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

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