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Wed, 18 Feb 2026 05:27:19 -0800 (PST) X-Received: by 2002:a05:600c:871b:b0:483:7783:5382 with SMTP id 5b1f17b1804b1-48398ae5e18mr33798785e9.27.1771421238797; Wed, 18 Feb 2026 05:27:18 -0800 (PST) From: Hanna Czenczek To: qemu-block@nongnu.org Cc: qemu-devel@nongnu.org, Hanna Czenczek , Kevin Wolf , Brian Song Subject: [PATCH v4 14/24] fuse: Explicitly handle non-grow post-EOF accesses Date: Wed, 18 Feb 2026 14:26:23 +0100 Message-ID: <20260218132633.29748-15-hreitz@redhat.com> X-Mailer: git-send-email 2.53.0 In-Reply-To: <20260218132633.29748-1-hreitz@redhat.com> References: <20260218132633.29748-1-hreitz@redhat.com> MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Received-SPF: pass (zohomail.com: domain of gnu.org designates 209.51.188.17 as permitted sender) client-ip=209.51.188.17; envelope-from=qemu-devel-bounces+importer=patchew.org@nongnu.org; helo=lists.gnu.org; Received-SPF: pass client-ip=170.10.129.124; envelope-from=hreitz@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.043, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, RCVD_IN_VALIDITY_SAFE_BLOCKED=0.001, SPF_HELO_PASS=-0.001, SPF_PASS=-0.001 autolearn=unavailable autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: qemu development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+importer=patchew.org@nongnu.org Sender: qemu-devel-bounces+importer=patchew.org@nongnu.org X-ZohoMail-DKIM: pass (identity @redhat.com) X-ZM-MESSAGEID: 1771421702895158500 Content-Type: text/plain; charset="utf-8" When reading to / writing from non-growable exports, we cap the I/O size by `offset - blk_len`. This will underflow for accesses that are completely past the disk end. Check and handle that case explicitly. This is also enough to ensure that `offset + size` will not overflow; blk_len is int64_t, offset is uint32_t, `offset < blk_len`, so from `INT64_MAX + UINT32_MAX < UINT64_MAX` it follows that `offset + size` cannot overflow. Just one catch: We have to allow write accesses to growable exports past the EOF, so then we cannot rely on `offset < blk_len`, but have to verify explicitly that `offset + size` does not overflow. The negative consequences of not having this commit are luckily limited because blk_pread() and blk_pwrite() will reject post-EOF requests anyway, so a `size` underflow post-EOF will just result in an I/O error. So: - Post-EOF reads will incorrectly result in I/O errors instead of just 0-length reads. We will also attempt to allocate a very large buffer, which is wrong and not good, but not terrible. - Post-EOF writes on non-growable exports will result in I/O errors instead of 0-length writes (which generally indicate ENOSPC). - Post-EOF writes on growable exports can theoretically overflow on EOF and truncate the export down to a much too small size, but in practice, FUSE will never send an offset greater than signed INT_MAX, preventing a uint64_t overflow. (fuse_write_args_fill() in the kernel uses loff_t for the offset, which is signed.) Signed-off-by: Hanna Czenczek --- block/export/fuse.c | 20 +++++++++++++++++++- tests/qemu-iotests/308 | 35 ++++++++++++++++++++++++++++++----- tests/qemu-iotests/308.out | 10 ++++++++++ 3 files changed, 59 insertions(+), 6 deletions(-) diff --git a/block/export/fuse.c b/block/export/fuse.c index d45c6b814f..af0a8de17b 100644 --- a/block/export/fuse.c +++ b/block/export/fuse.c @@ -657,6 +657,16 @@ static void fuse_read(fuse_req_t req, fuse_ino_t inode, return; } =20 + if (offset >=3D blk_len) { + /* + * Technically libfuse does not allow returning a zero error code = for + * read requests, but in practice this is a 0-length read (and a f= uture + * commit will change this code anyway) + */ + fuse_reply_err(req, 0); + return; + } + if (offset + size > blk_len) { size =3D blk_len - offset; } @@ -717,7 +727,15 @@ static void