Message-ID: <drh2ikzku37tblmpkjbcdqjllynb3shsfogljfkhq2vcsej2ga@4ex36yehg2e6>
Date: Thu, 23 Jan 2025 13:13:32 +0200
From: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To: Ekansh Gupta <quic_ekangupt@...cinc.com>
Cc: srinivas.kandagatla@...aro.org, linux-arm-msm@...r.kernel.org, 
	gregkh@...uxfoundation.org, quic_bkumar@...cinc.com, linux-kernel@...r.kernel.org, 
	quic_chennak@...cinc.com, dri-devel@...ts.freedesktop.org, arnd@...db.de
Subject: Re: [PATCH v1 1/4] misc: fastrpc: Add CRC support using invokeV2
 request

On Thu, Jan 23, 2025 at 03:19:21PM +0530, Ekansh Gupta wrote:
> 
> 
> 
> On 1/23/2025 1:18 PM, Dmitry Baryshkov wrote:
> > On Thu, Jan 23, 2025 at 11:16:41AM +0530, Ekansh Gupta wrote:
> >>
> >>
> >> On 10/7/2024 7:27 PM, Dmitry Baryshkov wrote:
> >>> On Mon, Oct 07, 2024 at 02:15:15PM GMT, Ekansh Gupta wrote:
> >>>> InvokeV2 request is intended to support multiple enhanced invoke
> >>>> requests like CRC check, performance counter enablement and polling
> >>>> mode for RPC invocations. CRC check is getting enabled as part of
> >>>> this patch. CRC check for input and output argument helps in ensuring
> >>>> data consistency over a remote call. If user intends to enable CRC
> >>>> check, first local user CRC is calculated at user end and a CRC buffer
> >>>> is passed to DSP to capture remote CRC values. DSP is expected to
> >>>> write to the remote CRC buffer which is then compared at user level
> >>>> with the local CRC values.
> >>> This doesn't explain why this is necessary. Why do you need to checksum
> >>> arguments?
> >> This helps if the user suspects any data inconsistencies in the buffers passed to DSP over
> >> remote call. This is not enabled by default and user can enable it as per their reqirement.
> >> I'll add this information.
> > An inconsistency where? Between the kernel and the DSP? Between the user
> > and the DSP? Does it cover buffer contents or just the addresses?
> Inconsistency between user and DSP. crc_user is calculated at user library before
> making ioctl call and it is compared against the crc data which is filled by DSP and
> copied to user.
> This covers inconsistency in buffer contents.

What is the reason for possible inconsistencies? Is it a debugging
feature?

> >
> >>> Also, what if the DSP firmware doesn't support CRC? How should userspace
> >>> know that?
> >> CRC support on DSP is there since long time(>6years).
> > This doesn't give us a lot. Upstream kernel supports fastrpc since
> > MSM8916 and MSM8996. Do those platforms support CRC?
> The metadata buffer as of today also carries space for CRC information:
> https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/tree/drivers/misc/fastrpc.c#n877
> 
> So this is common across all platforms.
> 
> In case CRC is not supported on any older platform, it would result in crc mismatch at user library.
> As of now a warning is getting logged there, I can add the information suggesting the failure might
> also occur if CRC is not supported.

Logs go to /dev/null, they are ignored by users, etc. So either there
should be an actual error being returned by the kernel / library, or it
can be completely ignored and skipped.

So, do MSM8916 / MSM8996 / SDM845 support CRC? If not, that must be
handled somehow.

> > And if they do, why do we need the invoke_v2? Can we modify existing
> > code instead?
> invoke_v2 is needed because there is a need to pass user crc pointer over ioctl call which
> cannot be achieved using existing code. Also there are plans to add more features to this
> invoke_v2 request which will carry some information from user.

Is it really extensible without breaking the ABI?

> >
> >> From user space CRC check failure is
> >> not fatal and is printed as a warning. But if copy of CRC to user fails, it will result in remote
> >> call failure. Should I keep it as fatal considering that ever very old DSP support this or should
> >> I consider the copy failure as non-fatal as userspace is treating this as a warning?
> > warnings can remain unseen for a long time. Consider a GUI app. Nobody
> > is there to view kernel warnings or library output.
> Let me see if this can be done. Are you suggesting that the app will be somewhat tracking
> if there is any crc check mismatch failures?

I suggest returning -EIO to the app.

