[linux-next] mm/i915: i915_gemfs_init() NULL dereference

[Sergey Senozhatsky <sergey.senozhatsky@gmail.com>]

Hi,

My laptop oopses early on with nothing on the screen;
after some debugging I managed to obtain a backtrace:

 BUG: kernel NULL pointer dereference, address: 0000000000000000
 #PF: supervisor instruction fetch in kernel mode
 #PF: error_code(0x0010) - not-present page
 PGD 0 P4D 0 
 Oops: 0010 [#1] PREEMPT SMP PTI
 RIP: 0010:0x0
 Code: Bad RIP value.
 [..]
 Call Trace:
  i915_gemfs_init+0x6e/0xa0 [i915]
  i915_gem_init_early+0x76/0x90 [i915]
  i915_driver_probe+0x30a/0x1640 [i915]
  ? kernfs_activate+0x5a/0x80
  ? kernfs_add_one+0xdd/0x130
  pci_device_probe+0x9e/0x110
  really_probe+0xce/0x230
  driver_probe_device+0x4b/0xc0
  device_driver_attach+0x4e/0x60
  __driver_attach+0x47/0xb0
  ? device_driver_attach+0x60/0x60
  bus_for_each_dev+0x61/0x90
  bus_add_driver+0x167/0x1b0
  driver_register+0x67/0xaa
  ? 0xffffffffc0522000
  do_one_initcall+0x37/0x13f
  ? kmem_cache_alloc+0x11a/0x150
  do_init_module+0x51/0x200
  __se_sys_init_module+0xef/0x100
  do_syscall_64+0x49/0x250
  entry_SYSCALL_64_after_hwframe+0x44/0xa9

RIP is at 0x00, which is never good

It sort of boils down to commit 144df3b288c4 (vfs: Convert ramfs,
shmem, tmpfs, devtmpfs, rootfs to use the new mount API), which
removed ->remount_fs from tmpfs' ops:

===
@@ -3736,7 +3849,6 @@ static const struct super_operations shmem_ops = {
        .destroy_inode  = shmem_destroy_inode,
 #ifdef CONFIG_TMPFS
        .statfs         = shmem_statfs,
-       .remount_fs     = shmem_remount_fs,
        .show_options   = shmem_show_options,
 #endif
        .evict_inode    = shmem_evict_inode,
===

So i915 init executes NULL

	get_fs_type("tmpfs");
	sb->s_op->remount_fs(sb, &flags, options);

For the time being the following (obvious and wrong) patch
at least boots -next:

---

diff --git a/drivers/gpu/drm/i915/gem/i915_gemfs.c b/drivers/gpu/drm/i915/gem/i915_gemfs.c
index 099f3397aada..1f95d9ea319a 100644
--- a/drivers/gpu/drm/i915/gem/i915_gemfs.c
+++ b/drivers/gpu/drm/i915/gem/i915_gemfs.c
@@ -39,6 +39,9 @@ int i915_gemfs_init(struct drm_i915_private *i915)
 		int flags = 0;
 		int err;
 
+		if (!sb->s_op->remount_fs)
+			return -ENODEV;
+
 		err = sb->s_op->remount_fs(sb, &flags, options);
 		if (err) {
 			kern_unmount(gemfs);
---

	-ss