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Subject: Re: [PATCH RFC v2 5/6] staging: r8188eu: add error handling of
 rtw_read32
From:   Pavel Skripkin <paskripkin@gmail.com>
To:     "Fabio M. De Francesco" <fmdefrancesco@gmail.com>,
        Phillip Potter <phil@philpotter.co.uk>
Cc:     Larry Finger <Larry.Finger@lwfinger.net>,
        Greg KH <gregkh@linuxfoundation.org>,
        Michael Straube <straube.linux@gmail.com>,
        "open list:STAGING SUBSYSTEM" <linux-staging@lists.linux.dev>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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Message-ID: <96e3703e-a5e2-3c6d-ea3c-b5d3892849b2@gmail.com>
Date:   Tue, 24 Aug 2021 11:53:35 +0300
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On 8/24/21 11:47 AM, Pavel Skripkin wrote:
> On 8/24/21 11:38 AM, Fabio M. De Francesco wrote:
>> On Tuesday, August 24, 2021 8:40:18 AM CEST Pavel Skripkin wrote:
>>> On 8/24/21 3:10 AM, Fabio M. De Francesco wrote:
>>> > On Tuesday, August 24, 2021 1:33:46 AM CEST Phillip Potter wrote:
>>> >> On Sun, 22 Aug 2021 at 15:36, Pavel Skripkin <paskripkin@gmail.com> wrote:
>>> >> > -static u32 usb_read32(struct intf_hdl *pintfhdl, u32 addr)
>>> >> > +static int usb_read32(struct intf_hdl *pintfhdl, u32 addr, u32 *data)
>>> >> >  {
>>> >> >         u8 requesttype;
>>> >> >         u16 wvalue;
>>> >> >         u16 len;
>>> >> > -       __le32 data;
>>> >> > +       int res;
>>> >> > +       __le32 tmp;
>>> >> > +
>>> >> > +       if (WARN_ON(unlikely(!data)))
>>> >> > +               return -EINVAL;
>>> >> >
>>> >> >         requesttype = 0x01;/* read_in */
>>> >> >
>>> >> >         wvalue = (u16)(addr & 0x0000ffff);
>>> >> >         len = 4;
>>> >> >
>>> >> > -       usbctrl_vendorreq(pintfhdl, wvalue, &data, len, requesttype);
>>> >> > +       res = usbctrl_vendorreq(pintfhdl, wvalue, &data, len, requesttype);
>>> >> > +       if (res < 0) {
>>> >> > +               dev_err(dvobj_to_dev(pintfhdl->pintf_dev), "Failed to read 32 bytes: %d\n", res);
>>> >> > +       } else {
>>> >> > +               /* Noone cares about positive return value */
>>> >> > +               *data = le32_to_cpu(tmp);
>>> >> > +               res = 0;
>>> >> > +       }
>>> >> >
>>> >> > -       return le32_to_cpu(data);
>>> >> > +       return res;
>>> >> >  }
>>> >> 
>>> >> Dear Pavel,
>>> >> 
>>> >> OK, found the issue with decoded stack trace after reviewing this
>>> >> usb_read32 function. Your line:
>>> >> res = usbctrl_vendorreq(pintfhdl, wvalue, &data, len, requesttype);
>>> >> 
>>> >> should read:
>>> >> res = usbctrl_vendorreq(pintfhdl, wvalue, &tmp, len, requesttype);
>>> > 
>>> > Dear Philip,
>>> > 
>>> > No, it should read:
>>> > 
>>> > res = usbctrl_vendorreq(pintfhdl, wvalue, data, len, requesttype);
>>> > 
>>> > I suspect that Pavel didn't notice he was reusing a line of the old code
>>> > wth no due changes.
