[PATCH v6 0/5] vvfat: Fix write bugs for large files and add iotests

[PATCH v6 0/5] vvfat: Fix write bugs for large files and add iotests

[Amjad Alsharafi]

These patches fix some bugs found when modifying files in vvfat.
First, there was a bug when writing to the cluster 2 or above of a file, it
will copy the cluster before it instead, so, when writing to cluster=2, the
content of cluster=1 will be copied into disk instead in its place.

Another issue was modifying the clusters of a file and adding new
clusters, this showed 2 issues:
- If the new cluster is not immediately after the last cluster, it will
cause issues when reading from this file in the future.
- Generally, the usage of info.file.offset was incorrect, and the
system would crash on abort() when the file is modified and a new
cluster was added.

Also, added some iotests for vvfat, covering the this fix and also
general behavior such as reading, writing, and creating files on the filesystem.
Including tests for reading/writing the first cluster which
would pass even before this patch.

v6:
  - small fixes and code improvements when switching between active vfat mappings
  - small fixes and code improvements for the fa16.py driver used for iotests

v5:
  - Fix a bug in reading non-contiguous clusters in vvfat for more than 2 mappings.
  - Added a test for adding more clusters where they are non-contiguous and
    result in 3 mappings (for the above fix).
  - Split PATCH 2/4 into 2 patches and a better fix for the `abort` issue (now in PATCH 3/5).
  - Other fixes and improvements in `fat16.py` module used for iotests.

v4:
  Applied some suggestions from Kevin Wolf <kwolf@redhat.com>:
  - Fixed code formatting by following the coding style in `scripts/checkpatch.pl`
  - Reduced changes related to `iotests` by setting `vvfat` format as non-generic.
  - Added another test to cover the fix done in `PATCH 2/4` and `PATCH 3/4` for 
    handling reading/writing files with non-continuous clusters.

v3:
  Added test for creating new files in vvfat.

v2:
  Added iotests for `vvfat` driver along with a simple `fat16` module to run the tests.

v1:
  https://patchew.org/QEMU/20240327201231.31046-1-amjadsharafi10@gmail.com/
  Fix the issue of writing to the middle of the file in vvfat

Amjad Alsharafi (5):
  vvfat: Fix bug in writing to middle of file
  vvfat: Fix usage of `info.file.offset`
  vvfat: Fix wrong checks for cluster mappings invariant
  vvfat: Fix reading files with non-continuous clusters
  iotests: Add `vvfat` tests

 block/vvfat.c                      |  34 +-
 tests/qemu-iotests/check           |   2 +-
 tests/qemu-iotests/fat16.py        | 675 +++++++++++++++++++++++++++++
 tests/qemu-iotests/testenv.py      |   2 +-
 tests/qemu-iotests/tests/vvfat     | 458 +++++++++++++++++++
 tests/qemu-iotests/tests/vvfat.out |   5 +
 6 files changed, 1157 insertions(+), 19 deletions(-)
 create mode 100644 tests/qemu-iotests/fat16.py
 create mode 100755 tests/qemu-iotests/tests/vvfat
 create mode 100755 tests/qemu-iotests/tests/vvfat.out

-- 
2.45.2

[PATCH v6 1/5] vvfat: Fix bug in writing to middle of file

[Amjad Alsharafi]

Before this commit, the behavior when calling `commit_one_file` for
example with `offset=0x2000` (second cluster), what will happen is that
we won't fetch the next cluster from the fat, and instead use the first
cluster for the read operation.

This is due to off-by-one error here, where `i=0x2000 !< offset=0x2000`,
thus not fetching the next cluster.

Signed-off-by: Amjad Alsharafi <amjadsharafi10@gmail.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Tested-by: Kevin Wolf <kwolf@redhat.com>
---
 block/vvfat.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/block/vvfat.c b/block/vvfat.c
index 9d050ba3ae..19da009a5b 100644
--- a/block/vvfat.c
+++ b/block/vvfat.c
@@ -2525,8 +2525,9 @@ commit_one_file(BDRVVVFATState* s, int dir_index, uint32_t offset)
         return -1;
     }
 
-    for (i = s->cluster_size; i < offset; i += s->cluster_size)
+    for (i = 0; i < offset; i += s->cluster_size) {
         c = modified_fat_get(s, c);
+    }
 
     fd = qemu_open_old(mapping->path, O_RDWR | O_CREAT | O_BINARY, 0666);
     if (fd < 0) {
-- 
2.45.2

[PATCH v6 2/5] vvfat: Fix usage of `info.file.offset`

[Amjad Alsharafi]

The field is marked as "the offset in the file (in clusters)", but it
was being used like this
`cluster_size*(nums)+mapping->info.file.offset`, which is incorrect.

Signed-off-by: Amjad Alsharafi <amjadsharafi10@gmail.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
---
 block/vvfat.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/block/vvfat.c b/block/vvfat.c
index 19da009a5b..247b232608 100644
--- a/block/vvfat.c
+++ b/block/vvfat.c
@@ -1408,7 +1408,9 @@ read_cluster_directory:
 
         assert(s->current_fd);
 
-        offset=s->cluster_size*(cluster_num-s->current_mapping->begin)+s->current_mapping->info.file.offset;
+        offset = s->cluster_size *
+            ((cluster_num - s->current_mapping->begin)
+            + s->current_mapping->info.file.offset);
         if(lseek(s->current_fd, offset, SEEK_SET)!=offset)
             return -3;
         s->cluster=s->cluster_buffer;
@@ -1929,8 +1931,9 @@ get_cluster_count_for_direntry(BDRVVVFATState* s, direntry_t* direntry, const ch
                         (mapping->mode & MODE_DIRECTORY) == 0) {
 
                     /* was modified in qcow */
-                    if (offset != mapping->info.file.offset + s->cluster_size
-                            * (cluster_num - mapping->begin)) {
+                    if (offset != s->cluster_size
+                            * ((cluster_num - mapping->begin)
+                            + mapping->info.file.offset)) {
                         /* offset of this cluster in file chain has changed */
                         abort();
                         copy_it = 1;
@@ -2404,7 +2407,7 @@ static int commit_mappings(BDRVVVFATState* s,
                         (mapping->end - mapping->begin);
             } else
                 next_mapping->info.file.offset = mapping->info.file.offset +
-                        mapping->end - mapping->begin;
+                        (mapping->end - mapping->begin);
 
             mapping = next_mapping;
         }
-- 
2.45.2

[PATCH v6 3/5] vvfat: Fix wrong checks for cluster mappings invariant

[Amjad Alsharafi]

How this `abort` was intended to check for was:
- if the `mapping->first_mapping_index` is not the same as
  `first_mapping_index`, which **should** happen only in one case,
  when we are handling the first mapping, in that case
  `mapping->first_mapping_index == -1`, in all other cases, the other
  mappings after the first should have the condition `true`.
- From above, we know that this is the first mapping, so if the offset
  is not `0`, then abort, since this is an invalid state.

The issue was that `first_mapping_index` is not set if we are
checking from the middle, the variable `first_mapping_index` is
only set if we passed through the check `cluster_was_modified` with the
first mapping, and in the same function call we checked the other
mappings.

One approach is to go into the loop even if `cluster_was_modified`
is not true so that we will be able to set `first_mapping_index` for the
first mapping, but since `first_mapping_index` is only used here,
another approach is to just check manually for the
`mapping->first_mapping_index != -1` since we know that this is the
value for the only entry where `offset == 0` (i.e. first mapping).

