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UNRM Hacking the filesystem Jessica Yu WHOOPS... GMAIL: UNDO SEND - PowerPoint PPT Presentation

May 16, 2023 •295 likes •888 views

UNRM Hacking the filesystem Jessica Yu WHOOPS... GMAIL: UNDO SEND unrm operates on the same idea as Gmail's undo send. UNRM: USAGE Goal : after an rm , want to be able to recover all removed files under a certain directory within a set period

  1. UNRM Hacking the filesystem Jessica Yu

  2. WHOOPS...

  3. GMAIL: UNDO SEND unrm operates on the same idea as Gmail's undo send.

  4. UNRM: USAGE Goal : after an rm , want to be able to recover all removed files under a certain directory within a set period of time. $ ls document.pdf image.png file.txt dir/ $ rm -r * # rm all files $ ls -l # all gone! total 0 $ unrm # undo $ ls document.pdf image.png file.txt dir/ # all back!

  5. RM: UNDER THE HOOD What happens during an rm? Application Application Application C-Standard Library (libc) USERSPACE KERNELSPACE Virtual Filesystem (VFS) Ext4 Btrfs XFS

  6. RM: UNDER THE HOOD What happens during an rm? Application rm Application $ strace rm file ... unlinkat(AT_FDCWD, "file", 0) C-Standard Library (libc) ... USERSPACE KERNELSPACE Virtual Filesystem (VFS) Ext4 Btrfs XFS

  7. RM: UNDER THE HOOD What happens during an rm? Application rm Application $ strace rm file ... unlinkat(AT_FDCWD, "file", 0) C-Standard Library (libc) ... USERSPACE KERNELSPACE do_unlinkat(...) Virtual Filesystem (VFS) → vfs_unlink(...) Ext4 Btrfs XFS

  8. RM: UNDER THE HOOD What happens during an rm? Application rm Application $ strace rm file ... unlinkat(AT_FDCWD, "file", 0) C-Standard Library (libc) ... USERSPACE KERNELSPACE do_unlinkat(...) Virtual Filesystem (VFS) → vfs_unlink(...) → dir->i_op->unlink(...) Ext4 Btrfs XFS

  9. RM: UNDER THE HOOD What happens during an rm? Application rm Application $ strace rm file ... unlinkat(AT_FDCWD, "file", 0) C-Standard Library (libc) ... USERSPACE KERNELSPACE do_unlinkat(...) Virtual Filesystem (VFS) → vfs_unlink(...) → dir->i_op->unlink(...) → ext4_unlink(....) Ext4 Btrfs XFS

  10. UNRM: UNDER THE HOOD What happens during an rm? Application rm Application $ strace rm file ... unlinkat(AT_FDCWD, "file", 0) C-Standard Library (libc) ... USERSPACE KERNELSPACE do_unlinkat(...) Virtual Filesystem (VFS) → vfs_unlink(...) → dir->i_op->unlink(...) → ext4_unlink(....) Ext4 Btrfs XFS → ext4_unrm_save(...)

  11. UNRM: OVERVIEW ○ Idea : Intercept unlinking process at the fs layer ○ When we run rm , we want it to look like the file has been removed already ( 'ls' shouldn't show file) □ But file should still exist somewhere □ And the file is finally removed after a period of time ○ accomplish this via work queues

  12. UNRM: OVERVIEW ext4_sb_info ○ Create a tree ... struct radix_tree_root s_unrm ; "second chance tree" □ ... radix tree rooted at ext4 superblock info □ ... struct dir 123 . c e s 0 indexed by directory inode # ○ 3 file each node in the tree points to a linked list □ . c e s 0 3 file . c ○ Linked lists e s 0 3 file For each directory, use a linked list to keep track □ of individual rm'd files from that dir ○ Work queues Finish unlinking process after set period of time □

  13. UNRM: OVERVIEW ext4_sb_info ○ Create a tree ... struct radix_tree_root s_unrm ; "second chance tree" □ ... radix tree rooted at ext4 superblock info □ ... struct dir 123 ! ! G N I indexed by directory inode # R ○ file each node in the tree points to a linked list □ . c e s 0 3 file . c ○ Linked lists e s 0 3 file For each directory, use a linked list to keep track □ of individual rm'd files from that dir ○ Work queues Finish unlinking process after set period of time □

  14. UNRM: OVERVIEW ext4_sb_info ○ Create a tree ... struct radix_tree_root s_unrm ; "second chance tree" □ ... radix tree rooted at ext4 superblock info □ ... struct goodbye dir 123 world :( indexed by directory inode # ○ file each node in the tree points to a linked list □ . c e s 0 3 file . c ○ Linked lists e s 0 3 file For each directory, use a linked list to keep track □ of individual rm'd files from that dir ○ Work queues Finish unlinking process after set period of time □ File is removed + entry from linked list removed □

  15. UNRM: OVERVIEW ext4_sb_info ○ Create a tree ... struct radix_tree_root s_unrm ; "second chance tree" □ ... radix tree rooted at ext4 superblock info □ ... struct dir 123 indexed by directory inode # ○ each node in the tree points to a linked list □ . c e s 0 3 file . c ○ Linked lists e s 0 3 file For each directory, use a linked list to keep track □ of individual rm'd files from that dir ○ Work queues Finish unlinking process after set period of time □ File is removed + entry from linked list removed □

  16. UNRM: ext4_unrm_save() ○ Idea : Intercept unlinking process at the fs layer Want to invalidate associated dentry (negate it) □ So that lookups fail ○ $ ls testfile testfile: No such file or directory $ ▌ ...but keep around the inode secretly (link count > 0) □ keep around the negative dentry too □ we'll need to "instantiate" it again if we call unrm ○ ...i.e., make the path lookup succeed again! ○

