runtime: add mmap of null page just in case

R=r, iant, robert.swiecki, rsc1
CC=golang-dev
https://golang.org/cl/1904044

7 files changed, 20 insertions, 3 deletions

diff --git a/src/pkg/runtime/linux/386/defs.h b/src/pkg/runtime/linux/386/defs.h
index ef8ef05d03..c1f58b2a08 100644
--- a/src/pkg/runtime/linux/386/defs.h
+++ b/src/pkg/runtime/linux/386/defs.h
@@ -10,6 +10,7 @@ enum {
 	PROT_EXEC = 0x4,
 	MAP_ANON = 0x20,
 	MAP_PRIVATE = 0x2,
+	MAP_FIXED = 0x10,
 	SA_RESTART = 0x10000000,
 	SA_ONSTACK = 0x8000000,
 	SA_RESTORER = 0x4000000,
diff --git a/src/pkg/runtime/linux/amd64/defs.h b/src/pkg/runtime/linux/amd64/defs.h
index c08e6b25d1..3e3d32f0d4 100644
--- a/src/pkg/runtime/linux/amd64/defs.h
+++ b/src/pkg/runtime/linux/amd64/defs.h
@@ -10,6 +10,7 @@ enum {
 	PROT_EXEC = 0x4,
 	MAP_ANON = 0x20,
 	MAP_PRIVATE = 0x2,
+	MAP_FIXED = 0x10,
 	SA_RESTART = 0x10000000,
 	SA_ONSTACK = 0x8000000,
 	SA_RESTORER = 0x4000000,
diff --git a/src/pkg/runtime/linux/arm/defs.h b/src/pkg/runtime/linux/arm/defs.h
index b139851715..ff4ba3e7c9 100644
--- a/src/pkg/runtime/linux/arm/defs.h
+++ b/src/pkg/runtime/linux/arm/defs.h
@@ -10,6 +10,7 @@ enum {
 	PROT_EXEC = 0x4,
 	MAP_ANON = 0x20,
 	MAP_PRIVATE = 0x2,
+	MAP_FIXED = 0x10,
 	SA_RESTART = 0x10000000,
 	SA_ONSTACK = 0x8000000,
 	SA_RESTORER = 0x4000000,
diff --git a/src/pkg/runtime/linux/defs.c b/src/pkg/runtime/linux/defs.c
index f3bdb61fa2..2044fd60c9 100644
--- a/src/pkg/runtime/linux/defs.c
+++ b/src/pkg/runtime/linux/defs.c
@@ -27,6 +27,7 @@ enum {
 
 	$MAP_ANON = MAP_ANONYMOUS,
 	$MAP_PRIVATE = MAP_PRIVATE,
+	$MAP_FIXED = MAP_FIXED,
 
 	$SA_RESTART = SA_RESTART,
 	$SA_ONSTACK = SA_ONSTACK,
diff --git a/src/pkg/runtime/linux/defs2.c b/src/pkg/runtime/linux/defs2.c
index 4cfe4a7ed0..3c0b110fc9 100644
--- a/src/pkg/runtime/linux/defs2.c
+++ b/src/pkg/runtime/linux/defs2.c
@@ -47,6 +47,7 @@ enum {
 
 	$MAP_ANON = MAP_ANONYMOUS,
 	$MAP_PRIVATE = MAP_PRIVATE,
+	$MAP_FIXED = MAP_FIXED,
 
 	$SA_RESTART = SA_RESTART,
 	$SA_ONSTACK = SA_ONSTACK,
diff --git a/src/pkg/runtime/linux/defs_arm.c b/src/pkg/runtime/linux/defs_arm.c
index 2b197272c6..d30c5bec43 100644
--- a/src/pkg/runtime/linux/defs_arm.c
+++ b/src/pkg/runtime/linux/defs_arm.c
@@ -31,6 +31,7 @@ enum {
 
 	$MAP_ANON = MAP_ANONYMOUS,
 	$MAP_PRIVATE = MAP_PRIVATE,
+	$MAP_FIXED = MAP_FIXED,
 
 	$SA_RESTART = SA_RESTART,
 	$SA_ONSTACK = SA_ONSTACK,
diff --git a/src/pkg/runtime/linux/mem.c b/src/pkg/runtime/linux/mem.c
index bad67001c4..925e7fd342 100644
--- a/src/pkg/runtime/linux/mem.c
+++ b/src/pkg/runtime/linux/mem.c
@@ -14,10 +14,10 @@ SysAlloc(uintptr n)
 		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);
 		}
-		exit(2);
+		printf("mmap: errno=%p\n", p);
+		throw("mmap");
 	}
 	return p;
 }
@@ -37,3 +37,14 @@ SysFree(void *v, uintptr n)
 	runtime_munmap(v, n);
 }
 
+void
+SysMemInit(void)
+{
+	// Code generators assume that references to addresses
+	// on the first page will fault.  Map the page explicitly with
+	// no permissions, to head off possible bugs like the system
+	// allocating that page as the virtual address space fills.
+	// Ignore any error, since other systems might be smart
+	// enough to never allow anything there.
+	runtime_mmap(nil, 4096, PROT_NONE, MAP_FIXED|MAP_ANON|MAP_PRIVATE, -1, 0);
+}