Signed-off-by: Amjad Alsharafi <amjadsharafi10@gmail.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
---
 block/vvfat.c | 10 ++--------
 1 file changed, 2 insertions(+), 8 deletions(-)

diff --git a/block/vvfat.c b/block/vvfat.c
index 247b232608..b63ac5d045 100644
--- a/block/vvfat.c
+++ b/block/vvfat.c
@@ -1880,7 +1880,6 @@ get_cluster_count_for_direntry(BDRVVVFATState* s, direntry_t* direntry, const ch
 
     uint32_t cluster_num = begin_of_direntry(direntry);
     uint32_t offset = 0;
-    int first_mapping_index = -1;
     mapping_t* mapping = NULL;
     const char* basename2 = NULL;
 
@@ -1942,14 +1941,9 @@ get_cluster_count_for_direntry(BDRVVVFATState* s, direntry_t* direntry, const ch
 
                         if (strcmp(basename, basename2))
                             copy_it = 1;
-                        first_mapping_index = array_index(&(s->mapping), mapping);
-                    }
-
-                    if (mapping->first_mapping_index != first_mapping_index
-                            && mapping->info.file.offset > 0) {
-                        abort();
-                        copy_it = 1;
                     }
+                    assert(mapping->first_mapping_index == -1
+                            || mapping->info.file.offset > 0);
 
                     /* need to write out? */
                     if (!was_modified && is_file(direntry)) {
-- 
2.45.2

[PATCH v6 4/5] vvfat: Fix reading files with non-continuous clusters

[Amjad Alsharafi]

When reading with `read_cluster` we get the `mapping` with
`find_mapping_for_cluster` and then we call `open_file` for this
mapping.
The issue appear when its the same file, but a second cluster that is
not immediately after it, imagine clusters `500 -> 503`, this will give
us 2 mappings one has the range `500..501` and another `503..504`, both
point to the same file, but different offsets.

When we don't open the file since the path is the same, we won't assign
`s->current_mapping` and thus accessing way out of bound of the file.

From our example above, after `open_file` (that didn't open anything) we
will get the offset into the file with
`s->cluster_size*(cluster_num-s->current_mapping->begin)`, which will
give us `0x2000 * (504-500)`, which is out of bound for this mapping and
will produce some issues.

Signed-off-by: Amjad Alsharafi <amjadsharafi10@gmail.com>
---
 block/vvfat.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/block/vvfat.c b/block/vvfat.c
index b63ac5d045..d2b879705e 100644
--- a/block/vvfat.c
+++ b/block/vvfat.c
@@ -1360,15 +1360,17 @@ static int open_file(BDRVVVFATState* s,mapping_t* mapping)
 {
     if(!mapping)
         return -1;
-    if(!s->current_mapping ||
-            strcmp(s->current_mapping->path,mapping->path)) {
+    if (s->current_mapping != mapping) {
         /* open file */
         int fd = qemu_open_old(mapping->path,
-                               O_RDONLY | O_BINARY | O_LARGEFILE);
-        if(fd<0)
+                            O_RDONLY | O_BINARY | O_LARGEFILE);
+        if (fd < 0) {
             return -1;
+        }
         vvfat_close_current_file(s);
+
         s->current_fd = fd;
+        assert(s->current_fd);
         s->current_mapping = mapping;
     }
     return 0;
-- 
2.45.2

[PATCH v6 5/5] iotests: Add `vvfat` tests

[Amjad Alsharafi]

Added several tests to verify the implementation of the vvfat driver.

We needed a way to interact with it, so created a basic `fat16.py` driver
that handled writing correct sectors for us.

Added `vvfat` to the non-generic formats, as its not a normal image format.

Signed-off-by: Amjad Alsharafi <amjadsharafi10@gmail.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Tested-by: Kevin Wolf <kwolf@redhat.com>
---
 tests/qemu-iotests/check           |   2 +-
 tests/qemu-iotests/fat16.py        | 675 +++++++++++++++++++++++++++++
 tests/qemu-iotests/testenv.py      |   2 +-
 tests/qemu-iotests/tests/vvfat     | 458 +++++++++++++++++++
 tests/qemu-iotests/tests/vvfat.out |   5 +
 5 files changed, 1140 insertions(+), 2 deletions(-)
 create mode 100644 tests/qemu-iotests/fat16.py
 create mode 100755 tests/qemu-iotests/tests/vvfat
 create mode 100755 tests/qemu-iotests/tests/vvfat.out

diff --git a/tests/qemu-iotests/check b/tests/qemu-iotests/check
index 56d88ca423..545f9ec7bd 100755
--- a/tests/qemu-iotests/check
+++ b/tests/qemu-iotests/check
@@ -84,7 +84,7 @@ def make_argparser() -> argparse.ArgumentParser:
     p.set_defaults(imgfmt='raw', imgproto='file')
 
