[PATCH v3] ext4: test reserved GID block allocation

[PATCH v3] ext4: test reserved GID block allocation

[Sahaj Chaudhari]

Create a small ext4 filesystem with a nonzero reserved block pool and a
reserved GID stored in the superblock. Fill ordinary free space as an
unprivileged user outside that group, verify that another allocation is
denied, then verify that the same user can allocate a block with the
reserved group as its primary and only supplementary group.

This exercises loading s_resgid from the superblock rather than overriding
it with a resgid mount option, and detects the regression fixed by:

commit c87abbab6147 ("ext4: fix incorrect function call when initializing s_resgid")

Link: https://lore.kernel.org/r/20261008065651.3517824-1-sahaj123.sc@gmail.com
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Sahaj Chaudhari <sahaj123.sc@gmail.com>
---
Changes since v2:
- Use fsgqa2 instead of root for the fill group, and require it explicitly.
- Do not inherit external MKFS_OPTIONS; exclude journal-dependent mount
  options for the test's no-journal filesystem.
- Check tune2fs and su -g/-G support, and use the validated su executable
  consistently.
- Require the fill operation to write data and reach ENOSPC; check that
  ordinary allocation also fails specifically with ENOSPC.
- Let the check harness unmount scratch and finish with _exit 0.
- Add Jan Kara's Reviewed-by.

Testing:
- ./check ext4/309 in isolated x86-64 QEMU with fresh TEST_DEV and
  SCRATCH_DEV images passes on the fixed kernel and fails specifically
  at reserved-GID allocation on the pre-c87abbab6147 kernel.
- Both filesystems pass e2fsck -fn after each run.
- bash -n and git diff --check pass. Validation is limited to ext4/309,
  not the full xfstests suite.

 tests/ext4/309     | 75 ++++++++++++++++++++++++++++++++++++++++++++++
 tests/ext4/309.out |  2 ++
 2 files changed, 77 insertions(+)
 create mode 100755 tests/ext4/309
 create mode 100644 tests/ext4/309.out

