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Fri, 18 Dec 2020 14:49:26 +0100 (CET) From: Markus Armbruster To: Max Reitz Subject: Re: [PATCH 1/9] block/vpc: Make vpc_open() read the full dynamic header References: <20201217162003.1102738-1-armbru@redhat.com> <20201217162003.1102738-2-armbru@redhat.com> Date: Fri, 18 Dec 2020 14:49:26 +0100 In-Reply-To: (Max Reitz's message of "Fri, 18 Dec 2020 11:19:46 +0100") Message-ID: <878s9v9qc9.fsf@dusky.pond.sub.org> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/27.1 (gnu/linux) MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=armbru@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Received-SPF: pass client-ip=216.205.24.124; envelope-from=armbru@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=-0.001, 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_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: kwolf@redhat.com, mrezanin@redhat.com, qemu-devel@nongnu.org, qemu-block@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Max Reitz writes: > On 17.12.20 17:19, Markus Armbruster wrote: >> The dynamic header's size is 1024 bytes. >>=20 >> vpc_open() reads only the 512 bytes of the dynamic header into buf[]. >> Works, because it doesn't actually access the second half. However, a >> colleague told me that GCC 11 warns: >>=20 >> ../block/vpc.c:358:51: error: array subscript 'struct VHDDynDiskHea= der[0]' is partly outside array bounds of 'uint8_t[512]' [-Werror=3Darray-b= ounds] >>=20 >> Clean up to read the full header. >>=20 >> Rename buf[] to dyndisk_header_buf[] while there. >>=20 >> Signed-off-by: Markus Armbruster >> --- >> block/vpc.c | 8 ++++---- >> 1 file changed, 4 insertions(+), 4 deletions(-) >>=20 >> diff --git a/block/vpc.c b/block/vpc.c >> index 1ab55f9287..2fcf3f6283 100644 >> --- a/block/vpc.c >> +++ b/block/vpc.c >> @@ -220,7 +220,7 @@ static int vpc_open(BlockDriverState *bs, QDict *opt= ions, int flags, >> QemuOpts *opts =3D NULL; >> Error *local_err =3D NULL; >> bool use_chs; >> - uint8_t buf[HEADER_SIZE]; >> + uint8_t dyndisk_header_buf[1024]; > > Perhaps this should be heap-allocated, but I don=E2=80=99t know whether q= emu has=20 > ever established a (perhaps just inofficial) threshold on what goes on=20 > the stack and what goes on the heap. There is no official per-function limit. I generally don't worry about a few kilobytes unless it's recursive. We have many, many functions with stack frames in the order of a kilobyte. Our coroutine and thread stacks are reasonable (corotine stacks are 1MiB, the default thread stack size 2MiB on x86-64, and I can't see code that sets a different size). >> uint32_t checksum; >> uint64_t computed_size; >> uint64_t pagetable_size; >> @@ -340,14 +340,14 @@ static int vpc_open(BlockDriverState *bs, QDict *o= ptions, int flags, >> } >> =20 >> if (disk_type =3D=3D VHD_DYNAMIC) { >> - ret =3D bdrv_pread(bs->file, be64_to_cpu(footer->data_offset), = buf, >> - HEADER_SIZE); >> + ret =3D bdrv_pread(bs->file, be64_to_cpu(footer->data_offset), >> + dyndisk_header_buf, 1024); > > sizeof() would be better, but I see that=E2=80=99s what patch 6 is for. Yes, but sizeof what? VHDDynDiskHeader becomes usable for that only in PATCH 5. I chose to separate the buffers first, and only then tidy up their use. >> if (ret < 0) { >> error_setg(errp, "Error reading dynamic VHD header"); >> goto fail; >> } >> =20 >> - dyndisk_header =3D (VHDDynDiskHeader *) buf; >> + dyndisk_header =3D (VHDDynDiskHeader *)dyndisk_header_buf; >> =20 >> if (strncmp(dyndisk_header->magic, "cxsparse", 8)) { >> error_setg(errp, "Invalid header magic"); > > Reviewed-by: Max Reitz Thanks!