     format_list = ['raw', 'bochs', 'cloop', 'parallels', 'qcow', 'qcow2',
-                   'qed', 'vdi', 'vpc', 'vhdx', 'vmdk', 'luks', 'dmg']
+                   'qed', 'vdi', 'vpc', 'vhdx', 'vmdk', 'luks', 'dmg', 'vvfat']
     g_fmt = p.add_argument_group(
         '  image format options',
         'The following options set the IMGFMT environment variable. '
diff --git a/tests/qemu-iotests/fat16.py b/tests/qemu-iotests/fat16.py
new file mode 100644
index 0000000000..0acc987da6
--- /dev/null
+++ b/tests/qemu-iotests/fat16.py
@@ -0,0 +1,675 @@
+# A simple FAT16 driver that is used to test the `vvfat` driver in QEMU.
+#
+# Copyright (C) 2024 Amjad Alsharafi <amjadsharafi10@gmail.com>
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 2 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+from typing import List
+import string
+
+SECTOR_SIZE = 512
+DIRENTRY_SIZE = 32
+ALLOWED_FILE_CHARS = set(
+    "!#$%&'()-@^_`{}~" + string.digits + string.ascii_uppercase
+)
+
+
+class MBR:
+    def __init__(self, data: bytes):
+        assert len(data) == 512
+        self.partition_table = []
+        for i in range(4):
+            partition = data[446 + i * 16 : 446 + (i + 1) * 16]
+            self.partition_table.append(
+                {
+                    "status": partition[0],
+                    "start_head": partition[1],
+                    "start_sector": partition[2] & 0x3F,
+                    "start_cylinder": ((partition[2] & 0xC0) << 2)
+                                      | partition[3],
+                    "type": partition[4],
+                    "end_head": partition[5],
+                    "end_sector": partition[6] & 0x3F,
+                    "end_cylinder": ((partition[6] & 0xC0) << 2) | partition[7],
+                    "start_lba": int.from_bytes(partition[8:12], "little"),
+                    "size": int.from_bytes(partition[12:16], "little"),
+                }
+            )
+
+    def __str__(self):
+        return "\n".join(
+            [
+                f"{i}: {partition}"
+                for i, partition in enumerate(self.partition_table)
+            ]
+        )
+
+
+class FatBootSector:
+    def __init__(self, data: bytes):
+        assert len(data) == 512
+        self.bytes_per_sector = int.from_bytes(data[11:13], "little")
+        self.sectors_per_cluster = data[13]
+        self.reserved_sectors = int.from_bytes(data[14:16], "little")
+        self.fat_count = data[16]
+        self.root_entries = int.from_bytes(data[17:19], "little")
+        total_sectors_16 = int.from_bytes(data[19:21], "little")
+        self.media_descriptor = data[21]
+        self.sectors_per_fat = int.from_bytes(data[22:24], "little")
+        self.sectors_per_track = int.from_bytes(data[24:26], "little")
+        self.heads = int.from_bytes(data[26:28], "little")
+        self.hidden_sectors = int.from_bytes(data[28:32], "little")
+        total_sectors_32 = int.from_bytes(data[32:36], "little")
+        assert (
+            total_sectors_16 == 0 or total_sectors_32 == 0
+        ), "Both total sectors (16 and 32) fields are non-zero"
+        self.total_sectors = total_sectors_16 or total_sectors_32
+        self.drive_number = data[36]
+        self.volume_id = int.from_bytes(data[39:43], "little")
+        self.volume_label = data[43:54].decode("ascii").strip()
+        self.fs_type = data[54:62].decode("ascii").strip()
+
+    def root_dir_start(self):
+        """
+        Calculate the start sector of the root directory.
+        """
+        return self.reserved_sectors + self.fat_count * self.sectors_per_fat
+
+    def root_dir_size(self):
+        """
+        Calculate the size of the root directory in sectors.
+        """
+        return (
+            self.root_entries * DIRENTRY_SIZE + self.bytes_per_sector - 1
+        ) // self.bytes_per_sector
+
+    def data_sector_start(self):
+        """
+        Calculate the start sector of the data region.
+        """
+        return self.root_dir_start() + self.root_dir_size()
+
+    def first_sector_of_cluster(self, cluster: int):
+        """
+        Calculate the first sector of the given cluster.
+        """
+        return (
+            self.data_sector_start() + (cluster - 2) * self.sectors_per_cluster
+        )
+
+    def cluster_bytes(self):
+        """
+        Calculate the number of bytes in a cluster.
+        """
+        return self.bytes_per_sector * self.sectors_per_cluster
+
+    def __str__(self):
+        return (
+            f"Bytes per sector: {self.bytes_per_sector}\n"
+            f"Sectors per cluster: {self.sectors_per_cluster}\n"
+            f"Reserved sectors: {self.reserved_sectors}\n"
+            f"FAT count: {self.fat_count}\n"
+            f"Root entries: {self.root_entries}\n"
+            f"Total sectors: {self.total_sectors}\n"
+            f"Media descriptor: {self.media_descriptor}\n"
+            f"Sectors per FAT: {self.sectors_per_fat}\n"
+            f"Sectors per track: {self.sectors_per_track}\n"
+            f"Heads: {self.heads}\n"
+            f"Hidden sectors: {self.hidden_sectors}\n"
+            f"Drive number: {self.drive_number}\n"
+            f"Volume ID: {self.volume_id}\n"
+            f"Volume label: {self.volume_label}\n"
+            f"FS type: {self.fs_type}\n"
+        )
+
+
+class FatDirectoryEntry:
+    def __init__(self, data: bytes, sector: int, offset: int):
+        self.name = data[0:8].decode("ascii").strip()
+        self.ext = data[8:11].decode("ascii").strip()
+        self.attributes = data[11]
+        self.reserved = data[12]
+        self.create_time_tenth = data[13]
+        self.create_time = int.from_bytes(data[14:16], "little")
+        self.create_date = int.from_bytes(data[16:18], "little")
+        self.last_access_date = int.from_bytes(data[18:20], "little")
+        high_cluster = int.from_bytes(data[20:22], "little")
+        self.last_mod_time = int.from_bytes(data[22:24], "little")
+        self.last_mod_date = int.from_bytes(data[24:26], "little")
+        low_cluster = int.from_bytes(data[26:28], "little")
+        self.cluster = (high_cluster << 16) | low_cluster
+        self.size_bytes = int.from_bytes(data[28:32], "little")
+
+        # extra (to help write back to disk)
+        self.sector = sector
+        self.offset = offset
+
+    def as_bytes(self) -> bytes:
+        return (
+            self.name.ljust(8, " ").encode("ascii")
+            + self.ext.ljust(3, " ").encode("ascii")
+            + self.attributes.to_bytes(1, "little")
+            + self.reserved.to_bytes(1, "little")
+            + self.create_time_tenth.to_bytes(1, "little")
+            + self.create_time.to_bytes(2, "little")
+            + self.create_date.to_bytes(2, "little")
+            + self.last_access_date.to_bytes(2, "little")
+            + (self.cluster >> 16).to_bytes(2, "little")
+            + self.last_mod_time.to_bytes(2, "little")
+            + self.last_mod_date.to_bytes(2, "little")
+            + (self.cluster & 0xFFFF).to_bytes(2, "little")
+            + self.size_bytes.to_bytes(4, "little")
+        )
+
+    def whole_name(self):
+        if self.ext:
+            return f"{self.name}.{self.ext}"
+        else:
+            return self.name
+
+    def __str__(self):
+        return (
+            f"Name: {self.name}\n"
+            f"Ext: {self.ext}\n"
+            f"Attributes: {self.attributes}\n"
+            f"Reserved: {self.reserved}\n"
+            f"Create time tenth: {self.create_time_tenth}\n"
+            f"Create time: {self.create_time}\n"
+            f"Create date: {self.create_date}\n"
+            f"Last access date: {self.last_access_date}\n"
+            f"Last mod time: {self.last_mod_time}\n"
+            f"Last mod date: {self.last_mod_date}\n"
+            f"Cluster: {self.cluster}\n"
+            f"Size: {self.size_bytes}\n"
+        )
+
+    def __repr__(self):
+        # convert to dict
+        return str(vars(self))
+
+
+class Fat16:
+    def __init__(
+        self,
+        start_sector: int,
+        size: int,
+        sector_reader: callable,
+        sector_writer: callable,
+    ):
+        self.start_sector = start_sector
+        self.size_in_sectors = size
+        self.sector_reader = sector_reader
+        self.sector_writer = sector_writer
+
+        self.boot_sector = FatBootSector(self.sector_reader(start_sector))