diff --git a/tests/ext4/309 b/tests/ext4/309
new file mode 100755
index 00000000..e5dcd215
--- /dev/null
+++ b/tests/ext4/309
@@ -0,0 +1,75 @@
+#! /bin/bash
+# SPDX-License-Identifier: GPL-2.0
+# Copyright (c) 2026 Sahaj Chaudhari.  All Rights Reserved.
+#
+# FS QA Test No. 309
+#
+# Verify that the ext4 reserved GID can allocate blocks after ordinary
+# free space is exhausted.
+#
+. ./common/preamble
+_begin_fstest auto enospc quick
+
+_exclude_fs ext2
+_exclude_fs ext3
+_exclude_scratch_mount_option "resuid=" "resgid=" "data=" \
+	"data_err=abort" "commit=" "journal_async_commit" "journal_checksum"
+
+_fixed_by_kernel_commit c87abbab6147 \
+	"ext4: fix incorrect function call when initializing s_resgid"
+
+_require_scratch
+_require_command "$TUNE2FS_PROG" tune2fs
+_require_xfs_io_command "falloc"
+_require_user
+_require_group
+RES_GROUP=$qa_group
+_require_group fsgqa2
+FILL_GROUP=fsgqa2
+SU_PROG=$(command -v su) || _notrun "su utility required"
+"$SU_PROG" -s /bin/bash -g "$FILL_GROUP" -G "$FILL_GROUP" "$qa_user" \
+	-c true >/dev/null 2>&1 ||
+	_notrun "su with -g and -G support required"
+
+echo "Silence is golden"
+
+RESGID=$(_cat_group | awk -F: -v group="$RES_GROUP" \
+	'$1 == group { print $3 }')
+FILLGID=$(_cat_group | awk -F: -v group="$FILL_GROUP" \
+	'$1 == group { print $3 }')
+[ "$RESGID" != "$FILLGID" ] || _notrun "reserved and fill groups are identical"
+[ "$(id -u "$qa_user")" -ne 0 ] || _notrun "test user is root"
+TEST_FILE="$SCRATCH_MNT/ext4_resgid.fill"
+
+MKFS_OPTIONS="-m 10 -N 1024 -O ^has_journal" \
+	_scratch_mkfs_sized $((16 * 1024 * 1024)) 4096 >>"$seqres".full 2>&1
+"$TUNE2FS_PROG" -u 0 -g "$RESGID" "$SCRATCH_DEV" >>"$seqres".full 2>&1 ||
+	_fail "could not set scratch filesystem reserved GID"
+_scratch_mount
+
+touch "$TEST_FILE"
+chmod 0666 "$TEST_FILE"
+
+FILL_OUTPUT=$("$SU_PROG" -s /bin/bash -g "$FILL_GROUP" -G "$FILL_GROUP" \
+	"$qa_user" -c "$XFS_IO_PROG -f -c 'pwrite 0 16M' '$TEST_FILE'" 2>&1)
+FILL_STATUS=$?
+printf '%s\n' "$FILL_OUTPUT" >>"$seqres".full
+printf '%s\n' "$FILL_OUTPUT" | grep -q "pwrite: No space left on device" ||
+	_fail "space-fill operation did not reach ENOSPC (status $FILL_STATUS)"
+FILL_SIZE=$(_get_filesize "$TEST_FILE") ||
+	_fail "could not determine space-fill file size"
+[ "$FILL_SIZE" -gt 0 ] || _fail "space-fill operation wrote no data"
+
+if ALLOC_OUTPUT=$("$SU_PROG" -s /bin/bash -g "$FILL_GROUP" -G "$FILL_GROUP" \
+	"$qa_user" -c "$XFS_IO_PROG -c 'falloc 16M 4k' '$TEST_FILE'" 2>&1); then
+	_fail "ordinary user allocated from the reserved pool"
+fi
+printf '%s\n' "$ALLOC_OUTPUT" >>"$seqres".full
+printf '%s\n' "$ALLOC_OUTPUT" | grep -q "fallocate: No space left on device" ||
+	_fail "ordinary allocation failed for a reason other than ENOSPC"
+
+"$SU_PROG" -s /bin/bash -g "$RES_GROUP" -G "$RES_GROUP" "$qa_user" -c \
+	"$XFS_IO_PROG -c 'falloc 16M 4k' '$TEST_FILE'" \
+	>>"$seqres".full 2>&1 || _fail "reserved GID failed to allocate a block"
+
+_exit 0
diff --git a/tests/ext4/309.out b/tests/ext4/309.out
new file mode 100644
index 00000000..56330d65
--- /dev/null
+++ b/tests/ext4/309.out
@@ -0,0 +1,2 @@
+QA output created by 309
+Silence is golden

base-commit: d07dca72d2455f4ba8c660ad4c008541ccccb707
-- 
2.43.0

Re: [PATCH v3] ext4: test reserved GID block allocation

[sashiko-bot]

> Create a small ext4 filesystem with a nonzero reserved block pool and a
> reserved GID stored in the superblock. Fill ordinary free space as an
> unprivileged user outside that group, verify that another allocation is
> denied, then verify that the same user can allocate a block with the
> reserved group as its primary and only supplementary group.
> 
> This exercises loading s_resgid from the superblock rather than overriding
> it with a resgid mount option, and detects the regression fixed by:
> 
> commit c87abbab6147 ("ext4: fix incorrect function call when initializing s_resgid")
> 
> Link: https://lore.kernel.org/r/20261008065651.3517824-1-sahaj123.sc@gmail.com
> Reviewed-by: Jan Kara <jack@suse.cz>
> Signed-off-by: Sahaj Chaudhari <sahaj123.sc@gmail.com>

Sashiko has reviewed this patch and found no issues.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261110182040.798390-1-sahaj123.sc@gmail.com?part=1