[PATCH 02/15] xfs: introduce new file range exchange ioctl

Darrick J. Wong
2024-03-27T01:53:21Z

Introduce a new ioctl to handle exchanging ranges of bytes
between files. The goal here is to perform the exchange atomically with
respect to applications -- either they see the file contents before the
exchange or they see that A-B is now B-A, even if the kernel crashes.

My original goal with all this code was to make it so that online repair
can build a replacement directory or xattr structure in a temporary file
and commit the repair by atomically exchanging all the data blocks
between the two files. However, I needed a way to test this mechanism
thoroughly, so I've been evolving an ioctl interface since then.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
---
fs/xfs/Makefile | 1
fs/xfs/libxfs/xfs_fs.h | 41 ++++++
fs/xfs/xfs_exchrange.c | 304 ++++++++++++++++++++++++++++++++++++++++++++++++
fs/xfs/xfs_exchrange.h | 34 +++++
fs/xfs/xfs_ioctl.c | 38 ++++++
5 files changed, 418 insertions(+)
create mode 100644 fs/xfs/xfs_exchrange.c
create mode 100644 fs/xfs/xfs_exchrange.h

diff --git a/fs/xfs/Makefile b/fs/xfs/Makefile
index 76674ad5833e3..2474242f5a05f 100644
--- a/fs/xfs/Makefile
+++ b/fs/xfs/Makefile
@@ -67,6 +67,7 @@ xfs-y += xfs_aops.o \
xfs_dir2_readdir.o \
xfs_discard.o \
xfs_error.o \
+ xfs_exchrange.o \
xfs_export.o \
xfs_extent_busy.o \
xfs_file.o \
diff --git a/fs/xfs/libxfs/xfs_fs.h b/fs/xfs/libxfs/xfs_fs.h
index ca1b17d014377..8a1e30cf4dc88 100644
--- a/fs/xfs/libxfs/xfs_fs.h
+++ b/fs/xfs/libxfs/xfs_fs.h
@@ -772,6 +772,46 @@ struct xfs_scrub_metadata {
# define XFS_XATTR_LIST_MAX 65536
#endif

+/*
+ * Exchange part of file1 with part of the file that this ioctl that is being
+ * called against (which we'll call file2). Filesystems must be able to
+ * restart and complete the operation even after the system goes down.
+ */
+struct xfs_exchange_range {
+ __s32 file1_fd;
+ __u32 pad; /* must be zeroes */
+ __u64 file1_offset; /* file1 offset, bytes */
+ __u64 file2_offset; /* file2 offset, bytes */
+ __u64 length; /* bytes to exchange */
+
+ __u64 flags; /* see XFS_EXCHANGE_RANGE_* below */
+};
+
+/*
+ * Exchange file data all the way to the ends of both files, and then exchange
+ * the file sizes. This flag can be used to replace a file's contents with a
+ * different amount of data. length will be ignored.
+ */
+#define XFS_EXCHANGE_RANGE_TO_EOF (1ULL << 0)
+
+/* Flush all changes in file data and file metadata to disk before returning. */
+#define XFS_EXCHANGE_RANGE_DSYNC (1ULL << 1)
+
+/* Dry run; do all the parameter verification but do not change anything. */
+#define XFS_EXCHANGE_RANGE_DRY_RUN (1ULL << 2)
+
+/*
+ * Exchange only the parts of the two files where the file allocation units
+ * mapped to file1's range have been written to. This can accelerate
+ * scatter-gather atomic writes with a temp file if all writes are aligned to
+ * the file allocation unit.
+ */
+#define XFS_EXCHANGE_RANGE_FILE1_WRITTEN (1ULL << 3)
+
+#define XFS_EXCHANGE_RANGE_ALL_FLAGS (XFS_EXCHANGE_RANGE_TO_EOF | \
+ XFS_EXCHANGE_RANGE_DSYNC | \
+ XFS_EXCHANGE_RANGE_DRY_RUN | \
+ XFS_EXCHANGE_RANGE_FILE1_WRITTEN)

