runtime: fix grsec support

Changeset 36c9c7810f14 broke support for grsec-patched kernels.
Those do not give back the address requested without MAP_FIXED,
so when verifying an mmap without this flag for success, the
resulting address must not be compared against the requested
address since it may have succeeded at a different location.

R=golang-dev, rsc, gustavo, iant
CC=golang-dev
https://golang.org/cl/5650072

diff --git a/src/pkg/runtime/mem_linux.c b/src/pkg/runtime/mem_linux.c
index 47287939ad9e0d4d17dcd1290cad3a4421189fc0..b3e79cc412c9691042100f5bb34e1bcabe02f864 100644 (file)
--- a/src/pkg/runtime/mem_linux.c
+++ b/src/pkg/runtime/mem_linux.c
@@ -34,6 +34,21 @@ addrspace_free(void *v, uintptr n)
        return 1;
 }
 
+static void *
+mmap_fixed(byte *v, uintptr n, int32 prot, int32 flags, int32 fd, uint32 offset)
+{
+       void *p;
+
+       p = runtime·mmap(v, n, prot, flags, fd, offset);
+       if(p != v && addrspace_free(v, n)) {
+               // On some systems, mmap ignores v without
+               // MAP_FIXED, so retry if the address space is free.
+               if(p > (void*)4096)
+                       runtime·munmap(p, n);
+               p = runtime·mmap(v, n, prot, flags|MAP_FIXED, fd, offset);
+       }
+       return p;
+}
 
 void*
 runtime·SysAlloc(uintptr n)
@@ -76,20 +91,16 @@ runtime·SysReserve(void *v, uintptr n)
        // if we can reserve at least 64K and check the assumption in SysMap.
        // Only user-mode Linux (UML) rejects these requests.
        if(sizeof(void*) == 8 && (uintptr)v >= 0xffffffffU) {
-               p = runtime·mmap(v, 64<<10, PROT_NONE, MAP_ANON|MAP_PRIVATE, -1, 0);
-               if (p != v) {
+               p = mmap_fixed(v, 64<<10, PROT_NONE, MAP_ANON|MAP_PRIVATE, -1, 0);
+               if (p != v)
                        return nil;
-               }
                runtime·munmap(p, 64<<10);
-               
-               
                return v;
        }
        
        p = runtime·mmap(v, n, PROT_NONE, MAP_ANON|MAP_PRIVATE, -1, 0);
-       if((uintptr)p < 4096 || -(uintptr)p < 4096) {
+       if((uintptr)p < 4096 || -(uintptr)p < 4096)
                return nil;
-       }
        return p;
 }
 
@@ -102,15 +113,7 @@ runtime·SysMap(void *v, uintptr n)
 
        // On 64-bit, we don't actually have v reserved, so tread carefully.
        if(sizeof(void*) == 8 && (uintptr)v >= 0xffffffffU) {
-               p = runtime·mmap(v, n, PROT_READ|PROT_WRITE|PROT_EXEC, MAP_ANON|MAP_PRIVATE, -1, 0);
-               if(p != v && addrspace_free(v, n)) {
-                       // On some systems, mmap ignores v without
-                       // MAP_FIXED, so retry if the address space is free.
-                       if(p > (void*)4096) {
-                               runtime·munmap(p, n);
-                       }
-                       p = runtime·mmap(v, n, PROT_READ|PROT_WRITE|PROT_EXEC, MAP_ANON|MAP_FIXED|MAP_PRIVATE, -1, 0);
-               }
+               p = mmap_fixed(v, n, PROT_READ|PROT_WRITE|PROT_EXEC, MAP_ANON|MAP_PRIVATE, -1, 0);
                if(p == (void*)ENOMEM)
                        runtime·throw("runtime: out of memory");
                if(p != v) {