2, a3, 0, 0, 0)
 }
 
+//go:uintptrkeepalive
+//go:nosplit
+//go:linkname Syscall
+func Syscall(trap, a1, a2, a3 uintptr) (r1, r2 uintptr, err Errno) {
+	runtime_entersyscall()
+	// N.B. Calling RawSyscall here is unsafe with atomic coverage
+	// instrumentation and race mode.
+	//
+	// Coverage instrumentation will add a sync/atomic call to RawSyscall.
+	// Race mode will add race instrumentation to sync/atomic. Race
+	// instrumentation requires a P, which we no longer have.
+	//
+	// RawSyscall6 is fine because it is implemented in assembly and thus
+	// has no coverage instrumentation.
+	//
+	// This is typically not a problem in the runtime because cmd/go avoids
+	// adding coverage instrumentation to the runtime in race mode.
+	r1, r2, err = RawSyscall6(trap, a1, a2, a3, 0, 0, 0)
+	runtime_exitsyscall()
+	return
+}
+
 func rawSyscallNoError(trap, a1, a2, a3 uintptr) (r1, r2 uintptr)
 
 /*