/*
* ioctl commands that are used by Linux filesystems
@@ -843,6 +883,7 @@ struct xfs_scrub_metadata {
#define XFS_IOC_FSGEOMETRY _IOR ('X', 126, struct xfs_fsop_geom)
#define XFS_IOC_BULKSTAT _IOR ('X', 127, struct xfs_bulkstat_req)
#define XFS_IOC_INUMBERS _IOR ('X', 128, struct xfs_inumbers_req)
+#define XFS_IOC_EXCHANGE_RANGE _IOWR('X', 129, struct xfs_exchange_range)
/* XFS_IOC_GETFSUUID ---------- deprecated 140 */

diff --git a/fs/xfs/xfs_exchrange.c b/fs/xfs/xfs_exchrange.c
new file mode 100644
index 0000000000000..a575e26ae1a58
--- /dev/null
+++ b/fs/xfs/xfs_exchrange.c
@@ -0,0 +1,304 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Copyright (c) 2020-2024 Oracle. All Rights Reserved.
+ * Author: Darrick J. Wong <djwong@kernel.org>
+ */
+#include "xfs.h"
+#include "xfs_shared.h"
+#include "xfs_format.h"
+#include "xfs_log_format.h"
+#include "xfs_trans_resv.h"
+#include "xfs_mount.h"
+#include "xfs_defer.h"
+#include "xfs_inode.h"
+#include "xfs_trans.h"
+#include "xfs_exchrange.h"
+#include <linux/fsnotify.h>
+
+/*
+ * Generic code for exchanging ranges of two files via XFS_IOC_EXCHANGE_RANGE.
+ * This part deals with struct file objects and byte ranges and does not deal
+ * with XFS-specific data structures such as xfs_inodes and block ranges. This
+ * separation may some day facilitate porting to another filesystem.
+ *
+ * The goal is to exchange fxr.length bytes starting at fxr.file1_offset in
+ * file1 with the same number of bytes starting at fxr.file2_offset in file2.
+ * Implementations must call xfs_exchange_range_prep to prepare the two
+ * files prior to taking locks; and they must update the inode change and mod
+ * times of both files as part of the metadata update. The timestamp update
+ * and freshness checks must be done atomically as part of the data exchange
+ * operation to ensure correctness of the freshness check.
+ * xfs_exchange_range_finish must be called after the operation completes
+ * successfully but before locks are dropped.
+ */
+
+/* Verify that we have security clearance to perform this operation. */
+static int
+xfs_exchange_range_verify_area(
+ struct xfs_exchrange *fxr)
+{
+ int ret;
+
+ ret = remap_verify_area(fxr->file1, fxr->file1_offset, fxr->length,
+ true);
+ if (ret)
+ return ret;
+
+ return remap_verify_area(fxr->file2, fxr->file2_offset, fxr->length,
+ true);
+}
+
+/*
+ * Performs necessary checks before doing a range exchange, having stabilized
+ * mutable inode attributes via i_rwsem.
+ */
+static inline int
+xfs_exchange_range_checks(
+ struct xfs_exchrange *fxr,
+ unsigned int alloc_unit)
+{
+ struct inode *inode1 = file_inode(fxr->file1);
+ struct inode *inode2 = file_inode(fxr->file2);
+ uint64_t allocmask = alloc_unit - 1;
+ int64_t test_len;
+ uint64_t blen;
+ loff_t size1, size2, tmp;
+ int error;
+
+ /* Don't touch certain kinds of inodes */
+ if (IS_IMMUTABLE(inode1) || IS_IMMUTABLE(inode2))
+ return -EPERM;
+ if (IS_SWAPFILE(inode1) || IS_SWAPFILE(inode2))
+ return -ETXTBSY;
+
+ size1 = i_size_read(inode1);
+ size2 = i_size_read(inode2);
+
+ /* Ranges cannot start after EOF. */
+ if (fxr->file1_offset > size1 || fxr->file2_offset > size2)
+ return -EINVAL;
+
+ /*
+ * If the caller said to exchange to EOF, we set the length of the
+ * request large enough to cover everything to the end of both files.
+ */
+ if (fxr->flags & XFS_EXCHANGE_RANGE_TO_EOF) {
+ fxr->length = max_t(int64_t, size1 - fxr->file1_offset,
+ size2 - fxr->file2_offset);
+
+ error = xfs_exchange_range_verify_area(fxr);
+ if (error)
+ return error;
+ }
+
+ /*
+ * The start of both ranges must be aligned to the file allocation
