runtime: warn about SELinux based mmap failures on Linux.

SELinux will cause mmap to fail when we request w+x memory unless the
user has configured their policies. We have a warning in make.bash,
but it's quite likely that the policy will be reset at some point and
then all their binaries start failing.

This patch prints a warning on Linux when mmap fails with EACCES.

R=rsc
CC=golang-dev
https://golang.org/cl/152086

src/pkg/runtime/linux/386/sys.s

@@ -69,8 +69,9 @@ TEXT runtime·mmap(SB),7,$0
 	SHRL	$12, BP
 	INT	$0x80
 	CMPL	AX, $0xfffff001
-	JLS	2(PC)
-	INT	$3
+	JLS	3(PC)
+	NOTL	AX
+	INCL	AX
 	RET
 
 // int32 futex(int32 *uaddr, int32 op, int32 val,

src/pkg/runtime/linux/amd64/sys.s

@@ -81,8 +81,9 @@ TEXT	runtime·mmap(SB),7,$0-32
 	MOVL	$9, AX			// syscall entry
 	SYSCALL
 	CMPQ	AX, $0xfffffffffffff001
-	JLS	2(PC)
-	CALL	notok(SB)
+	JLS	3(PC)
+	NOTQ	AX
+	INCQ	AX
 	RET
 
 TEXT	notok(SB),7,$0

src/pkg/runtime/malloc.cgo

@@ -208,8 +208,19 @@ mallocinit(void)
 void*
 SysAlloc(uintptr n)
 {
+	void *p;
 	mstats.sys += n;
-	return runtime_mmap(nil, n, PROT_READ|PROT_WRITE|PROT_EXEC, MAP_ANON|MAP_PRIVATE, -1, 0);
+	p = runtime_mmap(nil, n, PROT_READ|PROT_WRITE|PROT_EXEC, MAP_ANON|MAP_PRIVATE, -1, 0);
+	if(p < (void*)4096) {
+		if(p == (void*)EACCES) {
+			printf("mmap: access denied\n");
+			printf("If you're running SELinux, enable execmem for this process.\n");
+		} else {
+			printf("mmap: errno=%p\n", p);
+		}
+		exit(2);
+	}
+	return p;
 }
 
 void

src/pkg/runtime/mem.c

@@ -20,6 +20,10 @@ brk(uint32 n)
 	byte *v;
 
 	v = runtime_mmap(nil, n, PROT_READ|PROT_WRITE|PROT_EXEC, MAP_ANON|MAP_PRIVATE, 0, 0);
+	if(v < (void *)4096) {
+		printf("mmap: errno=%p\n", v);
+		exit(2);
+	}
 	m->mem.nmmap += n;
 	return v;
 }
@@ -56,6 +60,9 @@ oldmal(uint32 n)
 			m->mem.hunk =
 				runtime_mmap(nil, NHUNK, PROT_READ|PROT_WRITE|PROT_EXEC,
 					MAP_ANON|MAP_PRIVATE, 0, 0);
+			if(m->mem.hunk < (void*)4096) {
+				*(uint32*)0xf1 = 0;
+			}
 			m->mem.nhunk = NHUNK;
 			m->mem.nmmap += NHUNK;
 		}

src/pkg/runtime/runtime.h

@@ -446,6 +446,13 @@ void	notewakeup(Note*);
 #define runtime_setcallerpc runtime·setcallerpc
 #endif
 
+/*
+ * This is consistent across Linux and BSD.
+ * If a new OS is added that is different, move this to
+ * $GOOS/$GOARCH/defs.h.
+ */
+#define EACCES		13
+
 /*
  * low level go-called
  */