On 12/5/21 4:50 PM, Giovanni Mascellani wrote:
> reassign 1000974 xfslibs-dev
> severity 1000974 important
> retitle 1000974 xfs/linux.h defines common word "fallthrough" breaking
> unrelated headers
> thanks
>
> Hi,
>
> On 04/12/21 23:36, Thomas Goirand wrote:
>> On 12/4/21 5:11 PM, Giovanni Mascellani wrote:
>>> Could you try running that compilation command with g++ -E, so you can
>>> see what BOOST_FALLTHROUGH is actually begin replaced with?
>>
>> Oh, sorry, now I get what you meant (did a c++ --help to understand what
>> -E was doing). Here's the result in
>> CMakeFiles/seastar.dir/src/core/file.cc.o:
>>
>> __attribute__((__attribute__((__fallthrough__))));
>>
>> Probably, there's a mistake, and it should really be:
>>
>> __attribute__((__fallthrough__));
>>
>> instead, which is the source of the trouble?
>
> It seems that the problem goes this way:
>
> * Boost defines in /usr/include/boost/config/compiler/gcc.hpp:
>
> #define BOOST_FALLTHROUGH __attribute__((fallthrough))
>
> * But /usr/include/xfs/linux.h defines:
>
> #define fallthrough __attribute__((__fallthrough__))
>
> So the "fallthrough" in Boost's definition is expanded again and causes
> the problem.
>
> I think xfs/linux.h is at fault here, because it aggressively defines a
> common word (while Boost namespaces all its definitions with a BOOST_
> prefix). Unfortunately the C/C++ preprocessor makes it easy to break
> other headers if you define stuff too liberally. This wold also, for
> example, break any program that use the name "fallthrough" for a
> variable, which is a completely reasonable name to use. A simple example
> could be:
> ---
> #include <xfs/linux.h>
>
> int main() {
> int fallthrough = 0;
> return fallthrough;
> }
> ---
> which fails compilation with:
> ---
> $ LANG=C gcc test.c
> test.c: In function 'main':
> test.c:4:5: warning: 'fallthrough' attribute not followed by ';'
> [-Wattributes]
> 4 | int fallthrough = 0;
> | ^~~
> test.c:4:21: error: expected identifier or '(' before '=' token
> 4 | int fallthrough = 0;
> | ^
> In file included from test.c:1:
> test.c:5:12: error: expected expression before '__attribute__'
> 5 | return fallthrough;
> | ^~~~~~~~~~~
> ---
>
> You can probably work around this problem by undefining "fallthrough"
> just after the xfs/linux.h header. In the meantime I am reassigning this
> bug to xfslibs-dev.
>
> Giovanni.
Hi,
I can confirm that commenting away line 373 to 381 of xfs/linux.h solve
the troubles when building Ceph. Downgrading to 5.13.0-1 (using
snapshot.d.o) also solved the trouble, showing that 5.14.0 really is the
trouble here.
Thanks Giovanni for finding this out.
Cheers,
Thomas Goirand (zigo)
