Re: [PATCH][next] rtc: msc313: Fix unintentional sign extension issue on left shift of a u16

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



Hi Colin,

On Tue, 28 Sept 2021 at 21:39, Colin King <colin.king@xxxxxxxxxxxxx> wrote:
>Shifting the u16 value returned by readw by 16 bits to the left
>will be promoted to a 32 bit signed int and then sign-extended
>to an unsigned long. If the top bit of the readw is set then
>the shifted value will be sign extended and the top 32 bits of
>the result will be set.

Ah,.. C is fun in all the wrong places. :)
These chips are full of 32bit registers that are split into two 16
registers 4 bytes apart when seen from the ARM CPU so we probably have
this same mistake in a few other places.

A similar pattern is used a bit later on in the same file to read the counter:

seconds = readw(priv->rtc_base + REG_RTC_CNT_VAL_L)
| (readw(priv->rtc_base + REG_RTC_CNT_VAL_H) << 16);

I guess it works at the moment because the top bit won't be set until 2038.

Thanks,

Daniel



[Index of Archives]     [Kernel Development]     [Kernel Announce]     [Kernel Newbies]     [Linux Networking Development]     [Share Photos]     [IDE]     [Security]     [Git]     [Netfilter]     [Yosemite News]     [MIPS Linux]     [ARM Linux]     [Device Mapper]

  Powered by Linux