linux-kernel - Re: [PATCH] IB/rdmavt: Remove err declaration in if statement in rvt_create

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Date:   Tue, 9 Jul 2019 15:38:57 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Nathan Chancellor <natechancellor@...il.com>
Cc:     Dennis Dalessandro <dennis.dalessandro@...el.com>,
        Mike Marciniszyn <mike.marciniszyn@...el.com>,
        Doug Ledford <dledford@...hat.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Kamenee Arumugam <kamenee.arumugam@...el.com>,
        linux-rdma@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH] IB/rdmavt: Remove err declaration in if statement in rvt_create_cq

On Tue, Jul 9, 2019 at 3:13 PM Nathan Chancellor
<natechancellor@...il.com> wrote:
>
> clang warns:
>
> drivers/infiniband/sw/rdmavt/cq.c:260:7: warning: variable 'err' is used

Oh, !$*@, this is a tricky one.  While the if scoped `err` declared on
L250 is initialized when used at L260, the function scoped `err`
declared on L211 is not initialized when it is used on L310 when
control flow enters the if on L249 then the goto on L255 or L261.  So
this is a bug due to the if scoped `err` "shadowing" the function
scoped `err`.

Maybe not important enough to send a v2, but I feel like the commit
message should say something along the lines of `fix a potential use
of uninitialized memory due to shadowing`.  Either way, this fixes a
real bug, so thanks for the patch.
Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>

> uninitialized whenever 'if' condition is true
> [-Wsometimes-uninitialized]
>                 if (err)
>                     ^~~
> drivers/infiniband/sw/rdmavt/cq.c:310:9: note: uninitialized use occurs
> here
>         return err;
>                ^~~
> drivers/infiniband/sw/rdmavt/cq.c:260:3: note: remove the 'if' if its
> condition is always false
>                 if (err)
>                 ^~~~~~~~
> drivers/infiniband/sw/rdmavt/cq.c:253:7: warning: variable 'err' is used
> uninitialized whenever 'if' condition is true
> [-Wsometimes-uninitialized]
>                 if (!cq->ip) {
>                     ^~~~~~~
> drivers/infiniband/sw/rdmavt/cq.c:310:9: note: uninitialized use occurs
> here
>         return err;
>                ^~~
> drivers/infiniband/sw/rdmavt/cq.c:253:3: note: remove the 'if' if its
> condition is always false
>                 if (!cq->ip) {
>                 ^~~~~~~~~~~~~~
> drivers/infiniband/sw/rdmavt/cq.c:211:9: note: initialize the variable
> 'err' to silence this warning
>         int err;
>                ^
>                 = 0
> 2 warnings generated.
>
> There are two err declarations in this function: at the top and within
> an if statement; clang warns because the assignments to err in the if
> statement are local to the if statement so err will be used
> uninitialized if this error handling is used. Remove the if statement's
> err declaration so that everything works properly.
>
> Fixes: 239b0e52d8aa ("IB/hfi1: Move rvt_cq_wc struct into uapi directory")
> Link: https://github.com/ClangBuiltLinux/linux/issues/594
> Signed-off-by: Nathan Chancellor <natechancellor@...il.com>
> ---
>  drivers/infiniband/sw/rdmavt/cq.c | 2 --
>  1 file changed, 2 deletions(-)
>
> diff --git a/drivers/infiniband/sw/rdmavt/cq.c b/drivers/infiniband/sw/rdmavt/cq.c
> index fac87b13329d..a85571a4cf57 100644
> --- a/drivers/infiniband/sw/rdmavt/cq.c
> +++ b/drivers/infiniband/sw/rdmavt/cq.c
> @@ -247,8 +247,6 @@ int rvt_create_cq(struct ib_cq *ibcq, const struct ib_cq_init_attr *attr,
>          * See rvt_mmap() for details.
>          */
>         if (udata && udata->outlen >= sizeof(__u64)) {
> -               int err;
> -

-- 
Thanks,
~Nick Desaulniers