memory: fetch pmem size in get_file_size()

Neither stat(2) nor lseek(2) report the size of Linux devdax pmem
character device nodes.  Commit 314aec4a6e06844937f1677f6cba21981005f389
("hostmem-file: reject invalid pmem file sizes") added code to
hostmem-file.c to fetch the size from sysfs and compare against the
user-provided size=NUM parameter:

  if (backend->size > size) {
      error_setg(errp, "size property %" PRIu64 " is larger than "
                 "pmem file \"%s\" size %" PRIu64, backend->size,
                 fb->mem_path, size);
      return;
  }

It turns out that exec.c:qemu_ram_alloc_from_fd() already has an
equivalent size check but it skips devdax pmem character devices because
lseek(2) returns 0:

  if (file_size > 0 && file_size < size) {
      error_setg(errp, "backing store %s size 0x%" PRIx64
                 " does not match 'size' option 0x" RAM_ADDR_FMT,
                 mem_path, file_size, size);
      return NULL;
  }

This patch moves the devdax pmem file size code into get_file_size() so
that we check the memory size in a single place:
qemu_ram_alloc_from_fd().  This simplifies the code and makes it more
general.

This also fixes the problem that hostmem-file only checks the devdax
pmem file size when the pmem=on parameter is given.  An unchecked
size=NUM parameter can lead to SIGBUS in QEMU so we must always fetch
the file size for Linux devdax pmem character device nodes.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20190830093056.12572-1-stefanha@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

backends/hostmem-file.c

@@ -58,28 +58,6 @@ file_backend_memory_alloc(HostMemoryBackend *backend, Error **errp)
         return;
     }
 
-    /*
-     * Verify pmem file size since starting a guest with an incorrect size
-     * leads to confusing failures inside the guest.
-     */
-    if (fb->is_pmem) {
-        Error *local_err = NULL;
-        uint64_t size;
-
-        size = qemu_get_pmem_size(fb->mem_path, &local_err);
-        if (local_err) {
-            error_propagate(errp, local_err);
-            return;
-        }
-
-        if (size && backend->size > size) {
-            error_setg(errp, "size property %" PRIu64 " is larger than "
-                       "pmem file \"%s\" size %" PRIu64, backend->size,
-                       fb->mem_path, size);
-            return;
-        }
-    }
-
     backend->force_prealloc = mem_prealloc;
     name = host_memory_backend_get_name(backend);
     memory_region_init_ram_from_file(&backend->mr, OBJECT(backend),

exec.c

@@ -1791,7 +1791,39 @@ long qemu_maxrampagesize(void)
 #ifdef CONFIG_POSIX
 static int64_t get_file_size(int fd)
 {
-    int64_t size = lseek(fd, 0, SEEK_END);
+    int64_t size;
+#if defined(__linux__)
+    struct stat st;
+
+    if (fstat(fd, &st) < 0) {
+        return -errno;
+    }
+
+    /* Special handling for devdax character devices */
+    if (S_ISCHR(st.st_mode)) {
+        g_autofree char *subsystem_path = NULL;
+        g_autofree char *subsystem = NULL;
+
+        subsystem_path = g_strdup_printf("/sys/dev/char/%d:%d/subsystem",
+                                         major(st.st_rdev), minor(st.st_rdev));
+        subsystem = g_file_read_link(subsystem_path, NULL);
+
+        if (subsystem && g_str_has_suffix(subsystem, "/dax")) {
+            g_autofree char *size_path = NULL;
+            g_autofree char *size_str = NULL;
+
+            size_path = g_strdup_printf("/sys/dev/char/%d:%d/size",
+                                    major(st.st_rdev), minor(st.st_rdev));
+
+            if (g_file_get_contents(size_path, &size_str, NULL, NULL)) {
+                return g_ascii_strtoll(size_str, NULL, 0);
+            }
+        }
+    }
+#endif /* defined(__linux__) */
+
+    /* st.st_size may be zero for special files yet lseek(2) works */
+    size = lseek(fd, 0, SEEK_END);
     if (size < 0) {
         return -errno;
     }

include/qemu/osdep.h

@@ -570,19 +570,6 @@ void qemu_set_tty_echo(int fd, bool echo);
 void os_mem_prealloc(int fd, char *area, size_t sz, int smp_cpus,
                      Error **errp);
 
-/**
- * qemu_get_pmem_size:
- * @filename: path to a pmem file
- * @errp: pointer to a NULL-initialized error object
- *
- * Determine the size of a persistent memory file.  Besides supporting files on
- * DAX file systems, this function also supports Linux devdax character
- * devices.
- *
- * Returns: the size or 0 on failure
- */
-uint64_t qemu_get_pmem_size(const char *filename, Error **errp);
-
 /**
  * qemu_get_pid_name:
  * @pid: pid of a process

util/oslib-posix.c

@@ -514,60 +514,6 @@ void os_mem_prealloc(int fd, char *area, size_t memory, int smp_cpus,
     }
 }
 
-uint64_t qemu_get_pmem_size(const char *filename, Error **errp)
-{
-    struct stat st;
-
-    if (stat(filename, &st) < 0) {
-        error_setg(errp, "unable to stat pmem file \"%s\"", filename);
-        return 0;
-    }
-
-#if defined(__linux__)
-    /* Special handling for devdax character devices */
-    if (S_ISCHR(st.st_mode)) {
-        char *subsystem_path = NULL;
-        char *subsystem = NULL;
-        char *size_path = NULL;
-        char *size_str = NULL;
-        uint64_t ret = 0;
-
-        subsystem_path = g_strdup_printf("/sys/dev/char/%d:%d/subsystem",
-                                         major(st.st_rdev), minor(st.st_rdev));
-        subsystem = g_file_read_link(subsystem_path, NULL);
-        if (!subsystem) {
-            error_setg(errp, "unable to read subsystem for pmem file \"%s\"",
-                       filename);
-            goto devdax_err;
-        }
-
-        if (!g_str_has_suffix(subsystem, "/dax")) {
-            error_setg(errp, "pmem file \"%s\" is not a dax device", filename);
-            goto devdax_err;
-        }
-
-        size_path = g_strdup_printf("/sys/dev/char/%d:%d/size",
-                                    major(st.st_rdev), minor(st.st_rdev));
-        if (!g_file_get_contents(size_path, &size_str, NULL, NULL)) {
-            error_setg(errp, "unable to read size for pmem file \"%s\"",
-                       size_path);
-            goto devdax_err;
-        }
-
-        ret = g_ascii_strtoull(size_str, NULL, 0);
-
-devdax_err:
-        g_free(size_str);
-        g_free(size_path);
-        g_free(subsystem);
-        g_free(subsystem_path);
-        return ret;
-    }
-#endif /* defined(__linux__) */
-
-    return st.st_size;
-}
-
 char *qemu_get_pid_name(pid_t pid)
 {
     char *name = NULL;

util/oslib-win32.c

@@ -562,12 +562,6 @@ void os_mem_prealloc(int fd, char *area, size_t memory, int smp_cpus,
     }
 }
 
-uint64_t qemu_get_pmem_size(const char *filename, Error **errp)
-{
-    error_setg(errp, "pmem support not available");
-    return 0;
-}
-
 char *qemu_get_pid_name(pid_t pid)
 {
     /* XXX Implement me */