Re: [Qemu-devel] [PATCH v2] hostmem-file: reject invalid pmem file sizes

[Igor Mammedov <imammedo@redhat.com>]

On Wed, 13 Feb 2019 16:53:54 +0800
Wei Yang <richardw.yang@linux.intel.com> wrote:

> On Wed, Feb 13, 2019 at 03:14:01PM +0800, Stefan Hajnoczi wrote:
> >Guests started with NVDIMMs larger than the underlying host file produce
> >confusing errors inside the guest.  This happens because the guest
> >accesses pages beyond the end of the file.
> >
> >Check the pmem file size on startup and print a clear error message if
> >the size is invalid.
> >
> >Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1669053
> >Cc: Haozhong Zhang <haozhong.zhang@intel.com>
> >Cc: Zhang Yi <yi.z.zhang@linux.intel.com>
> >Cc: Eduardo Habkost <ehabkost@redhat.com>
> >Cc: Igor Mammedov <imammedo@redhat.com>
> >Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> >---
> >v2:
> > * Propagate qemu_get_pmem_size() errors [Igor]
> >
> > include/qemu/osdep.h    | 13 ++++++++++
> > backends/hostmem-file.c | 22 +++++++++++++++++
> > util/oslib-posix.c      | 53 +++++++++++++++++++++++++++++++++++++++++
> > util/oslib-win32.c      |  5 ++++
> > 4 files changed, 93 insertions(+)
> >
> >diff --git a/include/qemu/osdep.h b/include/qemu/osdep.h
> >index 840af09cb0..303d315c5d 100644
> >--- a/include/qemu/osdep.h
> >+++ b/include/qemu/osdep.h
> >@@ -570,6 +570,19 @@ 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
> >diff --git a/backends/hostmem-file.c b/backends/hostmem-file.c
> >index ba601ce940..d62689179b 100644
> >--- a/backends/hostmem-file.c
> >+++ b/backends/hostmem-file.c
> >@@ -46,6 +46,28 @@ file_backend_memory_alloc(HostMemoryBackend *backend, Error **errp)
> >     gchar *name;
> > #endif
> > 
> >+    /*
> >+     * Verify pmem file size since starting a guest with an incorrect size
> >+     * leads to confusing failures inside the guest.
> >+     */
> >+    if (fb->is_pmem && fb->mem_path) {
> >+        Error *local_err = NULL;
> >+        uint64_t size;
> >+
> >+        size = qemu_get_pmem_size(fb->mem_path, &local_err);
> >+        if (!size) {
> >+            error_propagate(errp, local_err);
> >+            return;
> >+        }
> >+
> >+        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;
> >+        }
> >+    }
> >+  
> 
> -mem-path is only supported on POSIX host.
> 
> So how about put this in the last code block before doing the real memory
> region init?
Indeed, evaluating file would be better done only if we didn't fail size/mempath checks first.

 
> BTW, I suggest to cleanup file_backend_memory_alloc() to make POSIX/non-POSIX
> situation clear to understand.
that's a separate patch, I'll reply with it here so Sefan could pick it (due to conflicts the change would create)

> 
> >     if (!backend->size) {
> >         error_setg(errp, "can't create backend with size 0");
> >         return;
> >diff --git a/util/oslib-posix.c b/util/oslib-posix.c
> >index 37c5854b9c..10d90d1783 100644
> >--- a/util/oslib-posix.c
> >+++ b/util/oslib-posix.c
> >@@ -500,6 +500,59 @@ 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)
> > {
> >diff --git a/util/oslib-win32.c b/util/oslib-win32.c
> >index b4c17f5dfa..bd633afab6 100644
> >--- a/util/oslib-win32.c
> >+++ b/util/oslib-win32.c
> >@@ -560,6 +560,11 @@ 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)
> > {
> >-- 
> >2.20.1
> >  
>