+
+        fat_size_in_sectors = (
+            self.boot_sector.sectors_per_fat * self.boot_sector.fat_count
+        )
+        self.fats = self.read_sectors(
+            self.boot_sector.reserved_sectors, fat_size_in_sectors
+        )
+        self.fats_dirty_sectors = set()
+
+    def read_sectors(self, start_sector: int, num_sectors: int) -> bytes:
+        return self.sector_reader(start_sector + self.start_sector, num_sectors)
+
+    def write_sectors(self, start_sector: int, data: bytes):
+        return self.sector_writer(start_sector + self.start_sector, data)
+
+    def directory_from_bytes(
+        self, data: bytes, start_sector: int
+    ) -> List[FatDirectoryEntry]:
+        """
+        Convert `bytes` into a list of `FatDirectoryEntry` objects.
+        Will ignore long file names.
+        Will stop when it encounters a 0x00 byte.
+        """
+
+        entries = []
+        for i in range(0, len(data), DIRENTRY_SIZE):
+            entry = data[i : i + DIRENTRY_SIZE]
+
+            current_sector = start_sector + (i // SECTOR_SIZE)
+            current_offset = i % SECTOR_SIZE
+
+            if entry[0] == 0:
+                break
+            elif entry[0] == 0xE5:
+                # Deleted file
+                continue
+
+            if entry[11] & 0xF == 0xF:
+                # Long file name
+                continue
+
+            entries.append(
+                FatDirectoryEntry(entry, current_sector, current_offset)
+            )
+        return entries
+
+    def read_root_directory(self) -> List[FatDirectoryEntry]:
+        root_dir = self.read_sectors(
+            self.boot_sector.root_dir_start(), self.boot_sector.root_dir_size()
+        )
+        return self.directory_from_bytes(
+            root_dir, self.boot_sector.root_dir_start()
+        )
+
+    def read_fat_entry(self, cluster: int) -> int:
+        """
+        Read the FAT entry for the given cluster.
+        """
+        fat_offset = cluster * 2  # FAT16
+        return int.from_bytes(self.fats[fat_offset : fat_offset + 2], "little")
+
+    def write_fat_entry(self, cluster: int, value: int):
+        """
+        Write the FAT entry for the given cluster.
+        """
+        fat_offset = cluster * 2
+        self.fats = (
+            self.fats[:fat_offset]
+            + value.to_bytes(2, "little")
+            + self.fats[fat_offset + 2 :]
+        )
+        self.fats_dirty_sectors.add(fat_offset // SECTOR_SIZE)
+
+    def flush_fats(self):
+        """
+        Write the FATs back to the disk.
+        """
+        for sector in self.fats_dirty_sectors:
+            data = self.fats[sector * SECTOR_SIZE : (sector + 1) * SECTOR_SIZE]
+            sector = self.boot_sector.reserved_sectors + sector
+            self.write_sectors(sector, data)
+        self.fats_dirty_sectors = set()
+
+    def next_cluster(self, cluster: int) -> int | None:
+        """
+        Get the next cluster in the chain.
+        If its `None`, then its the last cluster.
+        The function will crash if the next cluster
+        is `FREE` (unexpected) or invalid entry.
+        """
+        fat_entry = self.read_fat_entry(cluster)
+        if fat_entry == 0:
+            raise Exception("Unexpected: FREE cluster")
+        elif fat_entry == 1:
+            raise Exception("Unexpected: RESERVED cluster")
+        elif fat_entry >= 0xFFF8:
+            return None
+        elif fat_entry >= 0xFFF7:
+            raise Exception("Invalid FAT entry")
+        else:
+            return fat_entry
+
+    def next_free_cluster(self) -> int:
+        """
+        Find the next free cluster.
+        """
+        # simple linear search
+        for i in range(2, 0xFFFF):
+            if self.read_fat_entry(i) == 0:
+                return i
+        raise Exception("No free clusters")
+
+    def next_free_cluster_non_continuous(self) -> int:
+        """
+        Find the next free cluster, but makes sure
+        that the cluster before and after it are not allocated.
+        """
+        # simple linear search
+        before = False
+        for i in range(2, 0xFFFF):
+            if self.read_fat_entry(i) == 0:
+                if before and self.read_fat_entry(i + 1) == 0:
+                    return i
+                else:
+                    before = True
+            else:
+                before = False
+
+        raise Exception("No free clusters")
+
+    def read_cluster(self, cluster: int) -> bytes:
+        """
+        Read the cluster at the given cluster.
+        """
+        return self.read_sectors(
+            self.boot_sector.first_sector_of_cluster(cluster),
+            self.boot_sector.sectors_per_cluster,
+        )
+
+    def write_cluster(self, cluster: int, data: bytes):
+        """
+        Write the cluster at the given cluster.
+        """
+        assert len(data) == self.boot_sector.cluster_bytes()
+        return self.write_sectors(
+            self.boot_sector.first_sector_of_cluster(cluster),
+            data,
+        )
+
+    def read_directory(self, cluster: int) -> List[FatDirectoryEntry]:
+        """
+        Read the directory at the given cluster.
+        """
+        entries = []
+        while cluster is not None:
+            data = self.read_cluster(cluster)
+            entries.extend(
+                self.directory_from_bytes(
+                    data, self.boot_sector.first_sector_of_cluster(cluster)
+                )
+            )
+            cluster = self.next_cluster(cluster)
+        return entries
+
+    def add_direntry(
+        self, cluster: int | None, name: str, ext: str, attributes: int
+    ):
+        """
+        Add a new directory entry to the given cluster.
+        If the cluster is `None`, then it will be added to the root directory.
+        """
+
+        def find_free_entry(data: bytes):
+            for i in range(0, len(data), DIRENTRY_SIZE):
+                entry = data[i : i + DIRENTRY_SIZE]
+                if entry[0] == 0 or entry[0] == 0xE5:
+                    return i
+            return None
+
+        assert len(name) <= 8, "Name must be 8 characters or less"
+        assert len(ext) <= 3, "Ext must be 3 characters or less"
+        assert attributes % 0x15 != 0x15, "Invalid attributes"
+
+        # initial dummy data
+        new_entry = FatDirectoryEntry(b"\0" * 32, 0, 0)
+        new_entry.name = name.ljust(8, " ")
+        new_entry.ext = ext.ljust(3, " ")
+        new_entry.attributes = attributes
+        new_entry.reserved = 0
+        new_entry.create_time_tenth = 0
+        new_entry.create_time = 0
+        new_entry.create_date = 0
+        new_entry.last_access_date = 0
+        new_entry.last_mod_time = 0
+        new_entry.last_mod_date = 0
+        new_entry.cluster = self.next_free_cluster()
+        new_entry.size_bytes = 0
+
+        # mark as EOF
+        self.write_fat_entry(new_entry.cluster, 0xFFFF)
+
+        if cluster is None:
+            for i in range(self.boot_sector.root_dir_size()):
+                sector_data = self.read_sectors(
+                    self.boot_sector.root_dir_start() + i, 1
+                )
+                offset = find_free_entry(sector_data)
+                if offset is not None:
+                    new_entry.sector = self.boot_sector.root_dir_start() + i
+                    new_entry.offset = offset
+                    self.update_direntry(new_entry)
+                    return new_entry
+        else:
+            while cluster is not None:
+                data = self.read_cluster(cluster)
+                offset = find_free_entry(data)
+                if offset is not None:
+                    new_entry.sector = self.boot_sector.first_sector_of_cluster(
+                        cluster
+                    ) + (offset // SECTOR_SIZE)
+                    new_entry.offset = offset % SECTOR_SIZE
+                    self.update_direntry(new_entry)
+                    return new_entry
+                cluster = self.next_cluster(cluster)
+
+        raise Exception("No free directory entries")
+
+    def update_direntry(self, entry: FatDirectoryEntry):
+        """
+        Write the directory entry back to the disk.
+        """
+        sector = self.read_sectors(entry.sector, 1)
+        sector = (
+            sector[: entry.offset]
+            + entry.as_bytes()
