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

Randy Dunlap
2021-08-25T17:49:24Z

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=’

--
~Randy