+ * unit.
+ */
+ if (!IS_ALIGNED(fxr->file1_offset, alloc_unit) ||
+ !IS_ALIGNED(fxr->file2_offset, alloc_unit))
+ return -EINVAL;
+
+ /* Ensure offsets don't wrap. */
+ if (check_add_overflow(fxr->file1_offset, fxr->length, &tmp) ||
+ check_add_overflow(fxr->file2_offset, fxr->length, &tmp))
+ return -EINVAL;
+
+ /*
+ * We require both ranges to end within EOF, unless we're exchanging
+ * to EOF.
+ */
+ if (!(fxr->flags & XFS_EXCHANGE_RANGE_TO_EOF) &&
+ (fxr->file1_offset + fxr->length > size1 ||
+ fxr->file2_offset + fxr->length > size2))
+ return -EINVAL;
+
+ /*
+ * Make sure we don't hit any file size limits. If we hit any size
+ * limits such that test_length was adjusted, we abort the whole
+ * operation.
+ */
+ test_len = fxr->length;
+ error = generic_write_check_limits(fxr->file2, fxr->file2_offset,
+ &test_len);
+ if (error)
+ return error;
+ error = generic_write_check_limits(fxr->file1, fxr->file1_offset,
+ &test_len);
+ if (error)
+ return error;
+ if (test_len != fxr->length)
+ return -EINVAL;
+
+ /*
+ * If the user wanted us to exchange up to the infile's EOF, round up
+ * to the next allocation unit boundary for this check. Do the same
+ * for the outfile.
+ *
+ * Otherwise, reject the range length if it's not aligned to an
+ * allocation unit.
+ */
+ if (fxr->file1_offset + fxr->length == size1)
+ blen = ALIGN(size1, alloc_unit) - fxr->file1_offset;
+ else if (fxr->file2_offset + fxr->length == size2)
+ blen = ALIGN(size2, alloc_unit) - fxr->file2_offset;
+ else if (!IS_ALIGNED(fxr->length, alloc_unit))
+ return -EINVAL;
+ else
+ blen = fxr->length;
+
+ /* Don't allow overlapped exchanges within the same file. */
+ if (inode1 == inode2 &&
+ fxr->file2_offset + blen > fxr->file1_offset &&
+ fxr->file1_offset + blen > fxr->file2_offset)
+ return -EINVAL;
+
+ /*
+ * Ensure that we don't exchange a partial EOF block into the middle of
+ * another file.
+ */
+ if ((fxr->length & allocmask) == 0)
+ return 0;
+
+ blen = fxr->length;
+ if (fxr->file2_offset + blen < size2)
+ blen &= ~allocmask;
+
+ if (fxr->file1_offset + blen < size1)
+ blen &= ~allocmask;
+
+ return blen == fxr->length ? 0 : -EINVAL;
+}
+
+/*
+ * Check that the two inodes are eligible for range exchanges, the ranges make
+ * sense, and then flush all dirty data. Caller must ensure that the inodes
+ * have been locked against any other modifications.
+ */
+static inline int
+xfs_exchange_range_prep(
+ struct xfs_exchrange *fxr,
+ unsigned int alloc_unit)
+{
+ struct inode *inode1 = file_inode(fxr->file1);
+ struct inode *inode2 = file_inode(fxr->file2);
+ bool same_inode = (inode1 == inode2);
+ int error;
+
+ /* Check that we don't violate system file offset limits. */
+ error = xfs_exchange_range_checks(fxr, alloc_unit);
+ if (error || fxr->length == 0)
+ return error;
+
+ /* Wait for the completion of any pending IOs on both files */
+ inode_dio_wait(inode1);
+ if (!same_inode)
+ inode_dio_wait(inode2);
+
+ error = filemap_write_and_wait_range(inode1->i_mapping,
+ fxr->file1_offset,
+ fxr->file1_offset + fxr->length - 1);
+ if (error)
+ return error;
+
+ error = filemap_write_and_wait_range(inode2->i_mapping,
+ fxr->file2_offset,
+ fxr->file2_offset + fxr->length - 1);
+ if (error)
+ return error;
+
+ /*
+ * If the files or inodes involved require synchronous writes, amend
+ * the request to force the filesystem to flush all data and metadata
+ * to disk after the operation completes.
+ */
+ if (((fxr->file1->f_flags | fxr->file2->f_flags) & O_SYNC) ||
+ IS_SYNC(inode1) || IS_SYNC(inode2))
+ fxr->flags |= XFS_EXCHANGE_RANGE_DSYNC;
+
+ return 0;
+}
+
+/*
