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@@ -0,0 +1,165 @@
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+ From bbd4bb4fd663da72d0a9806e53d84601d03532b2 Mon Sep 17 00:00:00 2001
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+ Message-Id: <bbd4bb4fd663da72d0a9806e53d84601d03532b2.1668799549.git.boris@bur.io>
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+ In-Reply-To: <cover.1668799549.git.boris@bur.io>
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+ References: <cover.1668799549.git.boris@bur.io>
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+ From: Filipe Manana <fdmanana@suse.com>
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+ Date: Tue, 15 Nov 2022 16:25:26 +0000
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+ Subject: [PATCH 3/3] btrfs-progs: receive: fix silent data loss after fall
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+ back from encoded write
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+
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+ When attempting an encoded write, if it fails for some specific reason
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+ like -EINVAL (when an offset is not sector size aligned) or -ENOSPC, we
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+ then fallback into decompressing the data and writing it using regular
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+ buffered IO. This logic however is not correct, one of the reasons is
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+ that it assumes the encoded offset is smaller than the unencoded file
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+ length and that they can be compared, but one is an offset and the other
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+ is a length, not an end offset, so they can't be compared to get correct
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+ results. This bad logic will often result in not copying all data, or even
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+ no data at all, resulting in a silent data loss. This is easily seen in
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+ with the following reproducer:
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+
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+ $ cat test.sh
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+ #!/bin/bash
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+
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+ DEV=/dev/sdj
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+ MNT=/mnt/sdj
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+
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+ umount $DEV &> /dev/null
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+ mkfs.btrfs -f $DEV > /dev/null
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+ mount -o compress $DEV $MNT
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+
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+ # File foo has a size of 33K, not aligned to the sector size.
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+ xfs_io -f -c "pwrite -S 0xab 0 33K" $MNT/foo
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+
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+ xfs_io -f -c "pwrite -S 0xcd 0 64K" $MNT/bar
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+
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+ # Now clone the first 32K of file bar into foo at offset 0.
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+ xfs_io -c "reflink $MNT/bar 0 0 32K" $MNT/foo
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+
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+ # Snapshot the default subvolume and create a full send stream (v2).
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+ btrfs subvolume snapshot -r $MNT $MNT/snap
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+
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+ btrfs send --compressed-data -f /tmp/test.send $MNT/snap
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+
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+ echo -e "\nFile bar in the original filesystem:"
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+ od -A d -t x1 $MNT/snap/bar
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+
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+ umount $MNT
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+ mkfs.btrfs -f $DEV > /dev/null
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+ mount $DEV $MNT
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+
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+ echo -e "\nReceiving stream in a new filesystem..."
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+ btrfs receive -f /tmp/test.send $MNT
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+
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+ echo -e "\nFile bar in the new filesystem:"
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+ od -A d -t x1 $MNT/snap/bar
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+
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+ umount $MNT
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+
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+ Running the test without this patch:
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+
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+ $ ./test.sh
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+ (...)
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+ File bar in the original filesystem:
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+ 0000000 cd cd cd cd cd cd cd cd cd cd cd cd cd cd cd cd
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+ *
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+ 0065536
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+
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+ Receiving stream in a new filesystem...
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+ At subvol snap
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+
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+ File bar in the new filesystem:
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+ 0000000 cd cd cd cd cd cd cd cd cd cd cd cd cd cd cd cd
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+ *
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+ 0033792
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+
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+ We end up with file bar having less data, and a smaller size, than in the
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+ original filesystem.
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+
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+ This happens because when processing file bar, send issues the following
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+ commands:
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+
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+ clone bar - source=foo source offset=0 offset=0 length=32768
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+ write bar - offset=32768 length=1024
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+ encoded_write bar - offset=33792, len=4096, unencoded_offset=33792, unencoded_file_len=31744, unencoded_len=65536, compression=1, encryption=0
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+
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+ The first 32K are cloned from file foo, as that file ranged is shared
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+ between the files.
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+
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+ Then there's a regular write operation for the file range [32K, 33K[,
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+ since file foo has different data from bar for that file range.
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+
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+ Finally for the remainder of file bar, the send side issues an encoded
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+ write since the extent is compressed in the source filesystem, for the
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+ file offset 33792 (33K), remaining 31K of data. The receiver will try the
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+ encoded write, but that fails with -EINVAL since the offset 33K is not
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+ sector size aligned, so it will fallback to decompressing the data and
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+ writing it using regular buffered writes. However that results in doing
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+ no writes at decompress_and_write() because 'pos' is initialized to the
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+ value of 33K (unencoded_offset) and unencoded_file_len is 31K, so the
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+ while loop has no iterations.
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+
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+ Another case where we can fallback to decompression plus regular buffered
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+ writes is when the destination filesystem has a sector size larger then
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+ the sector size of the source filesystem (for example when the source
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+ filesystem is on x86_64 with a 4K sector size and the destination
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+ filesystem is PowerPC with a 64K sector size). In that scenario encoded
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+ write attempts wil fail with -EINVAL due to offsets not being aligned with
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+ the sector size of the destination filesystem, and the receive will
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+ attempt the fallback of decompressing the buffer and writing the
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+ decompressed using regular buffered IO.
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+
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+ Fix this by tracking the number of written bytes instead, and increment
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+ it, and the unencoded offset, after each write.
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+
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+ Fixes: d20e759fc917 ("btrfs-progs: receive: encoded_write fallback to explicit decode and write")
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+ Signed-off-by: Filipe Manana <fdmanana@suse.com>
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+ Reviewed-by: Boris Burkov <boris@bur.io>
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+ ---
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+ cmds/receive.c | 15 ++++++++-------
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+ 1 file changed, 8 insertions(+), 7 deletions(-)
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+
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+ diff --git a/cmds/receive.c b/cmds/receive.c
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+ index 9f73b072..7756b42c 100644
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+ --- a/cmds/receive.c
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+ +++ b/cmds/receive.c
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+ @@ -1155,10 +1155,9 @@ static int decompress_and_write(struct btrfs_receive *rctx,
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+ u32 compression)
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+ {
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+ int ret = 0;
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+ - size_t pos;
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+ - ssize_t w;
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+ char *unencoded_data;
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+ int sector_shift = 0;
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+ + u64 written = 0;
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+
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+ unencoded_data = calloc(unencoded_len, 1);
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+ if (!unencoded_data) {
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+ @@ -1209,17 +1208,19 @@ static int decompress_and_write(struct btrfs_receive *rctx,
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+ goto out;
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+ }
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+
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+ - pos = unencoded_offset;
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+ - while (pos < unencoded_file_len) {
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+ - w = pwrite(rctx->write_fd, unencoded_data + pos,
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+ - unencoded_file_len - pos, offset);
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+ + while (written < unencoded_file_len) {
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+ + ssize_t w;
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+ +
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+ + w = pwrite(rctx->write_fd, unencoded_data + unencoded_offset,
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+ + unencoded_file_len - written, offset);
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+ if (w < 0) {
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+ ret = -errno;
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+ error("writing unencoded data failed: %m");
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+ goto out;
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+ }
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+ - pos += w;
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+ + written += w;
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+ offset += w;
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+ + unencoded_offset += w;
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+ }
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+ out:
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+ free(unencoded_data);
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+ --
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+ 2.38.1
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+
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borisb
commented 2 years ago
Backport 'btrfs-progs: receive: fix a silent data loss bug with encoded writes'
https://lore.kernel.org/linux-btrfs/abf83660318efd4ceafaa3cea98371c1e6e9fa25.camel@scientia.org/T/#ma2ff58f2ee932c39e01a018aa4b08ec8f42687c3