On Thu, Oct 27, 2016 at 12:22:58PM +0800, Haozhong Zhang wrote:
> 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 <haozhong.zh...@intel.com>
Reviewed-by: Eduardo Habkost <ehabk...@redhat.com>
But I would add comment near the get_file_size() call to indicate
that not stopping on get_file_size() errors is on purpose and not
a mistake.
> ---
> exec.c | 22 +++++++++++++++++++++-
> 1 file changed, 21 insertions(+), 1 deletion(-)
>
> diff --git a/exec.c b/exec.c
> index 587b489..a2b371a 100644
> --- a/exec.c
> +++ b/exec.c
> @@ -1224,6 +1224,15 @@ 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,
> @@ -1235,6 +1244,7 @@ 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,
> @@ -1297,6 +1307,8 @@ static void *file_ram_alloc(RAMBlock *block,
> }
> #endif
>
> + file_size = get_file_size(fd);
> +
> 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",
> @@ -1311,8 +1323,16 @@ static void *file_ram_alloc(RAMBlock *block,
> * hosts, so don't bother bailing out on errors.
> * If anything goes wrong with it under other filesystems,
> * mmap will fail.
> + *
> + * Do not truncate the non-empty backend file to avoid corrupting
> + * 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 (ftruncate(fd, memory)) {
> + if (!file_size && ftruncate(fd, memory)) {
> perror("ftruncate");
> }
>
> --
> 2.10.1
>
--
Eduardo
