[Xen-devel] [PATCH] libxc: Replace alloca() with mmap() in linux_privcmd_map_foreign_bulk()

[Aravindh Puthiyaparambil <aravindh@xxxxxxxxxxxx>]

When mapping in large amounts of pages (in the GB range) from a guest in to 
Dom0 using xc_map_foreign_bulk(), a segfault occurs in the libxc client 
application. This is because the pfn array in linux_privcmd_map_foreign_bulk() 
is being allocated using alloca() and the subsequent memcpy causes the stack to 
blow. This patch replaces the alloca() with mmap().

Signed-off-by: Aravindh Puthiyaparambil <aravindh@xxxxxxxxxxxx>
Acked-by: Andres Lagar-Cavilla <andres@xxxxxxxxxxxxxxxx>

diff -r 7c777cb8f705 -r 2f68aefc46c3 tools/libxc/xc_linux_osdep.c
--- a/tools/libxc/xc_linux_osdep.c      Wed Apr 18 16:49:55 2012 +0100
+++ b/tools/libxc/xc_linux_osdep.c      Thu Apr 19 15:21:43 2012 -0700
@@ -39,6 +39,7 @@
 #include "xenctrl.h"
 #include "xenctrlosdep.h"
 
+#define ROUNDUP(_x,_w) (((unsigned long)(_x)+(1UL<<(_w))-1) & ~((1UL<<(_w))-1))
 #define ERROR(_m, _a...)  xc_osdep_log(xch,XTL_ERROR,XC_INTERNAL_ERROR,_m , ## 
_a )
 #define PERROR(_m, _a...) xc_osdep_log(xch,XTL_ERROR,XC_INTERNAL_ERROR,_m \
                   " (%d = %s)", ## _a , errno, xc_strerror(xch, errno))
@@ -286,7 +287,14 @@ static void *linux_privcmd_map_foreign_b
          * IOCTL_PRIVCMD_MMAPBATCH.
          */
         privcmd_mmapbatch_t ioctlx;
-        xen_pfn_t *pfn = alloca(num * sizeof(*pfn));
+        xen_pfn_t *pfn = mmap(NULL, ROUNDUP((num * sizeof(*pfn)), 
XC_PAGE_SHIFT),
+                              PROT_READ | PROT_WRITE,
+                              MAP_PRIVATE | MAP_ANON | MAP_POPULATE, -1, 0);
+        if ( pfn == MAP_FAILED )
+        {
+            PERROR("xc_map_foreign_bulk: mmap of pfn array failed");
+            return NULL;
+        }
 
         memcpy(pfn, arr, num * sizeof(*arr));
 
@@ -328,6 +336,8 @@ static void *linux_privcmd_map_foreign_b
             break;
         }
 
+        munmap(pfn, ROUNDUP((num * sizeof(*pfn)), XC_PAGE_SHIFT));
+
         if ( rc == -ENOENT && i == num )
             rc = 0;
         else if ( rc )

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