Re: [PATCH RFC v2 5/6] staging: r8188eu: add error handling of rtw_read32

Pavel Skripkin
2021-08-24T08:47:15Z

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.

With regards,
Pavel Skripkin