[Qemu-devel] [PATCH 1/2] exec.c: do not truncate non-empty memory backend file

[Haozhong Zhang]

For '-object memory-backend-file,mem-path=foo,size=xyz', if the size of
file 'foo' does not match the given size 'xyz', the current QEMU will
truncate the file to the given size, which may corrupt the existing data
in that file. To avoid such data corruption, this patch disables
truncating non-empty backend files.

Signed-off-by: Haozhong Zhang <address@hidden>
---
 exec.c | 37 ++++++++++++++++++++++++++++++++++++-
 1 file changed, 36 insertions(+), 1 deletion(-)

diff --git a/exec.c b/exec.c
index e63c5a1..95983c9 100644
--- a/exec.c
+++ b/exec.c
@@ -1188,6 +1188,15 @@ void qemu_mutex_unlock_ramlist(void)
 }
 
 #ifdef __linux__
+static int64_t get_file_size(int fd)
+{
+    int64_t size = lseek(fd, 0, SEEK_END);
+    if (size < 0) {
+        return -errno;
+    }
+    return size;
+}
+
 static void *file_ram_alloc(RAMBlock *block,
                             ram_addr_t memory,
                             const char *path,
@@ -1199,6 +1208,7 @@ static void *file_ram_alloc(RAMBlock *block,
     char *c;
     void *area = MAP_FAILED;
     int fd = -1;
+    int64_t file_size;
 
     if (kvm_enabled() && !kvm_has_sync_mmu()) {
         error_setg(errp,
@@ -1256,6 +1266,14 @@ static void *file_ram_alloc(RAMBlock *block,
     block->page_size = qemu_fd_getpagesize(fd);
     block->mr->align = MAX(block->page_size, QEMU_VMALLOC_ALIGN);
 
+    file_size = get_file_size(fd);
+    if (file_size < 0) {
+        error_setg_errno(errp, file_size,
+                         "can't get size of backing store %s",
+                         path);
+        goto error;
+    }
+
     if (memory < block->page_size) {
         error_setg(errp, "memory size 0x" RAM_ADDR_FMT " must be equal to "
                    "or larger than page size 0x%zx",
@@ -1266,12 +1284,29 @@ static void *file_ram_alloc(RAMBlock *block,
     memory = ROUND_UP(memory, block->page_size);
 
     /*
+     * Do not extend/shrink the backend file if it's not empty, or its
+     * size does not match the aligned 'size=xxx' option. Otherwise,
+     * it is possible to corrupt the existing data in the file.
+     *
+     * Disabling shrinking is not enough. For example, the current
+     * vNVDIMM implementation stores the guest NVDIMM labels at the
+     * end of the backend file. If the backend file is later extended,
+     * QEMU will not be able to find those labels. Therefore,
+     * extending the non-empty backend file is disabled as well.
+     */
+    if (file_size && file_size != memory) {
+        error_setg(errp, "backing store %s size %"PRId64
+                   " does not math with aligned 'size' option %"PRIu64,
+                   path, file_size, memory);
+        goto error;
+    }
+    /*
      * ftruncate is not supported by hugetlbfs in older
      * hosts, so don't bother bailing out on errors.
      * If anything goes wrong with it under other filesystems,
      * mmap will fail.
      */
-    if (ftruncate(fd, memory)) {
+    if (!file_size && ftruncate(fd, memory)) {
         perror("ftruncate");
     }
 
-- 
2.10.1