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Tue, 09 Sep 2025 07:43:14 -0700 (PDT) Received: from sidongui-MacBookPro.local ([61.83.209.48]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-25a281e5620sm313525ad.63.2025.09.09.07.43.11 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 09 Sep 2025 07:43:13 -0700 (PDT) Date: Tue, 9 Sep 2025 23:43:08 +0900 From: Sidong Yang To: Caleb Sander Mateos Cc: Jens Axboe , Daniel Almeida , Benno Lossin , Miguel Ojeda , Arnd Bergmann , Greg Kroah-Hartman , rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org, io-uring@vger.kernel.org Subject: Re: [RFC PATCH v3 2/5] io_uring/cmd: zero-init pdu in io_uring_cmd_prep() to avoid UB Message-ID: References: <20250822125555.8620-1-sidong.yang@furiosa.ai> <20250822125555.8620-3-sidong.yang@furiosa.ai> Precedence: bulk X-Mailing-List: rust-for-linux@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: On Mon, Sep 08, 2025 at 12:45:58PM -0700, Caleb Sander Mateos wrote: > On Sat, Sep 6, 2025 at 7:28 AM Sidong Yang wrote: > > > > On Tue, Sep 02, 2025 at 08:31:00AM -0700, Caleb Sander Mateos wrote: > > > On Tue, Sep 2, 2025 at 3:23 AM Sidong Yang wrote: > > > > > > > > On Mon, Sep 01, 2025 at 05:34:28PM -0700, Caleb Sander Mateos wrote: > > > > > On Fri, Aug 22, 2025 at 5:56 AM Sidong Yang wrote: > > > > > > > > > > > > The pdu field in io_uring_cmd may contain stale data when a request > > > > > > object is recycled from the slab cache. Accessing uninitialized or > > > > > > garbage memory can lead to undefined behavior in users of the pdu. > > > > > > > > > > > > Ensure the pdu buffer is cleared during io_uring_cmd_prep() so that > > > > > > each command starts from a well-defined state. This avoids exposing > > > > > > uninitialized memory and prevents potential misinterpretation of data > > > > > > from previous requests. > > > > > > > > > > > > No functional change is intended other than guaranteeing that pdu is > > > > > > always zero-initialized before use. > > > > > > > > > > > > Signed-off-by: Sidong Yang > > > > > > --- > > > > > > io_uring/uring_cmd.c | 1 + > > > > > > 1 file changed, 1 insertion(+) > > > > > > > > > > > > diff --git a/io_uring/uring_cmd.c b/io_uring/uring_cmd.c > > > > > > index 053bac89b6c0..2492525d4e43 100644 > > > > > > --- a/io_uring/uring_cmd.c > > > > > > +++ b/io_uring/uring_cmd.c > > > > > > @@ -203,6 +203,7 @@ int io_uring_cmd_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe) > > > > > > if (!ac) > > > > > > return -ENOMEM; > > > > > > ioucmd->sqe = sqe; > > > > > > + memset(&ioucmd->pdu, 0, sizeof(ioucmd->pdu)); > > > > > > > > > > Adding this overhead to every existing uring_cmd() implementation is > > > > > unfortunate. Could we instead track the initialized/uninitialized > > > > > state by using different types on the Rust side? The io_uring_cmd > > > > > could start as an IoUringCmd, where the PDU field is MaybeUninit, > > > > > write_pdu() could return a new IoUringCmdPdu that guarantees the > > > > > PDU has been initialized. > > > > > > > > I've found a flag IORING_URING_CMD_REISSUE that we could initialize > > > > the pdu. In uring_cmd callback, we can fill zero when it's not reissued. > > > > But I don't know that we could call T::default() in miscdevice. If we > > > > make IoUringCmdPdu, MiscDevice also should be MiscDevice. > > > > > > > > How about assign a byte in pdu for checking initialized? In uring_cmd(), > > > > We could set a byte flag that it's not initialized. And we could return > > > > error that it's not initialized in read_pdu(). > > > > > > Could we do the zero-initialization (or T::default()) in > > > MiscdeviceVTable::uring_cmd() if the IORING_URING_CMD_REISSUE flag > > > isn't set (i.e. on the initial issue)? That way, we avoid any > > > performance penalty for the existing C uring_cmd() implementations. > > > I'm not quite sure what you mean by "assign a byte in pdu for checking > > > initialized". > > > > Sure, we could fill zero when it's the first time uring_cmd called with > > checking the flag. I would remove this commit for next version. I also > > suggests that we would provide the method that read_pdu() and write_pdu(). > > In read_pdu() I want to check write_pdu() is called before. So along the > > 20 bytes for pdu, maybe we could use a bytes for the flag that pdu is > > initialized? > > Not sure what you mean about "20 bytes for pdu". > It seems like it would be preferable to enforce that write_pdu() has > been called before read_pdu() using the Rust type system instead of a > runtime check. I was thinking a signature like fn write_pdu(cmd: > IoUringCmd, value: T) -> IoUringCmdPdu. Do you feel there's a > reason that wouldn't work and a runtime check would be necessary? I didn't think about make write_pdu() to return IoUringCmdPdu before. I think it's good way to pdu is safe without adding a new generic param for MiscDevice. write_pdu() would return IoUringCmdPdu and it could call IoUringCmdPdu::pdu(&mut self) -> &mut T safely maybe. > > > > > But maybe I would introduce a new struct that has Pin<&mut IoUringCmd> and > > issue_flags. How about some additional field for pdu is initialized like below? > > > > struct IoUringCmdArgs { > > ioucmd: Pin<&mut IoUringCmd>, > > issue_flags: u32, > > pdu_initialized: bool, > > } > > One other thing I realized is that issue_flags should come from the > *current* context rather than the context the uring_cmd() callback was > called in. For example, if io_uring_cmd_done() is called from task > work context, issue_flags should match the issue_flags passed to the > io_uring_cmd_tw_t callback, not the issue_flags originally passed to > the uring_cmd() callback. So it probably makes more sense to decouple > issue_flags from the (owned) IoUringCmd. I think you could pass it by > reference (&IssueFlags) or with a phantom reference lifetime > (IssueFlags<'_>) to the Rust uring_cmd() and task work callbacks to > ensure it can't be used after those callbacks have returned. I have had no idea about task work context. I agree with you that it would be better to separate issue_flags from IoUringCmd. So, IoUringCmdArgs would have a only field Pin<&mut IoUringCmd>? > > Best, > Caleb