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AJvYcCVPTslcova2e6G+1+Q1cs75byMzPGYmSLNy+QQ09cVQYBpqshHmcEWNCCAXGdY8sguUVKFK4I02Ng==@vger.kernel.org X-Gm-Message-State: AOJu0Yxlv0IoKDQ5fAjMflwohxa3zxFEwuQLunCBotuZNgLRGXwG/hHC q9vIkisrTZ7996qhE9CpHxLyKSQISSVBkpkVQ+yOV5+SyL3rTnw/MM02rrB5wzvEaqcNaDbJUA4 QtcnZAfrufkwUAiKjArya/NerUleBxPy9pf7LqaDKcQ== X-Gm-Gg: ASbGncscpOB8eQopO+wxef/skK3d38XHb0xoDjEOaUf1mM8gxqI4JnFlXPchC6tlubs ZdxAHaf56AMwebYEIwCi3rPBd20f0GoYcQq3vsdCmc+aINpeWlim4Du4g+H3Ak0Wy7WnuUht5Z4 aat+0LwsHMSAE6kFnMce3oV1BCUHkTltDxUNe4FzHK9Ce+QwZCnNwp+Z2bLpdQLMVbtCyr9EAID 8Eugm5xAPUlxEMxJkY9fE0RyDt3ZTHH48ryDTjR6iIy72eorxE= X-Google-Smtp-Source: AGHT+IHMAwA6cEL7MUPdfl7PV4ovMCbIEezc+HuF9md65upZSvgRXkEpjLiGdatB0iTxFeYFQgmYrxcSkTbFlYOukTU= X-Received: by 2002:a17:902:ec85:b0:267:bd8d:1b6 with SMTP id d9443c01a7336-267bd8d09c0mr3771115ad.6.1757955302499; Mon, 15 Sep 2025 09:55:02 -0700 (PDT) Precedence: bulk X-Mailing-List: io-uring@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 References: <20250822125555.8620-3-sidong.yang@furiosa.ai> In-Reply-To: From: Caleb Sander Mateos Date: Mon, 15 Sep 2025 09:54:50 -0700 X-Gm-Features: Ac12FXwmoh35IByx-sSqLbzTFKdFSm8mcEsw5Ger2WP3BbuSlGRbjwsXJOYIDJU Message-ID: Subject: Re: [RFC PATCH v3 2/5] io_uring/cmd: zero-init pdu in io_uring_cmd_prep() to avoid UB To: Sidong Yang 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 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Sat, Sep 13, 2025 at 5:42=E2=80=AFAM Sidong Yang wrote: > > On Fri, Sep 12, 2025 at 10:56:31AM -0700, Caleb Sander Mateos wrote: > > On Fri, Sep 12, 2025 at 9:42=E2=80=AFAM Sidong Yang wrote: > > > > > > On Tue, Sep 09, 2025 at 09:32:37AM -0700, Caleb Sander Mateos wrote: > > > > On Tue, Sep 9, 2025 at 7:43=E2=80=AFAM Sidong Yang wrote: > > > > > > > > > > On Mon, Sep 08, 2025 at 12:45:58PM -0700, Caleb Sander Mateos wro= te: > > > > > > On Sat, Sep 6, 2025 at 7:28=E2=80=AFAM 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=E2=80=AFAM Sidong Yang wrote: > > > > > > > > > > > > > > > > > > On Mon, Sep 01, 2025 at 05:34:28PM -0700, Caleb Sander Ma= teos wrote: > > > > > > > > > > On Fri, Aug 22, 2025 at 5:56=E2=80=AFAM 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 uni= nitialized or > > > > > > > > > > > garbage memory can lead to undefined behavior in user= s 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 a= voids exposing > > > > > > > > > > > uninitialized memory and prevents potential misinterp= retation of data > > > > > > > > > > > from previous requests. > > > > > > > > > > > > > > > > > > > > > > No functional change is intended other than guarantee= ing 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_cm= d.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_k= iocb *req, const struct io_uring_sqe *sqe) > > > > > > > > > > > if (!ac) > > > > > > > > > > > return -ENOMEM; > > > > > > > > > > > ioucmd->sqe =3D sqe; > > > > > > > > > > > + memset(&ioucmd->pdu, 0, sizeof(ioucmd->pdu)); > > > > > > > > > > > > > > > > > > > > Adding this overhead to every existing uring_cmd() impl= ementation is > > > > > > > > > > unfortunate. Could we instead track the initialized/uni= nitialized > > > > > > > > > > state by using different types on the Rust side? The io= _uring_cmd > > > > > > > > > > could start as an IoUringCmd, where the PDU field is Ma= ybeUninit, > > > > > > > > > > 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 miscd= evice. If we > > > > > > > > > make IoUringCmdPdu, MiscDevice also should be MiscDevi= ce. > > > > > > > > > > > > > > > > > > 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 w= e 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_REISS= UE flag > > > > > > > > isn't set (i.e. on the initial issue)? That way, we avoid a= ny > > > > > > > > performance penalty for the existing C uring_cmd() implemen= tations. > > > > > > > > I'm not quite sure what you mean by "assign a byte in pdu f= or checking > > > > > > > > initialized". > > > > > > > > > > > > > > Sure, we could fill zero when it's the first time uring_cmd c= alled with > > > > > > > checking the flag. I would remove this commit for next versio= n. 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. S= o along the > > > > > > > 20 bytes for pdu, maybe we could use a bytes for the flag tha= t 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 instea= d 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 necessar= y? > > > > > > > > > > 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 coul= d call > > > > > IoUringCmdPdu::pdu(&mut self) -> &mut T safely maybe. > > > > > > > > Yes, that's what I was thinking. > > > > > > Good, I'll change api in this way. Thanks! > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > But maybe I would introduce a new struct that has Pin<&mut Io= UringCmd> and > > > > > > > issue_flags. How about some additional field for pdu is initi= alized 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() callb= ack was > > > > > > called in. For example, if io_uring_cmd_done() is called from t= ask > > > > > > work context, issue_flags should match the issue_flags passed t= o the > > > > > > io_uring_cmd_tw_t