>>> > 
>>> >> With this change, the driver runs fine with no crashes/oopses. I will
>>> >> explain the issue but you can probably see already, so I hope I'm not
>>> >> coming across as patronising, just trying to be helpful :-)
>>> >> 
>>> >> Essentially, you are taking the address of the data function parameter
>>> >> on this line with &data, a pointer to u32, which is giving you a
>>> >> pointer to a pointer to u32 (u32 **) for this function parameter
>>> >> variable. When passed to usbctrl_vendorreq, it is being passed to
>>> >> memcpy inside this function as a void *, meaning that memcpy
>>> >> subsequently overwrites the value of the memory address inside data to
>>> >> point to a different location, which is problem when it is later
>>> >> deferenced at:
>>> >> *data = le32_to_cpu(tmp);
>>> >> causing the OOPS
>>> >> 
>>> >> Also, as written, you can probably see that tmp is uninitialised. This
>>> >> looks like a typo, so guessing this wasn't your intention. Anyhow,
>>> >> with that small change, usbctrl_vendorreq reads into tmp, which is
>>> >> then passed to le32_to_cpu whose return value is stored via the
>>> >> deferenced data ptr (which now has its original address within and not
>>> >> inadvertently modified). Hope this helps, and I'd be happy to Ack the
>>> >> series if you want to resend this patch. Many thanks.
>>> > 
>>> > I think that another typo is having 'tmp', because that variable is unnecessary
>>> > and "*data = le32_to_cpu(tmp);" is wrong too.
>>> > 
>>> > Now I also see that also usb_read16() is wrong, while usb_read8() (the one that
>>> > I had read yesterday) is the only correct function of the three usb_read*().
>>> > 
>>> 
>>> Hi, guys!
>>> 
>>> 
>>> Sorry for breaking your system, Phillip. This code was part of "last 
>>> minute" changes and yes, it's broken :)
>>> 
>>> I get what Phillip said, because I _should_ read into tmp variable 
>>> instead of directly to data, but I don't get Fabio's idea, sorry.
>> 
>> Hi Pavel,
>> 
>> I (wrongly?) assumed from the prototype of usb_read32() that u32 *data is in native
>> endianness. So, I didn't see the necessity of using _le32 tmp and then convert that tmp
>> with le32_to_cpu().
>> 
>> I simply thought that data could be passed to usbctrl_vendorreq as it-is.
>> 
>>> Data from chip comes in little-endian, so we _should_ convert it to 
>>> cpu's endian. Temp variable is needed to make smatch and all other 
>>> static anylis tools happy about this code.
>> 
>> Now that you explained that "Data from chip comes in little-endian", obviously
>> I must agree with you that the code needs tmp and that tmp must be
>> swapped by le32_to_cpu(), ahead of assigning it to *data.
>> 
>> Just a curiosity... Since I was not able to see that *data is returned in little endian,
>> can you please point me where in the code you found out that it is? There must
>> be some place in the code that I'm unable to find and see that *data is LE.
>> 
>> Thanks in advance,
>> 
>> Fabio
> 
> Hi, Fabio!
> 
> previous usb_read16() realization, which is 100% right:
> 
> 
> static u16 usb_read16(struct intf_hdl *pintfhdl, u32 addr)
> {
> 	u8 requesttype;
> 	u16 wvalue;
> 	u16 len;
> 	__le32 data;
> 
> 	requesttype = 0x01;/* read_in */
> 	wvalue = (u16)(addr & 0x0000ffff);
> 	len = 2;
> 	usbctrl_vendorreq(pintfhdl, wvalue, &data, len, requesttype);
> 
> 	return (u16)(le32_to_cpu(data) & 0xffff);
> }
> 
> 
> Bases on this code, I think, it's oblivious, that data comes in
> little-endian. That's why I leaved temp variable for casting le32 to
> cpu's endianess.
> 
> I could just read into u{16,32} * and then make smth like
> 
> *data = le32_to_cpu(*data)
> 
> but static analysis tools will complain about wrong data type passed to
>    le32_to_cpu()
> 
> + Phillip tested fixed v2 version and it worked well for him. I guess,
> Phillip was able to spot weird driver behavior, if this cast is wrong.
> 
		^^^^^&

I am wrong with this statement, I guess. Most likely, Phillip is testing 
on smth like x64 and this arch is le, so...


With regards,
Pavel Skripkin