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

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

On (07/21/19 23:29), Sergey Senozhatsky wrote:
> 
>  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


So "the new mount API" conversion probably looks like something below.
But I'm not 100% sure.

---
 drivers/gpu/drm/i915/gem/i915_gemfs.c | 32 +++++++++++++++++++++------
 1 file changed, 25 insertions(+), 7 deletions(-)

diff --git a/drivers/gpu/drm/i915/gem/i915_gemfs.c b/drivers/gpu/drm/i915/gem/i915_gemfs.c
index 099f3397aada..2e365b26f8ee 100644
--- a/drivers/gpu/drm/i915/gem/i915_gemfs.c
+++ b/drivers/gpu/drm/i915/gem/i915_gemfs.c
@@ -7,12 +7,14 @@
 #include <linux/fs.h>
 #include <linux/mount.h>
 #include <linux/pagemap.h>
+#include <linux/fs_context.h>
 
 #include "i915_drv.h"
 #include "i915_gemfs.h"
 
 int i915_gemfs_init(struct drm_i915_private *i915)
 {
+	struct fs_context *fc;
 	struct file_system_type *type;
 	struct vfsmount *gemfs;
 
@@ -36,19 +38,35 @@ int i915_gemfs_init(struct drm_i915_private *i915)
 		struct super_block *sb = gemfs->mnt_sb;
 		/* FIXME: Disabled until we get W/A for read BW issue. */
 		char options[] = "huge=never";
-		int flags = 0;
 		int err;
 
-		err = sb->s_op->remount_fs(sb, &flags, options);
-		if (err) {
-			kern_unmount(gemfs);
-			return err;
-		}
+		fc = fs_context_for_reconfigure(sb->s_root, 0, 0);
+		if (IS_ERR(fc))
+			goto err;
+
+		if (!fc->ops->parse_monolithic)
+			goto err;
+
+		err = fc->ops->parse_monolithic(fc, options);
+		if (err)
+			goto err;
+
+		if (!fc->ops->reconfigure)
+			goto err;
+
+		err = fc->ops->reconfigure(fc);
+		if (err)
+			goto err;
 	}
 
 	i915->mm.gemfs = gemfs;
-
 	return 0;
+
+err:
+	pr_err("i915 gemfs init() failed\n");
+	put_fs_context(fc);
+	kern_unmount(gemfs);
+	return -EINVAL;
 }
 
 void i915_gemfs_fini(struct drm_i915_private *i915)
-- 
2.22.0