callback, not the issue_flags originally pass= ed to > > > > > > the uring_cmd() callback. So it probably makes more sense to de= couple > > > > > > 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 callback= s 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>? > > > > > > > > "Task work" is a mechanism io_uring uses to queue work to run on th= e > > > > thread that submitted an io_uring operation. It's basically a > > > > per-thread atomic queue of callbacks that the thread will process > > > > whenever it returns from the kernel to userspace (after a syscall o= r > > > > an interrupt). This is the context where asynchronous uring_cmd > > > > completions are generally processed (see > > > > io_uring_cmd_complete_in_task() and io_uring_cmd_do_in_task_lazy())= . I > > > > can't speak to the history of why io_uring uses task work, but my > > > > guess would be that it provides a safe context to acquire the > > > > io_ring_ctx uring_lock mutex (e.g. nvme_uring_cmd_end_io() can be > > > > called from an interrupt handler, so it's not allowed to take a > > > > mutex). Processing all the task work at once also provides natural > > > > opportunities for batching. > > > > > > Thanks, I've checked io_uring_cmd_complete_in_task() that it receives > > > callback that has issue_flags different with io_uring_cmd(). I'll try= to add > > > a api that wrapping io_uring_cmd_complete_in_task() for next version. > > > > > > > > > > > Yes, we probably don't need to bundle anything else with the > > > > IoUringCmd after all. As I mentioned earlier, I don't think Pin<&mu= t > > > > IoUringCmd> will work for uring_cmds that complete asynchronously, = as > > > > they will need to outlive the uring_cmd() call. So uring_cmd() need= s > > > > to transfer ownership of the struct io_uring_cmd. > > > > > > I can't think that how to take ownership of struct io_uring_cmd. The > > > struct allocated with io_alloc_req() and should be freed with io_free= _req(). > > > If taking ownership means having pointer of struct io_uring_cmd, I th= ink > > > it's no difference with current version. Also could it be called with > > > mem::forget() if it has ownership? > > > > I don't mean ownership of the io_uring_cmd allocation; that's the > > responsibility of the io_uring layer. But once the io_uring_cmd is > > handed to the uring_cmd() implementation, it belongs to that layer > > until it completes the command back to io_uring. Maybe a better way to > > describe it would be as ownership of the "executing io_uring_cmd". The > > problem with Pin<&mut IoUringCmd> is that it is a borrowed reference > > to the io_uring_cmd, so it can't outlive the uring_cmd() callback. > > Yes, it's possible to leak the io_uring_cmd by never calling > > io_uring_cmd_done() to return it to the io_uring layer. > > Thanks, I understood that IoUringCmd could be outlive uring_cmd callback. > But it's sad that it could be leaked without any unsafe code. Safety in Rust doesn't require destructors to run, which means any resource can be safely leaked (https://faultlore.com/blah/everyone-poops/ has some historical background on why Rust decided leaks had to be considered safe). Leaking an io_uring_cmd is analogous to leaking a Box, both are perfectly possible in safe Rust. > > > > > I would imagine something like this: > > > > #[derive(Clone, Copy)] > > struct IssueFlags<'a>(c_uint, PhantomData<&'a ()>); > > > > // Indicates ownership of the io_uring_cmd between uring_cmd() and > > io_uring_cmd_done() > > struct IoUringCmd(NonNull); > > > > impl IoUringCmd { > > // ... > > > > fn done(self, ret: i32, res2: u64, issue_flags: IssueFlags<'_>)= { > > let cmd =3D self.0.as_ptr(); > > let issue_flags =3D issue_flags.0; > > unsafe { > > bindings::io_uring_cmd_done(cmd, ret, res2, iss= ue_flags) > > } > > } > > } > > > > // Can choose whether to complete the command synchronously or asynchro= nously. > > // If take_async() is called, IoUringCmd::done() needs to be called to > > complete the command. > > // If take_async() isn't called, the command is completed synchronously > > // with the return value from MiscDevice::uring_cmd(). > > struct UringCmdInput<'a>(&mut Option>); > > Thanks for a detailed example! > > But rather than this, We could introduce new return type that has a callb= ack that > user could take IoUringCmd instead of returning -EIOCBQUEUED. I'm not following what you're suggesting, maybe a code sample would help? > > But I prefer that we provide just one type IoUringCmd without UringCmdInp= ut. > Although UringCmdInput, user could call done() in uring_cmd callback and > it makes confusion that whether task_async() called and returning -EIOCBQ= UEUED > is mismatched that returns -EINVAL. We don't need to make it complex. Sure, if you only want to support asynchronous io_uring_cmd completions, than you can just pass IoUringCmd to MiscDevice::uring_cmd() and require it to call IoUringCmd::done() to complete the command. There's a small performance overhead to that over just returning the result from the uring_cmd() callback for synchronous completions (and it's more verbose), but I think that would be fine for an initial implementation. Best, Caleb