fuse_write(fuse_req_t req, fuse_ino_t inod= e, const char *buf, return; } =20 - if (offset + size > blk_len) { + if (offset >=3D blk_len && !exp->growable) { + fuse_reply_write(req, 0); + return; + } + + if (offset + size < offset) { + fuse_reply_err(req, EINVAL); + return; + } else if (offset + size > blk_len) { if (exp->growable) { ret =3D fuse_do_truncate(exp, offset + size, true, PREALLOC_MO= DE_OFF); if (ret < 0) { diff --git a/tests/qemu-iotests/308 b/tests/qemu-iotests/308 index 6ecb275555..a83c6fc01f 100755 --- a/tests/qemu-iotests/308 +++ b/tests/qemu-iotests/308 @@ -300,16 +300,34 @@ dd if=3D/dev/zero of=3D"$EXT_MP" bs=3D1 count=3D64k s= eek=3D$orig_len \ conv=3Dnotrunc 2>&1 \ | _filter_testdir | _filter_imgfmt =20 +# And one really squarely post-EOF write +dd if=3D/dev/zero of=3D"$EXT_MP" bs=3D1 count=3D1 seek=3D$((orig_len + 32 = * 1024)) \ + conv=3Dnotrunc 2>&1 \ + | _filter_testdir | _filter_imgfmt + +# Half-post-EOF reads +dd if=3D"$EXT_MP" of=3D/dev/null bs=3D1 count=3D64k skip=3D$((orig_len - 3= 2 * 1024)) \ + 2>&1 | _filter_testdir | _filter_imgfmt + +# And one really squarely post-EOF read +dd if=3D"$EXT_MP" of=3D/dev/null bs=3D1 count=3D1 skip=3D$((orig_len + 32 = * 1024)) \ + 2>&1 | _filter_testdir | _filter_imgfmt + echo echo '--- Resize export ---' =20 # But we can truncate it explicitly; even with fallocate -fallocate -o "$orig_len" -l 64k "$EXT_MP" +# (Make sure we extend it to a length not divisible by 128k, we need that = below) +bs=3D$((128 * 1024)) +extend_to=3D$(((orig_len + bs - 1) / bs * bs + bs / 2)) +extend_by=3D$((extend_to - orig_len)) + +fallocate -o "$orig_len" -l $extend_by "$EXT_MP" =20 new_len=3D$(get_proto_len "$EXT_MP" "$TEST_IMG") -if [ "$new_len" !=3D "$((orig_len + 65536))" ]; then +if [ "$new_len" !=3D "$extend_to" ]; then echo 'ERROR: Unexpected post-truncate image size:' - echo "$new_len !=3D $((orig_len + 65536))" + echo "$new_len !=3D $extend_to" else echo 'OK: Post-truncate image size is as expected' fi @@ -322,6 +340,13 @@ else echo "$orig_disk_usage =3D> $new_disk_usage" fi =20 +# Use this opportunity to test a read access across the (now no longer so = much +# aligned) EOF. dd can only do requests with a length of its block size, = and +# all of its seek/skip values are in bs units, so it is hard to do a reque= st +# across the EOF if the EOF is at a power of two (64M). +dd if=3D"$EXT_MP" of=3D/dev/null bs=3D$bs count=3D2 skip=3D$((extend_to / = bs)) \ + 2>&1 | _filter_testdir | _filter_imgfmt + echo echo '--- Try growing growable export ---' =20 @@ -338,9 +363,9 @@ dd if=3D/dev/zero of=3D"$EXT_MP" bs=3D1 count=3D64k see= k=3D$new_len conv=3Dnotrunc 2>&1 \ | _filter_testdir | _filter_imgfmt =20 new_len=3D$(get_proto_len "$EXT_MP" "$TEST_IMG") -if [ "$new_len" !=3D "$((orig_len + 131072))" ]; then +if [ "$new_len" !=3D "$((extend_to + 65536))" ]; then echo 'ERROR: Unexpected post-grow image size:' - echo "$new_len !=3D $((orig_len + 131072))" + echo "$new_len !=3D $((extend_to + 65536))" else echo 'OK: Post-grow image size is as expected' fi diff --git a/tests/qemu-iotests/308.out b/tests/qemu-iotests/308.out index 2d7a38d63d..ebeaf64b48 100644 --- a/tests/qemu-iotests/308.out +++ b/tests/qemu-iotests/308.out @@ -134,11 +134,21 @@ wrote 65536/65536 bytes at offset 1048576 dd: error writing 'TEST_DIR/t.IMGFMT.fuse': No space left on device 1+0 records in 0+0 records out +dd: error writing 'TEST_DIR/t.IMGFMT.fuse': No space left on device +1+0 records in +0+0 records out +32768+0 records in +32768+0 records out +dd: TEST_DIR/t.IMGFMT.fuse: cannot skip to specified offset +0+0 records in +0+0 records out =20 --- Resize export --- (OK: Lengths of export and original are the same) OK: Post-truncate image size is as expected OK: Disk usage grew with fallocate +0+1 records in +0+1 records out =20 --- Try growing growable export --- {'execute': 'block-export-del', --=20 2.53.0