syscall: Plan 9: use lightweight errstr in entersyscall mode

Change 231af8ac63aa (CL 7314062) made runtime.enteryscall()
set m->mcache = nil, which means that we can no longer use
syscall.errstr in syscall.Syscall and syscall.Syscall6, since it
requires a new buffer to be allocated for holding the error string.
Instead, we use pre-allocated per-M storage to hold error strings
from syscalls made while in entersyscall mode, and call
runtime.findnull to calculate the lengths.

Fixes #4994.

R=rsc, rminnich, ality, dvyukov, rminnich, r
CC=golang-dev
https://golang.org/cl/7567043

src/pkg/runtime/runtime.h

@@ -320,6 +320,7 @@ struct	M
 #endif
 #ifdef GOOS_plan9
 	int8*		notesig;
+	byte*	errstr;
 #endif
 	SEH*	seh;
 	uintptr	end[];

src/pkg/runtime/sys_plan9_386.s

@@ -170,3 +170,23 @@ TEXT runtime·sigtramp(SB),7,$0
 // Only used by the 64-bit runtime.
 TEXT runtime·setfpmasks(SB),7,$0
 	RET
+
+#define ERRMAX 128	/* from os_plan9.h */
+
+// func errstr() String
+// Only used by package syscall.
+// Grab error string due to a syscall made
+// in entersyscall mode, without going
+// through the allocator (issue 4994).
+// See ../syscall/asm_plan9_386.s:/·Syscall/
+TEXT runtime·errstr(SB),7,$0
+	get_tls(AX)
+	MOVL	m(AX), BX
+	MOVL	m_errstr(BX), CX
+	MOVL	CX, 4(SP)
+	MOVL	$ERRMAX, 8(SP)
+	MOVL	$41, AX
+	INT	$64
+	CALL	runtime·findnull(SB)
+	MOVL	AX, 8(SP)
+	RET

src/pkg/runtime/sys_plan9_amd64.s

@@ -206,3 +206,24 @@ TEXT runtime·setfpmasks(SB),7,$8
 	MOVL	AX, 0(SP)
 	LDMXCSR	0(SP)
 	RET
+
+#define ERRMAX 128	/* from os_plan9.h */
+
+// func errstr() String
+// Only used by package syscall.
+// Grab error string due to a syscall made
+// in entersyscall mode, without going
+// through the allocator (issue 4994).
+// See ../syscall/asm_plan9_386.s:/·Syscall/
+TEXT runtime·errstr(SB),7,$0
+	get_tls(AX)
+	MOVQ	m(AX), BX
+	MOVQ	m_errstr(BX), CX
+	MOVQ	CX, 8(SP)
+	MOVQ	$ERRMAX, 16(SP)
+	MOVQ	$0x8000, AX
+	MOVQ	$41, BP
+	SYSCALL
+	CALL	runtime·findnull(SB)
+	MOVQ	AX, 16(SP)
+	RET

src/pkg/runtime/thread_plan9.c

@@ -19,6 +19,10 @@ runtime·mpreinit(M *mp)
 	// Initialize stack and goroutine for note handling.
 	mp->gsignal = runtime·malg(32*1024);
 	mp->notesig = (int8*)runtime·malloc(ERRMAX*sizeof(int8));
+
+	// Initialize stack for handling strings from the
+	// errstr system call, as used in package syscall.
+	mp->errstr = (byte*)runtime·malloc(ERRMAX*sizeof(byte));
 }
 
 // Called to initialize a new m (including the bootstrap m).

src/pkg/syscall/asm_plan9_386.s

@@ -29,7 +29,7 @@ TEXT	·Syscall(SB),7,$0
 	JNE	ok3
 
 	SUBL	$8, SP
-	CALL	syscall·errstr(SB)
+	CALL	runtime·errstr(SB)
 	MOVL	SP, SI
 	ADDL	$8, SP
 	JMP	copyresult3
@@ -67,7 +67,7 @@ TEXT	·Syscall6(SB),7,$0
 	JNE	ok4
 	
 	SUBL	$8, SP
-	CALL	syscall·errstr(SB)
+	CALL	runtime·errstr(SB)
 	MOVL	SP, SI
 	ADDL	$8, SP
 	JMP	copyresult4

src/pkg/syscall/asm_plan9_amd64.s

@@ -32,7 +32,7 @@ TEXT	·Syscall(SB),7,$0
 	JNE	ok3
 
 	SUBQ	$16, SP
-	CALL	syscall·errstr(SB)
+	CALL	runtime·errstr(SB)
 	MOVQ	SP, SI
 	ADDQ	$16, SP
 	JMP	copyresult3
@@ -71,7 +71,7 @@ TEXT	·Syscall6(SB),7,$0
 	JNE	ok4
 	
 	SUBQ	$16, SP
-	CALL	syscall·errstr(SB)
+	CALL	runtime·errstr(SB)
 	MOVQ	SP, SI
 	ADDQ	$16, SP
 	JMP	copyresult4