  17. UNRM: ext4_unrm_save() ○ Idea : Intercept unlinking process at the fs layer ext4_unlink() □ ext4_unrm_save(): ○ Inserts new entry in our radix tree Save relevant info that will let us unrm □ i.e. Save pointers to the inode and dentry □ ○ But do not drop the inode's i_nlink (# of hard links)

  18. UNRM: ext4_unrm_save() ext4_sb_info ▌ radix_tree_root $ pwd s_unrm /home/jyu/ $ ls -lih /home/jyu 795 drwx------ 41 jyu jyu 4.0K Aug 12 22:37 /home/jyu $ ls testfile.pdf track01.mp3 important.doc $ ls -lih testfile.pdf 901 -rw-r--r-- 1 jyu jyu 94K Jun 23 18:04 testfile.pdf $ ▌

  19. UNRM: ext4_unrm_save() ext4_sb_info ▌ radix_tree_root $ pwd s_unrm /home/jyu/ $ ls -lih /home/jyu 795 drwx------ 41 jyu jyu 4.0K Aug 12 22:37 /home/jyu key: 795 ➞ Create a new list $ ls head ➞ Insert into radix testfile.pdf track01.mp3 important.doc tree with key 795 $ ls -lih testfile.pdf ➞ Create list node with: 901 -rw-r--r-- 1 jyu jyu 94K Jun 23 18:04 testfile.pdf $ rm testfile.pdf $ ▌

  20. UNRM: ext4_unrm_save() ext4_sb_info ▌ radix_tree_root $ pwd s_unrm /home/jyu/ $ ls -lih /home/jyu 795 drwx------ 41 jyu jyu 4.0K Aug 12 22:37 /home/jyu key: 795 ➞ Create a new list $ ls head ➞ Insert into radix testfile.pdf track01.mp3 important.doc tree with key 795 □ inode 901 $ ls -lih testfile.pdf ➞ Create list node □ dentry with: 901 -rw-r--r-- 1 jyu jyu 94K Jun 23 18:04 testfile.pdf □ pointer to inode □ pointer to dentry $ rm testfile.pdf □ allocated work $ ▌

  21. UNRM: ext4_unrm_save() ext4_sb_info ▌ radix_tree_root $ pwd s_unrm /home/jyu/ $ ls -lih /home/jyu ### inode 795 795 drwx------ 41 jyu jyu 4.0K Aug 12 22:37 /home/jyu key: 795 ➞ Create a new list $ ls head ➞ Insert into radix testfile.pdf track01.mp3 important.doc tree with key 795 □ inode 901 $ ls -lih testfile.pdf ➞ Create list node □ dentry with: 901 -rw-r--r-- 1 jyu jyu 94K Jun 23 18:04 testfile.pdf □ pointer to inode □ pointer to dentry $ rm testfile.pdf ### inode 901 □ allocated work $ rm track01.mp3 ### inode 941 □ inode 941 $ ▌ □ dentry

  22. UNRM: ext4_unrm_save() ext4_sb_info ▌ radix_tree_root $ pwd s_unrm /home/jyu/ $ ls -lih /home/jyu ### inode 795 795 drwx------ 41 jyu jyu 4.0K Aug 12 22:37 /home/jyu key: 795 ➞ Create a new list $ ls head ➞ Insert into radix testfile.pdf track01.mp3 important.doc tree with key 795 □ inode 901 $ ls -lih testfile.pdf ➞ Create list node □ dentry with: 901 -rw-r--r-- 1 jyu jyu 94K Jun 23 18:04 testfile.pdf □ pointer to inode □ pointer to dentry $ rm testfile.pdf ### inode 901 □ allocated work ➞ Negative dentries $ rm track01.mp3 ### inode 941 created as unlink □ inode 941 process finishes $ ls track01.mp3 □ dentry track01.mp3: No such file or directory $ ▌

  23. UNRM: RECURSIVE RM ext4_sb_info ▌ radix_tree_root $ pwd s_unrm /home/jyu/ $ ls testfile.pdf music/ $ ls music/ track01.mp3 $ ▌

  24. UNRM: RECURSIVE RM ext4_sb_info ▌ radix_tree_root $ pwd s_unrm /home/jyu/ $ ls testfile.pdf music/ $ ls music/ track01.mp3 $ rm -r * ### BOOM $ ▌

  25. UNRM: RECURSIVE RM ext4_sb_info ▌ radix_tree_root $ pwd s_unrm /home/jyu/ $ ls testfile.pdf music/ key:/home/jyu $ ls music/ track01.mp3 □ testfile.pdf $ rm -r * ### BOOM □ dentry $ ▌ □ music/ □ dentry * inode #'s replaced with filenames for simplicity

  26. UNRM: RECURSIVE RM ext4_sb_info ▌ radix_tree_root $ pwd s_unrm /home/jyu/ $ ls testfile.pdf music/ key:/home/jyu $ ls music/ track01.mp3 □ testfile.pdf $ echo * □ dentry testfile.pdf music ### rm -r * uses this order $ rm -r * ### BOOM $ ▌ □ music/ Mark dir entries so dir=1 that we can recurse □ dentry into them when unrm is called later * inode #'s replaced with filenames for simplicity

  27. UNRM: RECURSIVE RM ext4_sb_info ▌ radix_tree_root $ pwd s_unrm /home/jyu/ $ ls testfile.pdf music/ key:/home/jyu key: music/ $ ls music/ track01.mp3 □ testfile.pdf □ track01.mp3 $ echo * □ dentry □ dentry testfile.pdf music ### rm -r * uses this order $ rm -r * ### BOOM $ ▌ □ music/ dir=1 □ dentry * inode #'s replaced with filenames for simplicity

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