+            + sector[entry.offset + DIRENTRY_SIZE :]
+        )
+        self.write_sectors(entry.sector, sector)
+
+    def find_direntry(self, path: str) -> FatDirectoryEntry | None:
+        """
+        Find the directory entry for the given path.
+        """
+        assert path[0] == "/", "Path must start with /"
+
+        path = path[1:]  # remove the leading /
+        parts = path.split("/")
+        directory = self.read_root_directory()
+
+        current_entry = None
+
+        for i, part in enumerate(parts):
+            is_last = i == len(parts) - 1
+
+            for entry in directory:
+                if entry.whole_name() == part:
+                    current_entry = entry
+                    break
+            if current_entry is None:
+                return None
+
+            if is_last:
+                return current_entry
+            else:
+                if current_entry.attributes & 0x10 == 0:
+                    raise Exception(
+                        f"{current_entry.whole_name()} is not a directory"
+                    )
+                else:
+                    directory = self.read_directory(current_entry.cluster)
+
+    def read_file(self, entry: FatDirectoryEntry | None) -> bytes | None:
+        """
+        Read the content of the file at the given path.
+        """
+        if entry is None:
+            return None
+        if entry.attributes & 0x10 != 0:
+            raise Exception(f"{entry.whole_name()} is a directory")
+
+        data = b""
+        cluster = entry.cluster
+        while cluster is not None and len(data) <= entry.size_bytes:
+            data += self.read_cluster(cluster)
+            cluster = self.next_cluster(cluster)
+        return data[: entry.size_bytes]
+
+    def truncate_file(
+        self,
+        entry: FatDirectoryEntry,
+        new_size: int,
+        allocate_non_continuous: bool = False,
+    ):
+        """
+        Truncate the file at the given path to the new size.
+        """
+        if entry is None:
+            return Exception("entry is None")
+        if entry.attributes & 0x10 != 0:
+            raise Exception(f"{entry.whole_name()} is a directory")
+
+        def clusters_from_size(size: int):
+            return (
+                size + self.boot_sector.cluster_bytes() - 1
+            ) // self.boot_sector.cluster_bytes()
+
+        # First, allocate new FATs if we need to
+        required_clusters = clusters_from_size(new_size)
+        current_clusters = clusters_from_size(entry.size_bytes)
+
+        affected_clusters = set()
+
+        # Keep at least one cluster, easier to manage this way
+        if required_clusters == 0:
+            required_clusters = 1
+        if current_clusters == 0:
+            current_clusters = 1
+
+        if required_clusters > current_clusters:
+            # Allocate new clusters
+            cluster = entry.cluster
+            to_add = required_clusters
+            for _ in range(current_clusters - 1):
+                to_add -= 1
+                assert cluster is not None, "Cluster is None"
+                affected_clusters.add(cluster)
+                cluster = self.next_cluster(cluster)
+            assert required_clusters > 0, "No new clusters to allocate"
+            assert cluster is not None, "Cluster is None"
+            assert (
+                self.next_cluster(cluster) is None
+            ), "Cluster is not the last cluster"
+
+            # Allocate new clusters
+            for _ in range(to_add - 1):
+                if allocate_non_continuous:
+                    new_cluster = self.next_free_cluster_non_continuous()
+                else:
+                    new_cluster = self.next_free_cluster()
+                self.write_fat_entry(cluster, new_cluster)
+                self.write_fat_entry(new_cluster, 0xFFFF)
+                cluster = new_cluster
+
+        elif required_clusters < current_clusters:
+            # Truncate the file
+            cluster = entry.cluster
+            for _ in range(required_clusters - 1):
+                assert cluster is not None, "Cluster is None"
+                cluster = self.next_cluster(cluster)
+            assert cluster is not None, "Cluster is None"
+
+            next_cluster = self.next_cluster(cluster)
+            # mark last as EOF
+            self.write_fat_entry(cluster, 0xFFFF)
+            # free the rest
+            while next_cluster is not None:
+                cluster = next_cluster
+                next_cluster = self.next_cluster(next_cluster)
+                self.write_fat_entry(cluster, 0)
+
+        self.flush_fats()
+
+        # verify number of clusters
+        cluster = entry.cluster
+        count = 0
+        while cluster is not None:
+            count += 1
+            affected_clusters.add(cluster)
+            cluster = self.next_cluster(cluster)
+        assert (
+            count == required_clusters
+        ), f"Expected {required_clusters} clusters, got {count}"
+
+        # update the size
+        entry.size_bytes = new_size
+        self.update_direntry(entry)
+
+        # trigger every affected cluster
+        for cluster in affected_clusters:
+            first_sector = self.boot_sector.first_sector_of_cluster(cluster)
+            first_sector_data = self.read_sectors(first_sector, 1)
+            self.write_sectors(first_sector, first_sector_data)
+
+    def write_file(self, entry: FatDirectoryEntry, data: bytes):
+        """
+        Write the content of the file at the given path.
+        """
+        if entry is None:
+            return Exception("entry is None")
+        if entry.attributes & 0x10 != 0:
+            raise Exception(f"{entry.whole_name()} is a directory")
+
+        data_len = len(data)
+
+        self.truncate_file(entry, data_len)
+
+        cluster = entry.cluster
+        while cluster is not None:
+            data_to_write = data[: self.boot_sector.cluster_bytes()]
+            if len(data_to_write) < self.boot_sector.cluster_bytes():
+                old_data = self.read_cluster(cluster)
+                data_to_write += old_data[len(data_to_write) :]
+
+            self.write_cluster(cluster, data_to_write)
+            data = data[self.boot_sector.cluster_bytes() :]
+            if len(data) == 0:
+                break
+            cluster = self.next_cluster(cluster)
+
+        assert (
+            len(data) == 0
+        ), "Data was not written completely, clusters missing"
+
+    def create_file(self, path: str):
+        """
+        Create a new file at the given path.
+        """
+        assert path[0] == "/", "Path must start with /"
+
+        path = path[1:]  # remove the leading /
+
+        parts = path.split("/")
+
+        directory_cluster = None
+        directory = self.read_root_directory()
+
+        parts, filename = parts[:-1], parts[-1]
+
+        for _, part in enumerate(parts):
+            current_entry = None
+            for entry in directory:
+                if entry.whole_name() == part:
+                    current_entry = entry
+                    break
+            if current_entry is None:
+                return None
+
+            if current_entry.attributes & 0x10 == 0:
+                raise Exception(
+                    f"{current_entry.whole_name()} is not a directory"
+                )
+            else:
+                directory = self.read_directory(current_entry.cluster)
+                directory_cluster = current_entry.cluster
+
+        # add new entry to the directory
+
+        filename, ext = filename.split(".")
+
+        if len(ext) > 3:
+            raise Exception("Ext must be 3 characters or less")
+        if len(filename) > 8:
+            raise Exception("Name must be 8 characters or less")
+
+        for c in filename + ext:
+
+            if c not in ALLOWED_FILE_CHARS:
+                raise Exception("Invalid character in filename")
+
+        return self.add_direntry(directory_cluster, filename, ext, 0)
diff --git a/tests/qemu-iotests/testenv.py b/tests/qemu-iotests/testenv.py
index 588f30a4f1..4053d29de4 100644
--- a/tests/qemu-iotests/testenv.py
+++ b/tests/qemu-iotests/testenv.py
@@ -250,7 +250,7 @@ def __init__(self, source_dir: str, build_dir: str,
         self.qemu_img_options = os.getenv('QEMU_IMG_OPTIONS')
         self.qemu_nbd_options = os.getenv('QEMU_NBD_OPTIONS')
 
