3670e6451b
[BUG] When testing experimental subpage compressed write support, it hits a NULL pointer dereference inside read path: Unable to handle kernel NULL pointer dereference at virtual address 0000000000000018 pc : __pi_memcmp+0x28/0x1ec lr : check_data_csum+0xd0/0x274 [btrfs] Call trace: __pi_memcmp+0x28/0x1ec btrfs_verify_data_csum+0xf4/0x244 [btrfs] end_bio_extent_readpage+0x1d0/0x6b0 [btrfs] bio_endio+0x15c/0x1dc end_workqueue_fn+0x44/0x64 [btrfs] btrfs_work_helper+0x74/0x250 [btrfs] process_one_work+0x1d4/0x47c worker_thread+0x180/0x400 kthread+0x11c/0x120 ret_from_fork+0x10/0x30 Code: 54000261 d100044c d343fd8c f8408403 (f8408424) ---[ end trace 9e2c59f33ea40866 ]--- [CAUSE] When reading two compressed extents inside the same page, like the following layout, we trigger above crash: 0 32K 64K |-------|\\\\\\\| | \- Compressed extent (A) \--------- Compressed extent (B) For compressed read, we don't need to populate its io_bio->csum, as we rely on compressed_bio->csum to verify the compressed data, and then copy the decompressed to inode pages. Normally btrfs_verify_data_csum() skip such page by checking and clearing its PageChecked flag But since that flag is still for the full page, when endio for inode page range [0, 32K) gets executed, it clears PageChecked flag for the full page. Then when endio for inode page range [32K, 64K) gets executed, since the page no longer has PageChecked flag, it just continues checking, even though io_bio->csum is NULL. [FIX] Thankfully there are only two users of PageChecked bit: - Cow fixup Since subpage has its own way to trace page dirty (dirty_bitmap) and ordered bit (ordered_bitmap), it should never trigger cow fixup. - Compressed read We can distinguish such read by just checking io_bio->csum. So just check io_bio->csum before doing the verification to avoid such NULL pointer dereference. Signed-off-by: Qu Wenruo <wqu@suse.com> Signed-off-by: David Sterba <dsterba@suse.com>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.