linux-kernel - Re: -Wsometimes-uninitialized Clang warning in drivers/net/ethernet/broadcom/bnxt/bnxt.c

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Message-ID: <CACKFLikGY0=_FBosBdp2nq8F+Upmpo-Oi8kFeMH8diBEZP_0AQ@mail.gmail.com>
Date:   Thu, 25 Apr 2019 11:33:04 -0700
From:   Michael Chan <michael.chan@...adcom.com>
To:     Nathan Chancellor <natechancellor@...il.com>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        "David S. Miller" <davem@...emloft.net>,
        Networking <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux@...glegroups.com,
        Eddie Wai <eddie.wai@...adcom.com>,
        "jeffrey.huang@...adcom.com" <huangjw@...adcom.com>,
        "prashant.sreedharan@...adcom.com" <prashant@...adcom.com>,
        "michael.chan@...adcom.com" <mchan@...adcom.com>,
        Vasundhara Volam <vasundhara-v.volam@...adcom.com>
Subject: Re: -Wsometimes-uninitialized Clang warning in drivers/net/ethernet/broadcom/bnxt/bnxt.c

On Thu, Apr 25, 2019 at 11:14 AM Nathan Chancellor
<natechancellor@...il.com> wrote:
>
> Hi Arnd,
>
> On Fri, Mar 22, 2019 at 03:32:50PM +0100, Arnd Bergmann wrote:
> > On Wed, Mar 20, 2019 at 9:41 PM 'Nick Desaulniers' via Clang Built
> > Linux <clang-built-linux@...glegroups.com> wrote:
> > >
> > > + Broadcom folks from commit c0c050c58d84 ("bnxt_en: New Broadcom
> > > ethernet driver.").  Looks like Michael wrote and is still maintaining
> > > the driver.
> > >
> > > On Wed, Mar 20, 2019 at 12:08 PM Nathan Chancellor
> > > <natechancellor@...il.com> wrote:
> > > >
> > > > On Thu, Mar 07, 2019 at 05:57:35PM -0700, Nathan Chancellor wrote:
> > > > > Hi all,
> > > > >
> > > > > We are trying to get Clang's -Wsometimes-uninitialized turned on for the
> > > > > kernel as it can catch some bugs that GCC can't. This warning came up:
> > > > >
> > > > > drivers/net/ethernet/broadcom/bnxt/bnxt.c:1612:6: warning: variable 'len' is used uninitialized whenever 'if' condition is true [-Wsometimes-uninitialized]
> > > > >         if (rxcmp1->rx_cmp_cfa_code_errors_v2 & RX_CMP_L2_ERRORS) {
> > > > >             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > > > > drivers/net/ethernet/broadcom/bnxt/bnxt.c:1703:19: note: uninitialized use occurs here
> > > > >         cpr->rx_bytes += len;
> > > > >                          ^~~
> > > > > drivers/net/ethernet/broadcom/bnxt/bnxt.c:1612:2: note: remove the 'if' if its condition is always false
> > > > >         if (rxcmp1->rx_cmp_cfa_code_errors_v2 & RX_CMP_L2_ERRORS) {
> > > > >         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > > > > drivers/net/ethernet/broadcom/bnxt/bnxt.c:1540:18: note: initialize the variable 'len' to silence this warning
> > > > >         unsigned int len;
> > > > >                         ^
> > > > >                          = 0
> > > > > 1 warning generated.
> > > > >
> > > > > It seems like the logical change to make is this; however, I am not sure
> > > > > if this has any other unintended consequences since this is a rather
> > > > > dense function. I would much appreciate your input, especially if there
> > > > > is a better way to fix it.
> > >
> > > I agree that `goto next_rx_no_prod_no_len` appears to be most correct;
> > > though I don't understand why this function is a mix of early return
> > > codes, vs setting rc then updating *raw_cons.  The alternative is
> > > probably zero initializing len, but I'm not sure whether *raw_cons
> > > should be updated in that case or not.  Thanks for bringing this up
> > > and the patch.  Sorry for the delay in review.  Can folks at Broadcom
> > > please clarify?

Sorry, I somehow missed this email earlier.  "goto
next_rx_no_prod_no_len" is the most correct fix.  The "cpr->rx_bytes
+= len" logic was added about a year ago when we added the DIM
(Dynamic Interrupt Moderation) support and we missed fixing this goto.

I can prepare and send the patch later today.  Thanks.