-        is_generic = self.imgfmt not in ['bochs', 'cloop', 'dmg']
+        is_generic = self.imgfmt not in ['bochs', 'cloop', 'dmg', 'vvfat']
         self.imgfmt_generic = 'true' if is_generic else 'false'
 
         self.qemu_io_options = f'--cache {self.cachemode} --aio {self.aiomode}'
diff --git a/tests/qemu-iotests/tests/vvfat b/tests/qemu-iotests/tests/vvfat
new file mode 100755
index 0000000000..a6c536b886
--- /dev/null
+++ b/tests/qemu-iotests/tests/vvfat
@@ -0,0 +1,458 @@
+#!/usr/bin/env python3
+# group: rw vvfat
+#
+# Test vvfat driver implementation
+# Here, we use a simple FAT16 implementation and check the behavior of the vvfat driver.
+#
+# Copyright (C) 2024 Amjad Alsharafi <amjadsharafi10@gmail.com>
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 2 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+import os, shutil
+import iotests
+from iotests import imgfmt, QMPTestCase
+from fat16 import MBR, Fat16, DIRENTRY_SIZE
+
+filesystem = os.path.join(iotests.test_dir, "filesystem")
+
+nbd_sock = iotests.file_path("nbd.sock", base_dir=iotests.sock_dir)
+nbd_uri = "nbd+unix:///disk?socket=" + nbd_sock
+
+SECTOR_SIZE = 512
+
+
+class TestVVFatDriver(QMPTestCase):
+    def setUp(self) -> None:
+        if os.path.exists(filesystem):
+            if os.path.isdir(filesystem):
+                shutil.rmtree(filesystem)
+            else:
+                print(f"Error: {filesystem} exists and is not a directory")
+                exit(1)
+        os.mkdir(filesystem)
+
+        # Add some text files to the filesystem
+        for i in range(10):
+            with open(os.path.join(filesystem, f"file{i}.txt"), "w") as f:
+                f.write(f"Hello, world! {i}\n")
+
+        # Add 2 large files, above the cluster size (8KB)
+        with open(os.path.join(filesystem, "large1.txt"), "wb") as f:
+            # write 'A' * 1KB, 'B' * 1KB, 'C' * 1KB, ...
+            for i in range(8 * 2):  # two clusters
+                f.write(bytes([0x41 + i] * 1024))
+
+        with open(os.path.join(filesystem, "large2.txt"), "wb") as f:
+            # write 'A' * 1KB, 'B' * 1KB, 'C' * 1KB, ...
+            for i in range(8 * 3):  # 3 clusters
+                f.write(bytes([0x41 + i] * 1024))
+
+        self.vm = iotests.VM()
+
+        self.vm.add_blockdev(
+            self.vm.qmp_to_opts(
+                {
+                    "driver": imgfmt,
+                    "node-name": "disk",
+                    "rw": "true",
+                    "fat-type": "16",
+                    "dir": filesystem,
+                }
+            )
+        )
+
+        self.vm.launch()
+
+        self.vm.qmp_log("block-dirty-bitmap-add", **{"node": "disk", "name": "bitmap0"})
+
+        # attach nbd server
+        self.vm.qmp_log(
+            "nbd-server-start",
+            **{"addr": {"type": "unix", "data": {"path": nbd_sock}}},
+            filters=[],
+        )
+
+        self.vm.qmp_log(
+            "nbd-server-add",
+            **{"device": "disk", "writable": True, "bitmap": "bitmap0"},
+        )
+
+        self.qio = iotests.QemuIoInteractive("-f", "raw", nbd_uri)
+
+    def tearDown(self) -> None:
+        self.qio.close()
+        self.vm.shutdown()
+        # print(self.vm.get_log())
+        shutil.rmtree(filesystem)
+
+    def read_sectors(self, sector: int, num: int = 1) -> bytes:
+        """
+        Read `num` sectors starting from `sector` from the `disk`.
+        This uses `QemuIoInteractive` to read the sectors into `stdout` and then parse the output.
+        """
+        self.assertGreater(num, 0)
+        # The output contains the content of the sector in hex dump format
+        # We need to extract the content from it
+        output = self.qio.cmd(f"read -v {sector * SECTOR_SIZE} {num * SECTOR_SIZE}")
+        # Each row is 16 bytes long, and we are writing `num` sectors
+        rows = num * SECTOR_SIZE // 16
+        output_rows = output.split("\n")[:rows]
+
+        hex_content = "".join(
+            [(row.split(": ")[1]).split("  ")[0] for row in output_rows]
+        )
+        bytes_content = bytes.fromhex(hex_content)
+
+        self.assertEqual(len(bytes_content), num * SECTOR_SIZE)
+
+        return bytes_content
+
+    def write_sectors(self, sector: int, data: bytes):
+        """
+        Write `data` to the `disk` starting from `sector`.
+        This uses `QemuIoInteractive` to write the data into the disk.
+        """
+
+        self.assertGreater(len(data), 0)
+        self.assertEqual(len(data) % SECTOR_SIZE, 0)
+
+        temp_file = os.path.join(iotests.test_dir, "temp.bin")
+        with open(temp_file, "wb") as f:
+            f.write(data)
+
+        self.qio.cmd(f"write -s {temp_file} {sector * SECTOR_SIZE} {len(data)}")
+
+        os.remove(temp_file)
+
+    def init_fat16(self):
+        mbr = MBR(self.read_sectors(0))
+        return Fat16(
+            mbr.partition_table[0]["start_lba"],
+            mbr.partition_table[0]["size"],
+            self.read_sectors,
+            self.write_sectors,
+        )
+
+    # Tests
+
+    def test_fat_filesystem(self):
+        """
+        Test that vvfat produce a valid FAT16 and MBR sectors
+        """
+        mbr = MBR(self.read_sectors(0))
+
+        self.assertEqual(mbr.partition_table[0]["status"], 0x80)
+        self.assertEqual(mbr.partition_table[0]["type"], 6)
+
+        fat16 = Fat16(
+            mbr.partition_table[0]["start_lba"],
+            mbr.partition_table[0]["size"],
+            self.read_sectors,
+            self.write_sectors,
+        )
+        self.assertEqual(fat16.boot_sector.bytes_per_sector, 512)
+        self.assertEqual(fat16.boot_sector.volume_label, "QEMU VVFAT")
+
+    def test_read_root_directory(self):
+        """
+        Test the content of the root directory
+        """
+        fat16 = self.init_fat16()
+
+        root_dir = fat16.read_root_directory()
+
+        self.assertEqual(len(root_dir), 13)  # 12 + 1 special file
+
+        files = {
+            "QEMU VVF.AT": 0,  # special empty file
+            "FILE0.TXT": 16,
+            "FILE1.TXT": 16,
+            "FILE2.TXT": 16,
+            "FILE3.TXT": 16,
+            "FILE4.TXT": 16,
+            "FILE5.TXT": 16,
+            "FILE6.TXT": 16,
+            "FILE7.TXT": 16,
+            "FILE8.TXT": 16,
+            "FILE9.TXT": 16,
+            "LARGE1.TXT": 0x2000 * 2,
+            "LARGE2.TXT": 0x2000 * 3,
+        }
+
+        for entry in root_dir:
+            self.assertIn(entry.whole_name(), files)
+            self.assertEqual(entry.size_bytes, files[entry.whole_name()])
+
+    def test_direntry_as_bytes(self):
+        """
+        Test if we can convert Direntry back to bytes, so that we can write it back to the disk safely.
+        """
+        fat16 = self.init_fat16()
+
+        root_dir = fat16.read_root_directory()
+        first_entry_bytes = fat16.read_sectors(fat16.boot_sector.root_dir_start(), 1)
+        # The first entry won't be deleted, so we can compare it with the first entry in the root directory
+        self.assertEqual(root_dir[0].as_bytes(), first_entry_bytes[:DIRENTRY_SIZE])
+
+    def test_read_files(self):
+        """
+        Test reading the content of the files
+        """
+        fat16 = self.init_fat16()
+
+        for i in range(10):
+            file = fat16.find_direntry(f"/FILE{i}.TXT")
+            self.assertIsNotNone(file)
+            self.assertEqual(
+                fat16.read_file(file), f"Hello, world! {i}\n".encode("ascii")
+            )
+
+        # test large files
+        large1 = fat16.find_direntry("/LARGE1.TXT")
+        with open(os.path.join(filesystem, "large1.txt"), "rb") as f:
+            self.assertEqual(fat16.read_file(large1), f.read())
+
+        large2 = fat16.find_direntry("/LARGE2.TXT")
+        self.assertIsNotNone(large2)
+        with open(os.path.join(filesystem, "large2.txt"), "rb") as f:
+            self.assertEqual(fat16.read_file(large2), f.read())
+
+    def test_write_file_same_content_direct(self):
+        """
+        Similar to `test_write_file_in_same_content`, but we write the file directly clusters
+        and thus we don't go through the modification of direntry.
+        """
+        fat16 = self.init_fat16()
+
+        file = fat16.find_direntry("/FILE0.TXT")
+        self.assertIsNotNone(file)
+
