Re: [tboot-devel] tboot fails to build on GCC7+

[tony c. <tc...@re...>]

On 6/18/20 9:33 AM, Lukasz Hawrylko wrote:
> On Wed, 2020-06-17 at 08:54 -0400, Tony Camuso wrote:
>> Sorry for the noise, if this has already been reported or corrected.
>>
>> tboot is built with the -Wextra Cflag, which is an alias for a
>> collection of warning flags. tboot's make interprets warnings as errors.
>>
>>  From GCC 7 forward, the -Wextra Cflag includes -Wimplicit-fallthrough.
>>
>> The GCC 7+ switch statement requires a break statement after each case.
>> Without a break statement after a case, then an "implicit fallthrough"
>> condition exists, where the matched case is executed, and the following
>> case is also executed. If none of the fallthrough cases has a statement,
>> and if the last statement in the fallthrough cascade is a break, all is
>> forgiven, and the GCC 7+ compiler will move on.
>>
>> Otherwise, GCC 7+ with -Wextra will issue the following error when
>> -Werror is set, as it is in the tboot make.
>>
>> error: this statement may fall through [-Werror=implicit-fallthrough=]
>>
>> That means case statements with implicit fallthroughs will be flagged
>> as compile errors and crater the build. There exists a number of
>> compiler specific ways to tell the compiler that the fallthrough is
>> there by design, but the simplest way to avert this problem is to add
>> the -Wno-implicit-fallthrough flag to the CFLAGS_WARN variable in tboot's
>> Config.mk file.
>>
> 
> Hi Tony
> 
> Thank you for your patch. This is already fixed in a6180f9e9e86. In
> general we want to have -Wimplicit-fallthrough enabled, however there
> was a bug where different CFLAGS value was passed to safestringlib
> Makefile depends on building environment. Sometimes safestringlib was
> build with -Werror -Wextra, sometimes not. [1]
> 
> Could you please check if with that commit you can build TBOOT without
> errors?
> 
> [1] https://sourceforge.net/p/tboot/mailman/message/37005286/
> 
> Thanks,
> Lukasz
> 

Thanks, Lukasz! That patch fixes the build problem I was having.