> >
> >>>> Signed-off-by: Ekansh Gupta <quic_ekangupt@...cinc.com>
> >>>> ---
> >>>>  drivers/misc/fastrpc.c      | 161 ++++++++++++++++++++++++------------
> >>>>  include/uapi/misc/fastrpc.h |   7 ++
> >>>>  2 files changed, 116 insertions(+), 52 deletions(-)
> >>>>
> >>>> diff --git a/drivers/misc/fastrpc.c b/drivers/misc/fastrpc.c
> >>>> index 74181b8c386b..8e817a763d1d 100644
> >>>> --- a/drivers/misc/fastrpc.c
> >>>> +++ b/drivers/misc/fastrpc.c
> >>>> @@ -573,13 +573,15 @@ static void fastrpc_get_buff_overlaps(struct fastrpc_invoke_ctx *ctx)
> >>>>  
> >>>>  static struct fastrpc_invoke_ctx *fastrpc_context_alloc(
> >>>>  			struct fastrpc_user *user, u32 kernel, u32 sc,
> >>>> -			struct fastrpc_invoke_args *args)
> >>>> +			struct fastrpc_invoke_v2 *inv2)
> >>>>  {
> >>>>  	struct fastrpc_channel_ctx *cctx = user->cctx;
> >>>>  	struct fastrpc_invoke_ctx *ctx = NULL;
> >>>> +	struct fastrpc_invoke_args *args = NULL;
> >>> Why do you need to init to NULL if you are going to set it two lines
> >>> below?
> >>>
> >>>>  	unsigned long flags;
> >>>>  	int ret;
> >>>>  
> >>>> +	args = (struct fastrpc_invoke_args *)inv2->inv.args;
> >>> Why does it need a typecast?
> >>>
> >>>>  	ctx = kzalloc(sizeof(*ctx), GFP_KERNEL);
> >>>>  	if (!ctx)
> >>>>  		return ERR_PTR(-ENOMEM);
> >>>> @@ -611,6 +613,7 @@ static struct fastrpc_invoke_ctx *fastrpc_context_alloc(
> >>>>  	/* Released in fastrpc_context_put() */
> >>>>  	fastrpc_channel_ctx_get(cctx);
> >>>>  
> >>>> +	ctx->crc = (u32 *)(uintptr_t)inv2->crc;
> >>> Oh, but why? Also is it a user pointer or in-kernel data? If it's a
> >>> user-based pointer, where is the accessiblity check? Why isn't it
> >>> annotated properly?
> >> This is a user pointer where the crc data is expected to be copied. There is no
> >> other access to this pointer from kernel. I'm planning to change the data type
> >> for crc as (void __user*) inside fastrpc_invoke_ctx structure.
> > Yes, please. Also make sure that sparse doesn't add any warnings
> > regarding pointer conversions.
> Ack.
> >
> >>>>  	ctx->sc = sc;
> >>>>  	ctx->retval = -1;
> >>>>  	ctx->pid = current->pid;
> >>>> @@ -1070,6 +1073,7 @@ static int fastrpc_put_args(struct fastrpc_invoke_ctx *ctx,
> >>>>  	struct fastrpc_invoke_buf *list;
> >>>>  	struct fastrpc_phy_page *pages;
> >>>>  	u64 *fdlist;
> >>>> +	u32 *crclist;
> >>>>  	int i, inbufs, outbufs, handles;
> >>>>  
> >>>>  	inbufs = REMOTE_SCALARS_INBUFS(ctx->sc);
> >>>> @@ -1078,6 +1082,7 @@ static int fastrpc_put_args(struct fastrpc_invoke_ctx *ctx,
> >>>>  	list = fastrpc_invoke_buf_start(rpra, ctx->nscalars);
> >>>>  	pages = fastrpc_phy_page_start(list, ctx->nscalars);
> >>>>  	fdlist = (uint64_t *)(pages + inbufs + outbufs + handles);
> >>>> +	crclist = (u32 *)(fdlist + FASTRPC_MAX_FDLIST);
> >>> I think we should rewrite this parsing somehow. Is the format of data
> >>> documented somewhere?
> >> fdlist, crclist and poll(planned) are the only pointers that is being used. I'm planning
> >> to store these pointers to ctx structure and directly use it wherever needed. This will
> >> clean-up this unnecessary calculations at multiple places.
> >
> > Please do. Nevertheless, the format also must be documented.
> Ack.
> >
> >>>>  
> >>>>  	for (i = inbufs; i < ctx->nbufs; ++i) {
> >>>>  		if (!ctx->maps[i]) {
> >>>> @@ -1102,6 +1107,12 @@ static int fastrpc_put_args(struct fastrpc_invoke_ctx *ctx,
> >>>>  			fastrpc_map_put(mmap);
> >>>>  	}
> >>>>  
> >>>> +	if (ctx->crc && crclist && rpra) {
> >>>> +		if (copy_to_user((void __user *)ctx->crc, crclist,
> >>>> +				FASTRPC_MAX_CRCLIST * sizeof(u32)))
> >>> Oh, so it's a user pointer. Then u32* was completely incorrect.
> >>> Also you are copying FASTRPC_MAX_CRCLIST elements. Are all of them
> >>> filled? Or are we leaking some data to userspace?
> >> Yes, right. Planning clean-up in next patch.
> >>
> >> All of FASTRPC_MAX_CRCLIST is filled with crc data by DSP so copying should be fine.
> > Huh? I definitely want to see documentation for function arguments.
> Sure. I'll also modify the metadata layout doc here to add fdlist, CRC and other planned contents.
> https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/tree/drivers/misc/fastrpc.c#n842

This is not a documentation. E.g. I can not write code using that
description. For example, it mentions neither FDLIST nor CRC.

> >
> >>>> +			return -EFAULT;
> >>>> +	}
> >>>> +
> >>>>  	return 0;
> >>>>  }
> >>>>  

-- 
With best wishes
Dmitry

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