+        data = fat16.read_cluster(file.cluster)
+        fat16.write_cluster(file.cluster, data)
+
+        with open(os.path.join(filesystem, "file0.txt"), "rb") as f:
+            self.assertEqual(fat16.read_file(file), f.read())
+
+    def test_write_file_in_same_content(self):
+        """
+        Test writing the same content to the file back to it
+        """
+        fat16 = self.init_fat16()
+
+        file = fat16.find_direntry("/FILE0.TXT")
+        self.assertIsNotNone(file)
+
+        self.assertEqual(fat16.read_file(file), b"Hello, world! 0\n")
+
+        fat16.write_file(file, b"Hello, world! 0\n")
+        self.assertEqual(fat16.read_file(file), b"Hello, world! 0\n")
+
+        with open(os.path.join(filesystem, "file0.txt"), "rb") as f:
+            self.assertEqual(f.read(), b"Hello, world! 0\n")
+
+    def test_modify_content_same_clusters(self):
+        """
+        Test modifying the content of the file without changing the number of clusters
+        """
+        fat16 = self.init_fat16()
+
+        file = fat16.find_direntry("/FILE0.TXT")
+        self.assertIsNotNone(file)
+
+        new_content = b"Hello, world! Modified\n"
+        self.assertEqual(fat16.read_file(file), b"Hello, world! 0\n")
+
+        fat16.write_file(file, new_content)
+        self.assertEqual(fat16.read_file(file), new_content)
+
+        with open(os.path.join(filesystem, "file0.txt"), "rb") as f:
+            self.assertEqual(f.read(), new_content)
+
+    def test_truncate_file_same_clusters_less(self):
+        """
+        Test truncating the file without changing number of clusters
+        Test decreasing the file size
+        """
+        fat16 = self.init_fat16()
+
+        file = fat16.find_direntry("/FILE0.TXT")
+        self.assertIsNotNone(file)
+
+        self.assertEqual(fat16.read_file(file), b"Hello, world! 0\n")
+
+        fat16.truncate_file(file, 5)
+        new_content = fat16.read_file(file)
+        self.assertEqual(new_content, b"Hello")
+
+        with open(os.path.join(filesystem, "file0.txt"), "rb") as f:
+            self.assertEqual(f.read(), new_content)
+
+    def test_truncate_file_same_clusters_more(self):
+        """
+        Test truncating the file without changing number of clusters
+        Test increase the file size
+        """
+        fat16 = self.init_fat16()
+
+        file = fat16.find_direntry("/FILE0.TXT")
+        self.assertIsNotNone(file)
+
+        self.assertEqual(fat16.read_file(file), b"Hello, world! 0\n")
+
+        fat16.truncate_file(file, 20)
+        new_content = fat16.read_file(file)
+        self.assertIsNotNone(new_content)
+
+        # random pattern will be appended to the file, and its not always the same
+        self.assertEqual(new_content[:16], b"Hello, world! 0\n")
+        self.assertEqual(len(new_content), 20)
+
+        with open(os.path.join(filesystem, "file0.txt"), "rb") as f:
+            self.assertEqual(f.read(), new_content)
+
+    def test_write_large_file(self):
+        """
+        Test writing a large file
+        """
+        fat16 = self.init_fat16()
+
+        file = fat16.find_direntry("/LARGE1.TXT")
+        self.assertIsNotNone(file)
+
+        # The content of LARGE1 is A * 1KB, B * 1KB, C * 1KB, ..., P * 1KB
+        # Lets change it to be Z * 1KB, Y * 1KB, X * 1KB, ..., K * 1KB
+        # without changing the number of clusters or filesize
+        new_content = b"".join([bytes([0x5A - i] * 1024) for i in range(16)])
+        fat16.write_file(file, new_content)
+        self.assertEqual(fat16.read_file(file), new_content)
+
+        with open(os.path.join(filesystem, "large1.txt"), "rb") as f:
+            self.assertEqual(f.read(), new_content)
+
+    def test_truncate_file_change_clusters_less(self):
+        """
+        Test truncating a file by reducing the number of clusters
+        """
+        fat16 = self.init_fat16()
+
+        file = fat16.find_direntry("/LARGE1.TXT")
+        self.assertIsNotNone(file)
+
+        fat16.truncate_file(file, 1)
+        self.assertEqual(fat16.read_file(file), b"A")
+
+        with open(os.path.join(filesystem, "large1.txt"), "rb") as f:
+            self.assertEqual(f.read(), b"A")
+
+    def test_write_file_change_clusters_less(self):
+        """
+        Test truncating a file by reducing the number of clusters
+        """
+        fat16 = self.init_fat16()
+
+        file = fat16.find_direntry("/LARGE2.TXT")
+        self.assertIsNotNone(file)
+
+        new_content = b"X" * 8 * 1024 + b"Y" * 8 * 1024
+        fat16.write_file(file, new_content)
+        self.assertEqual(fat16.read_file(file), new_content)
+
+        with open(os.path.join(filesystem, "large2.txt"), "rb") as f:
+            self.assertEqual(f.read(), new_content)
+
+    def test_write_file_change_clusters_more(self):
+        """
+        Test truncating a file by increasing the number of clusters
+        """
+        fat16 = self.init_fat16()
+
+        file = fat16.find_direntry("/LARGE2.TXT")
+        self.assertIsNotNone(file)
+
+        # from 3 clusters to 4 clusters
+        new_content = (
+            b"W" * 8 * 1024 + b"X" * 8 * 1024 + b"Y" * 8 * 1024 + b"Z" * 8 * 1024
+        )
+        fat16.write_file(file, new_content)
+        self.assertEqual(fat16.read_file(file), new_content)
+
+        with open(os.path.join(filesystem, "large2.txt"), "rb") as f:
+            self.assertEqual(f.read(), new_content)
+
+    def test_write_file_change_clusters_more_non_contiguous_2_mappings(self):
+        """
+        Test truncating a file by increasing the number of clusters
+        Here we allocate the new clusters in a way that makes them non-contiguous
+        so that we will get 2 cluster mappings for the file
+        """
+        fat16 = self.init_fat16()
+
+        file = fat16.find_direntry("/LARGE1.TXT")
+        self.assertIsNotNone(file)
+
+        # from 2 clusters to 3 clusters with non-contiguous allocation
+        fat16.truncate_file(file, 3 * 0x2000, allocate_non_continuous=True)
+        new_content = b"X" * 8 * 1024 + b"Y" * 8 * 1024 + b"Z" * 8 * 1024
+        fat16.write_file(file, new_content)
+        self.assertEqual(fat16.read_file(file), new_content)
+
+        with open(os.path.join(filesystem, "large1.txt"), "rb") as f:
+            self.assertEqual(f.read(), new_content)
+
+    def test_write_file_change_clusters_more_non_contiguous_3_mappings(self):
+        """
+        Test truncating a file by increasing the number of clusters
+        Here we allocate the new clusters in a way that makes them non-contiguous
+        so that we will get 3 cluster mappings for the file
+        """
+        fat16 = self.init_fat16()
+
+        file = fat16.find_direntry("/LARGE1.TXT")
+        self.assertIsNotNone(file)
+
+        # from 2 clusters to 4 clusters with non-contiguous allocation
+        fat16.truncate_file(file, 4 * 0x2000, allocate_non_continuous=True)
+        new_content = (
+            b"W" * 8 * 1024 + b"X" * 8 * 1024 + b"Y" * 8 * 1024 + b"Z" * 8 * 1024
+        )
+        fat16.write_file(file, new_content)
+        self.assertEqual(fat16.read_file(file), new_content)
+
+        with open(os.path.join(filesystem, "large1.txt"), "rb") as f:
+            self.assertEqual(f.read(), new_content)
+
+    def test_create_file(self):
+        """
+        Test creating a new file
+        """
+        fat16 = self.init_fat16()
+
+        new_file = fat16.create_file("/NEWFILE.TXT")
+
+        self.assertIsNotNone(new_file)
+        self.assertEqual(new_file.size_bytes, 0)
+
+        new_content = b"Hello, world! New file\n"
+        fat16.write_file(new_file, new_content)
+        self.assertEqual(fat16.read_file(new_file), new_content)
+
+        with open(os.path.join(filesystem, "newfile.txt"), "rb") as f:
+            self.assertEqual(f.read(), new_content)
+
+    # TODO: support deleting files
+
+
+if __name__ == "__main__":
+    # This is a specific test for vvfat driver
+    iotests.main(supported_fmts=["vvfat"], supported_protocols=["file"])
diff --git a/tests/qemu-iotests/tests/vvfat.out b/tests/qemu-iotests/tests/vvfat.out
new file mode 100755
index 0000000000..b6f257674e
--- /dev/null
+++ b/tests/qemu-iotests/tests/vvfat.out
@@ -0,0 +1,5 @@
+................
+----------------------------------------------------------------------
+Ran 16 tests
+
+OK
-- 
2.45.2

