From d6af99c9f8d577927a52d00af3fa8e6ec0b2a4e2 Mon Sep 17 00:00:00 2001 From: Haozhong Zhang Date: Thu, 27 Oct 2016 12:22:58 +0800 Subject: [PATCH] exec.c: do not truncate non-empty memory backend file 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 Message-Id: <20161027042300.5929-2-haozhong.zhang@intel.com> Signed-off-by: Paolo Bonzini --- exec.c | 22 +++++++++++++++++++++- 1 file changed, 21 insertions(+), 1 deletion(-) diff --git a/exec.c b/exec.c index a19ed21bff..f471e7377a 100644 --- a/exec.c +++ b/exec.c @@ -1229,6 +1229,15 @@ 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, @@ -1240,6 +1249,7 @@ 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, @@ -1302,6 +1312,8 @@ 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", @@ -1316,8 +1328,16 @@ 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"); }