Re: linux-next: Tree for Aug 20 (Wno-alloc-size-larger-than)

Randy Dunlap
2021-08-26T05:10:35Z

On 8/25/21 8:54 PM, Kees Cook wrote:
> On Wed, Aug 25, 2021 at 10:49:19AM -0700, Randy Dunlap wrote:
>> On 8/25/21 10:04 AM, Kees Cook wrote:
>>> On Tue, Aug 24, 2021 at 11:58:59AM +1000, Stephen Rothwell wrote:
>>>> On Mon, 23 Aug 2021 18:24:44 -0700 Randy Dunlap <rdunlap@infradead.org> wrote:
>>>>>
>>>>> This is just weird. What I am seeing is that for every source file
>>>>> where gcc emits a warning: it then follows that up with this
>>>>>>> cc1: warning: unrecognized command line option '-Wno-alloc-size-larger-than'
>>>>
>>>> I see the same, as well as:
>>>>
>>>> <stdin>:1515:2: warning: #warning syscall clone3 not implemented [-Wcpp]
>>>> cc1: warning: unrecognized command line option '-Wno-alloc-size-larger-than'
>>>>
>>>> But only on my gcc 7.3.1 builds (the rest are gcc 10).
>>>>
>>>>> Smells like a gcc bug to me.
>>>>
>>>> Yes
>>>>
>>>> Also noted here: https://github.com/DynamoRIO/drmemory/issues/2099 (second comment)
>>>
>>> Okay, I think this work-around should work. I've been able to reproduce
>>> the weird conditions, and this seems to behave correctly. Andrew, can
>>> you fixup the fixup with this?
>>>
>>>
>>> diff --git a/Makefile b/Makefile
>>> index 26640899e7ca..c1842014a5de 100644
>>> --- a/Makefile
>>> +++ b/Makefile
>>> @@ -1094,8 +1094,13 @@ endif
>>> ifdef CONFIG_CC_IS_GCC
>>> # The allocators already balk at large sizes, so silence the compiler
>>> -# warnings for bounds checks involving those possible values.
>>> -KBUILD_CFLAGS += $(call cc-option, -Wno-alloc-size-larger-than)
>>> +# warnings for bounds checks involving those possible values. While
>>> +# -Wno-alloc-size-larger-than would normally be used here, some versions
>>> +# of gcc (<9.1) weirdly don't handle the option correctly when _other_
>>> +# warnings are produced (?!), so instead use SIZE_MAX to effectively
>>> +# disable it.
>>> +# https://lore.kernel.org/lkml/20210824115859.187f272f@canb.auug.org.au
>>> +KBUILD_CFLAGS += $(call cc-option, -Walloc-size-larger-than=SIZE_MAX)
>>> endif
>>> # disable invalid "can't wrap" optimizations for signed / pointers
>>>
>>
>> Hi Kees,
>>
>> I get a lot of these:
>>
>> ../include/linux/slab.h: In function ‘keyctl_instantiate_key_common’:
>> cc1: warning: invalid argument ‘SIZE_MAX’ to ‘-Walloc-size-larger-than=’
>
> O_o
>
> I love how the documentation on this option is consistently wrong. :)
>
> I haven't been able to exactly reproduce this error on godbolt.org, but
> I got close with trunk GCC:
> gcc: error: argument to '-Walloc-size-larger-than=' should be a non-negative integer optionally followed by a size unit
>
> Even though stdint.h is included. :(
>
> Okay. How about _this_ fix?
>
> diff --git a/Makefile b/Makefile
> index efa9bd36b158..141a851930e6 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -1096,8 +1096,17 @@ endif
>
> ifdef CONFIG_CC_IS_GCC
> # The allocators already balk at large sizes, so silence the compiler
> -# warnings for bounds checks involving those possible values.
> -KBUILD_CFLAGS += $(call cc-option, -Wno-alloc-size-larger-than)
> +# warnings for bounds checks involving those possible values. While
> +# -Wno-alloc-size-larger-than would normally be used here, earlier versions
> +# of gcc (<9.1) weirdly don't handle the option correctly when _other_
> +# warnings are produced (?!). Using -Walloc-size-larger-than=SIZE_MAX
> +# doesn't work (as it is documented to), silently resolving to "0" prior to
> +# version 9.1 (and producing an error more recently). Numeric values larger
> +# than PTRDIFF_MAX also don't work prior to version 9.1, which are silently
> +# ignored, continuing to default to PTRDIFF_MAX. So, left with no other
> +# choice, we must perform a versioned check to disable this warning.
> +# https://lore.kernel.org/lkml/20210824115859.187f272f@canb.auug.org.au
> +KBUILD_CFLAGS += $(call cc-ifversion, -ge, 0901, -Wno-alloc-size-larger-than)
> endif
>
> # disable invalid "can't wrap" optimizations for signed / pointers

Yes, this works for me. Thanks.

Tested-by: Randy Dunlap <rdunlap@infradead.org>

--
~Randy