GCC feature req: warn when bitops exceed type size (was: conntrack untracked match is broken)

GCC feature req: warn when bitops exceed type size (was: conntrack untracked match is broken)

[Jan Engelhardt]

Hi gcc list,


I am forwarding below's bugreport here(*), to implicitly make aware
of a feature that I deem important to have in a future gcc.



(*) should have posted to bugzilla instead? Don't feel like setting
up a bugmenot tho..

---------- Forwarded message ----------
Date: Mon, 29 Jun 2009 14:34:10
From: Patrick McHardy
To: Jan Engelhardt
Cc: Netfilter Developer Mailing List <netfilter-devel@vger.kernel.org>,
    Philip Craig
Subject: Re: conntrack untracked match is broken (kernel patch)

>On Monday 2009-06-22 08:31, Philip Craig wrote:
>>The problem is that state_mask in 'struct xt_conntrack_mtinfo1' is
>>only 8 bit, but XT_CONNTRACK_STATE_UNTRACKED == 256.
>>Unfortunately, gcc doesn't warn about this for '|=', only for '='.

[i.e. uint8_t x = 0; x |= 256; ]

>
>I smell a gcc-missing-feature there.

Re: GCC feature req: warn when bitops exceed type size (was: conntrack untracked match is broken)

[Richard Guenther]

On Mon, Jun 29, 2009 at 3:10 PM, Jan Engelhardt<jengelh@medozas.de> wrote:
> Hi gcc list,
>
>
> I am forwarding below's bugreport here(*), to implicitly make aware
> of a feature that I deem important to have in a future gcc.
>

-Wconversion should say

t.c:4: warning: conversion to ‘unsigned char’ from ‘int’ may alter its value

>
> (*) should have posted to bugzilla instead? Don't feel like setting
> up a bugmenot tho..
>
> ---------- Forwarded message ----------
> Date: Mon, 29 Jun 2009 14:34:10
> From: Patrick McHardy
> To: Jan Engelhardt
> Cc: Netfilter Developer Mailing List <netfilter-devel@vger.kernel.org>,
>    Philip Craig
> Subject: Re: conntrack untracked match is broken (kernel patch)
>
>>On Monday 2009-06-22 08:31, Philip Craig wrote:
>>>The problem is that state_mask in 'struct xt_conntrack_mtinfo1' is
>>>only 8 bit, but XT_CONNTRACK_STATE_UNTRACKED == 256.
>>>Unfortunately, gcc doesn't warn about this for '|=', only for '='.
>
> [i.e. uint8_t x = 0; x |= 256; ]
>
>>
>>I smell a gcc-missing-feature there.
>

Re: GCC feature req: warn when bitops exceed type size (was: conntrack untracked match is broken)

[Manuel López-Ibáñez]

2009/6/29 Richard Guenther <richard.guenther@gmail.com>:
> On Mon, Jun 29, 2009 at 3:10 PM, Jan Engelhardt<jengelh@medozas.de> wrote:
>> Hi gcc list,
>>
>>
>> I am forwarding below's bugreport here(*), to implicitly make aware
>> of a feature that I deem important to have in a future gcc.
>>
>
> -Wconversion should say
>
> t.c:4: warning: conversion to ‘unsigned char’ from ‘int’ may alter its value

From GCC 4.4.0 it says:

bug.c:6: warning: conversion to ‘uint8_t’ from ‘int’ may alter its value

More info abut new Wconversion: http://gcc.gnu.org/wiki/NewWconversion

Cheers,

Manuel.

Re: GCC feature req: warn when bitops exceed type size (was: conntrack untracked match is broken)

[Jan Engelhardt]

On Monday 2009-06-29 16:09, Manuel López-Ibáñez wrote:
>2009/6/29 Richard Guenther wrote:
>> On Mon, Jun 29, 2009 at 3:10 PM, Jan Engelhardt wrote:
>>> Hi gcc list,
>>>
>>>
>>> I am forwarding below's bugreport here(*), to implicitly make aware
>>> of a feature that I deem important to have in a future gcc.
>>>
>>
>> -Wconversion should say
>>
>> t.c:4: warning: conversion to ‘unsigned char’ from ‘int’ may alter its value

I added -Wconversion to the Linux kernel's global cflags (KBUILD_CFLAGS) 
just to see what would happen. As I expected, I get swamped with 
warnings, like "conversion to int from unsigned int". All legitimate in 
themselves, I would have preferred an option (or even no option at all, 
given that the "large integer implicitly truncated to unsigned type" 
warning is shown without any -W flags) that only flags up truncation 
problems with literals.

Re: GCC feature req: warn when bitops exceed type size (was: conntrack untracked match is broken)

[Manuel López-Ibáñez]

2009/6/29 Jan Engelhardt <jengelh@medozas.de>:
>
> On Monday 2009-06-29 16:09, Manuel López-Ibáñez wrote:
>>2009/6/29 Richard Guenther wrote:
>>> On Mon, Jun 29, 2009 at 3:10 PM, Jan Engelhardt wrote:
>>>> Hi gcc list,
>>>>
>>>>
>>>> I am forwarding below's bugreport here(*), to implicitly make aware
>>>> of a feature that I deem important to have in a future gcc.
>>>>
>>>
>>> -Wconversion should say
>>>
>>> t.c:4: warning: conversion to ‘unsigned char’ from ‘int’ may alter its value
>
> I added -Wconversion to the Linux kernel's global cflags (KBUILD_CFLAGS)
> just to see what would happen. As I expected, I get swamped with
> warnings, like "conversion to int from unsigned int". All legitimate in
> themselves, I would have preferred an option (or even no option at all,
> given that the "large integer implicitly truncated to unsigned type"
> warning is shown without any -W flags) that only flags up truncation
> problems with literals.

A lot of false warnings were fixed in GCC 4.4 and 4.5. And if only
truncation problems with literals were warned then your original
example wouldn't had been.

Cheers,

Manuel.