Thank you for your contribution! Sashiko AI review found 4 potential issue(s) to consider: - [High] fuse: missing inode_lock_shared() in DAX read path causes lockdep splats and races - [High] fuse: fuse_rw_clamp() truncates writes at EOF to 0 bytes, causing infinite loops - [High] fuse: O_APPEND writes bypass fuse_rw_clamp limits due to ordering - [High] fuse: ABBA deadlock between mmap_lock and i_rwsem in DAX page fault --
commit 900b63ca4cd5e837e1d2beda0ce7ca37d122ab2d Author: Miklos Szeredi <[email protected]> Subject: fuse: add extent map I/O support Wire up read, write, splice and mmap operations for extent-mapped files through the iomap/DAX infrastructure. > diff --git a/fs/fuse/ext_map.c b/fs/fuse/ext_map.c > index ab01d678d08e5..61cb22e821b60 100644 > --- a/fs/fuse/ext_map.c > +++ b/fs/fuse/ext_map.c [ ... ] > +static vm_fault_t fuse_ext_map_huge_fault(struct vm_fault *vmf, unsigned int > order) > +{ > + struct inode *inode = file_inode(vmf->vma->vm_file); > + bool write_fault = (vmf->flags & FAULT_FLAG_WRITE) && > (vmf->vma->vm_flags & VM_SHARED); > + vm_fault_t ret; > + unsigned long pfn; > + > + if (WARN_ON_ONCE(!IS_DAX(inode))) > + return VM_FAULT_SIGBUS; > + > + if (write_fault) { > + sb_start_pagefault(inode->i_sb); > + file_update_time(vmf->vma->vm_file); > + } > + > + filemap_invalidate_lock_shared(inode->i_mapping); > + > + ret = dax_iomap_fault(vmf, order, &pfn, NULL, &fuse_ext_map_iomap_ops); > + if (ret & VM_FAULT_NEEDDSYNC) > + ret = dax_finish_sync_fault(vmf, order, pfn); ^^^^ [Severity: High] Can this create an ABBA deadlock between mmap_lock and i_rwsem? This page fault handler is entered holding the VFS mmap_lock, and then takes filemap_invalidate_lock_shared(). If the fault requires a data sync (O_DSYNC), it calls dax_finish_sync_fault() which synchronously invokes vfs_fsync_range(). The filesystem's fsync handler, fuse_fsync(), unconditionally acquires inode_lock(inode). This creates an mmap_lock -> inode_lock dependency, which violates the standard kernel lock hierarchy and can deadlock if another thread holds the inode_lock while taking a page fault. > + > + filemap_invalidate_unlock_shared(inode->i_mapping); > + > + if (write_fault) > + sb_end_pagefault(inode->i_sb); > + > + return ret; > +} [ ... ] > +ssize_t fuse_ext_map_write_iter(struct kiocb *iocb, struct iov_iter *from) > +{ > + ssize_t res; > + > + fuse_rw_clamp(iocb, from); > + > + res = generic_write_checks(iocb, from); ^^^^ [Severity: High] Can O_APPEND writes bypass the file size limits checked in fuse_rw_clamp() due to this ordering? Because fuse_rw_clamp() runs before generic_write_checks(), kiocb->ki_pos is not yet updated to i_size for O_APPEND writes. Clamping will evaluate based on the old, smaller position, and generic_write_checks() will then advance ki_pos to i_size, allowing an out-of-bounds write via dax_iomap_rw(). [Severity: High] Also, does fuse_rw_clamp() cause writes at or past EOF to return 0 instead of a standard POSIX error? When a write starts at or past i_size, fuse_rw_clamp() truncates the iterator to 0. Then, generic_write_checks() returns 0, which propagates up to the VFS. This can cause standard utilities (like cp and dd) to retry indefinitely in an infinite loop instead of receiving -ENOSPC or -EFBIG. > + if (res <= 0) > + return res; > + > + res = kiocb_modified(iocb); > + if (res) > + return res; > + > + return dax_iomap_rw(iocb, from, &fuse_ext_map_iomap_ops); > +} > diff --git a/fs/fuse/passthrough.c b/fs/fuse/passthrough.c > index e9ab1aea34e27..040817ad76e94 100644 > --- a/fs/fuse/passthrough.c > +++ b/fs/fuse/passthrough.c > @@ -48,10 +47,10 @@ ssize_t fuse_passthrough_read_iter(struct kiocb *iocb, > struct iov_iter *iter) > if (!count) > return 0; > > - ret = backing_file_read_iter(backing_file, iter, iocb, iocb->ki_flags, > - &ctx); > + if (!backing_file) > + return fuse_ext_map_read_iter(iocb, iter); ^^^^ [Severity: High] Does this path miss acquiring the inode read lock before calling into the DAX read operations? When fuse_ext_map_read_iter() processes the read, it delegates to dax_iomap_rw() which expects the VFS inode lock to be held for synchronization against truncates. Skipping inode_lock_shared(&inode->i_rwsem) here could lead to lockdep splats or out-of-bounds reads during concurrent mapping changes. > > - return ret; > + return backing_file_read_iter(backing_file, iter, iocb, iocb->ki_flags, > &ctx); > } -- Sashiko AI review ยท https://sashiko.dev/#/patchset/[email protected]?part=7