+ * Finish a range exchange operation, if it was successful. Caller must ensure
+ * that the inodes are still locked against any other modifications.
+ */
+static inline int
+xfs_exchange_range_finish(
+ struct xfs_exchrange *fxr)
+{
+ int error;
+
+ error = file_remove_privs(fxr->file1);
+ if (error)
+ return error;
+ if (file_inode(fxr->file1) == file_inode(fxr->file2))
+ return 0;
+
+ return file_remove_privs(fxr->file2);
+}
+
+/* Exchange parts of two files. */
+int
+xfs_exchange_range(
+ struct xfs_exchrange *fxr)
+{
+ struct inode *inode1 = file_inode(fxr->file1);
+ struct inode *inode2 = file_inode(fxr->file2);
+ int ret;
+
+ BUILD_BUG_ON(XFS_EXCHANGE_RANGE_ALL_FLAGS &
+ XFS_EXCHANGE_RANGE_PRIV_FLAGS);
+
+ /* Both files must be on the same mount/filesystem. */
+ if (fxr->file1->f_path.mnt != fxr->file2->f_path.mnt)
+ return -EXDEV;
+
+ if (fxr->flags & ~XFS_EXCHANGE_RANGE_ALL_FLAGS)
+ return -EINVAL;
+
+ /* Userspace requests only honored for regular files. */
+ if (S_ISDIR(inode1->i_mode) || S_ISDIR(inode2->i_mode))
+ return -EISDIR;
+ if (!S_ISREG(inode1->i_mode) || !S_ISREG(inode2->i_mode))
+ return -EINVAL;
+
+ /* Both files must be opened for read and write. */
+ if (!(fxr->file1->f_mode & FMODE_READ) ||
+ !(fxr->file1->f_mode & FMODE_WRITE) ||
+ !(fxr->file2->f_mode & FMODE_READ) ||
+ !(fxr->file2->f_mode & FMODE_WRITE))
+ return -EBADF;
+
+ /* Neither file can be opened append-only. */
+ if ((fxr->file1->f_flags & O_APPEND) ||
+ (fxr->file2->f_flags & O_APPEND))
+ return -EBADF;
+
+ /*
+ * If we're not exchanging to EOF, we can check the areas before
+ * stabilizing both files' i_size.
+ */
+ if (!(fxr->flags & XFS_EXCHANGE_RANGE_TO_EOF)) {
+ ret = xfs_exchange_range_verify_area(fxr);
+ if (ret)
+ return ret;
+ }
+
+ /* Update cmtime if the fd/inode don't forbid it. */
+ if (!(fxr->file1->f_mode & FMODE_NOCMTIME) && !IS_NOCMTIME(inode1))
+ fxr->flags |= __XFS_EXCHANGE_RANGE_UPD_CMTIME1;
+ if (!(fxr->file2->f_mode & FMODE_NOCMTIME) && !IS_NOCMTIME(inode2))
+ fxr->flags |= __XFS_EXCHANGE_RANGE_UPD_CMTIME2;
+
+ file_start_write(fxr->file2);
+ ret = -EOPNOTSUPP; /* XXX call out to lower level code */
+ file_end_write(fxr->file2);
+ if (ret)
+ return ret;
+
+ fsnotify_modify(fxr->file1);
+ if (fxr->file2 != fxr->file1)
+ fsnotify_modify(fxr->file2);
+ return 0;
+}
diff --git a/fs/xfs/xfs_exchrange.h b/fs/xfs/xfs_exchrange.h
new file mode 100644
index 0000000000000..923c386cb9756
--- /dev/null
+++ b/fs/xfs/xfs_exchrange.h
@@ -0,0 +1,34 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+/*
+ * Copyright (c) 2020-2024 Oracle. All Rights Reserved.
+ * Author: Darrick J. Wong <djwong@kernel.org>
+ */
+#ifndef __XFS_EXCHRANGE_H__
+#define __XFS_EXCHRANGE_H__
+
+/* Update the mtime/cmtime of file1 and file2 */
+#define __XFS_EXCHANGE_RANGE_UPD_CMTIME1 (1ULL << 63)
+#define __XFS_EXCHANGE_RANGE_UPD_CMTIME2 (1ULL << 62)
+
+#define XFS_EXCHANGE_RANGE_PRIV_FLAGS (__XFS_EXCHANGE_RANGE_UPD_CMTIME1 | \
+ __XFS_EXCHANGE_RANGE_UPD_CMTIME2)
+
+struct xfs_exchrange {
+ struct file *file1;
+ struct file *file2;
+
+ loff_t file1_offset;
+ loff_t file2_offset;
+ u64 length;
+
+ u64 flags; /* XFS_EXCHANGE_RANGE flags */
+
+ /* file2 metadata for freshness checks if file1_ino != 0 */
+ u64 file2_ino;
+ struct timespec64 file2_mtime;
+ struct timespec64 file2_ctime;
+};
+
+int xfs_exchange_range(struct xfs_exchrange *fxr);
+
+#endif /* __XFS_EXCHRANGE_H__ */
diff --git a/fs/xfs/xfs_ioctl.c b/fs/xfs/xfs_ioctl.c
index 1397edea20f19..b11077b12f5fd 100644
--- a/fs/xfs/xfs_ioctl.c
+++ b/fs/xfs/xfs_ioctl.c
@@ -40,6 +40,7 @@
#include "xfs_xattr.h"
#include "xfs_rtbitmap.h"
#include "xfs_file.h"
+#include "xfs_exchrange.h"