Re: [PATCH v6 4/5] vvfat: Fix reading files with non-continuous clusters

[Kevin Wolf]

Am 20.07.2024 um 12:13 hat Amjad Alsharafi geschrieben:
> When reading with `read_cluster` we get the `mapping` with
> `find_mapping_for_cluster` and then we call `open_file` for this
> mapping.
> The issue appear when its the same file, but a second cluster that is
> not immediately after it, imagine clusters `500 -> 503`, this will give
> us 2 mappings one has the range `500..501` and another `503..504`, both
> point to the same file, but different offsets.
> 
> When we don't open the file since the path is the same, we won't assign
> `s->current_mapping` and thus accessing way out of bound of the file.
> 
> From our example above, after `open_file` (that didn't open anything) we
> will get the offset into the file with
> `s->cluster_size*(cluster_num-s->current_mapping->begin)`, which will
> give us `0x2000 * (504-500)`, which is out of bound for this mapping and
> will produce some issues.
> 
> Signed-off-by: Amjad Alsharafi <amjadsharafi10@gmail.com>
> ---
>  block/vvfat.c | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
> 
> diff --git a/block/vvfat.c b/block/vvfat.c
> index b63ac5d045..d2b879705e 100644
> --- a/block/vvfat.c
> +++ b/block/vvfat.c
> @@ -1360,15 +1360,17 @@ static int open_file(BDRVVVFATState* s,mapping_t* mapping)
>  {
>      if(!mapping)
>          return -1;
> -    if(!s->current_mapping ||
> -            strcmp(s->current_mapping->path,mapping->path)) {
> +    if (s->current_mapping != mapping) {
>          /* open file */
>          int fd = qemu_open_old(mapping->path,
> -                               O_RDONLY | O_BINARY | O_LARGEFILE);
> -        if(fd<0)
> +                            O_RDONLY | O_BINARY | O_LARGEFILE);
> +        if (fd < 0) {
>              return -1;
> +        }
>          vvfat_close_current_file(s);
> +
>          s->current_fd = fd;
> +        assert(s->current_fd);
>          s->current_mapping = mapping;
>      }
>      return 0;

Now we're reopening the file even if the mapping is actually referring
to the same file that was already open. So the result should be correct,
but wasteful in what is probably a common case.

However, this version of the patch simplified things enough that I think
I finally see what we really want:

diff --git a/block/vvfat.c b/block/vvfat.c
index e3a83fbc88..8ffe8b3b9b 100644
--- a/block/vvfat.c
+++ b/block/vvfat.c
@@ -1369,8 +1369,9 @@ static int open_file(BDRVVVFATState* s,mapping_t* mapping)
             return -1;
         vvfat_close_current_file(s);
         s->current_fd = fd;
-        s->current_mapping = mapping;
     }
+
+    s->current_mapping = mapping;
     return 0;
 }

That should be all that is needed, and it passes your test case.

I don't usually do this, but since time is running out for QEMU 9.1,
I'll just replace the code of this patch with the above and go ahead
with that. If you think it's wrong, let me know and we'll fix it on top
of this series.

Kevin

Re: [PATCH v6 0/5] vvfat: Fix write bugs for large files and add iotests

[Kevin Wolf]

Am 20.07.2024 um 12:13 hat Amjad Alsharafi geschrieben:
> These patches fix some bugs found when modifying files in vvfat.
> First, there was a bug when writing to the cluster 2 or above of a file, it
> will copy the cluster before it instead, so, when writing to cluster=2, the
> content of cluster=1 will be copied into disk instead in its place.
> 
> Another issue was modifying the clusters of a file and adding new
> clusters, this showed 2 issues:
> - If the new cluster is not immediately after the last cluster, it will
> cause issues when reading from this file in the future.
> - Generally, the usage of info.file.offset was incorrect, and the
> system would crash on abort() when the file is modified and a new
> cluster was added.
> 
> Also, added some iotests for vvfat, covering the this fix and also
> general behavior such as reading, writing, and creating files on the filesystem.
> Including tests for reading/writing the first cluster which
> would pass even before this patch.

Thanks, applied to the block branch (with patch 4 changed as indicated
in my comment there).

Kevin

Re: [PATCH v6 4/5] vvfat: Fix reading files with non-continuous clusters

[Amjad Alsharafi]

On Aug 5 2024, at 6:22 pm, Kevin Wolf <kwolf@redhat.com> wrote:

> Am 20.07.2024 um 12:13 hat Amjad Alsharafi geschrieben:
>> When reading with `read_cluster` we get the `mapping` with
>> `find_mapping_for_cluster` and then we call `open_file` for this
>> mapping.
>> The issue appear when its the same file, but a second cluster that is
>> not immediately after it, imagine clusters `500 -> 503`, this will give
>> us 2 mappings one has the range `500..501` and another `503..504`, both
>> point to the same file, but different offsets.
>> 
>> When we don't open the file since the path is the same, we won't assign
>> `s->current_mapping` and thus accessing way out of bound of the file.
>> 
>> From our example above, after `open_file` (that didn't open anything) we
>> will get the offset into the file with
>> `s->cluster_size*(cluster_num-s->current_mapping->begin)`, which will
>> give us `0x2000 * (504-500)`, which is out of bound for this mapping and
>> will produce some issues.
>> 
>> Signed-off-by: Amjad Alsharafi <amjadsharafi10@gmail.com>
>> ---
>>  block/vvfat.c | 10 ++++++----
>>  1 file changed, 6 insertions(+), 4 deletions(-)
>> 
>> diff --git a/block/vvfat.c b/block/vvfat.c
>> index b63ac5d045..d2b879705e 100644
>> --- a/block/vvfat.c
>> +++ b/block/vvfat.c
>> @@ -1360,15 +1360,17 @@ static int open_file(BDRVVVFATState*
>> s,mapping_t* mapping)
>>  {
>>      if(!mapping)
>>          return -1;
>> -    if(!s->current_mapping ||
>> -            strcmp(s->current_mapping->path,mapping->path)) {
>> +    if (s->current_mapping != mapping) {
>>          /* open file */
>>          int fd = qemu_open_old(mapping->path,
>> -                               O_RDONLY | O_BINARY | O_LARGEFILE);
>> -        if(fd<0)
>> +                            O_RDONLY | O_BINARY | O_LARGEFILE);
>> +        if (fd < 0) {
>>              return -1;
>> +        }
>>          vvfat_close_current_file(s);
>> +
>>          s->current_fd = fd;
>> +        assert(s->current_fd);
>>          s->current_mapping = mapping;
>>      }
>>      return 0;
> 
> Now we're reopening the file even if the mapping is actually referring
> to the same file that was already open. So the result should be correct,
> but wasteful in what is probably a common case.
> 
> However, this version of the patch simplified things enough that I think
> I finally see what we really want:
> 
> diff --git a/block/vvfat.c b/block/vvfat.c
> index e3a83fbc88..8ffe8b3b9b 100644
> --- a/block/vvfat.c
> +++ b/block/vvfat.c
> @@ -1369,8 +1369,9 @@ static int open_file(BDRVVVFATState*
> s,mapping_t* mapping)
>             return -1;
>         vvfat_close_current_file(s);
>         s->current_fd = fd;
> -        s->current_mapping = mapping;
>     }
> +
> +    s->current_mapping = mapping;
>     return 0;
> }

Ah, that indeed fixes it with such a simple change haha.

> 
> That should be all that is needed, and it passes your test case.
> 
> I don't usually do this, but since time is running out for QEMU 9.1,
> I'll just replace the code of this patch with the above and go ahead
> with that. If you think it's wrong, let me know and we'll fix it on top
> of this series.
> 
> Kevin
> 
> 

Thanks for that, and for publishing this. Looks good.

Amjad