#include <linux/mount.h>
#include <linux/namei.h>
@@ -1930,6 +1931,40 @@ xfs_ioctl_fs_counts(
return 0;
}

+static long
+xfs_ioc_exchange_range(
+ struct file *file,
+ struct xfs_exchange_range __user *argp)
+{
+ struct xfs_exchrange fxr = {
+ .file2 = file,
+ };
+ struct xfs_exchange_range args;
+ struct fd file1;
+ int error;
+
+ if (copy_from_user(&args, argp, sizeof(args)))
+ return -EFAULT;
+ if (memchr_inv(&args.pad, 0, sizeof(args.pad)))
+ return -EINVAL;
+ if (args.flags & ~XFS_EXCHANGE_RANGE_ALL_FLAGS)
+ return -EINVAL;
+
+ fxr.file1_offset = args.file1_offset;
+ fxr.file2_offset = args.file2_offset;
+ fxr.length = args.length;
+ fxr.flags = args.flags;
+
+ file1 = fdget(args.file1_fd);
+ if (!file1.file)
+ return -EBADF;
+ fxr.file1 = file1.file;
+
+ error = xfs_exchange_range(&fxr);
+ fdput(file1);
+ return error;
+}
+
/*
* These long-unused ioctls were removed from the official ioctl API in 5.17,
* but retain these definitions so that we can log warnings about them.
@@ -2170,6 +2205,9 @@ xfs_file_ioctl(
return error;
}

+ case XFS_IOC_EXCHANGE_RANGE:
+ return xfs_ioc_exchange_range(filp, arg);
+
default:
return -ENOTTY;
}