Re: [Minios-devel] [UNIKRAFT PATCH 2/2] lib/ukboot: move weak main in separate sub-lib

[Sharan Santhanam <sharan.santhanam@xxxxxxxxx>]

Hello Yuri,

I agree with the solution.

Maybe it is wise to add an "EXPERIMENTAL" tag around the LTO Config.uk 
option.
Thanks & Regards
Sharan

On 1/4/19 5:18 PM, Yuri Volchkov wrote:
> Link Time Optimization does not work well with incremental linking (ld
> -r), which is heavily used by Unikraft.
>
> The ukboot has a weak main() function, which is supposed to be called
> if no other library/application provided a proper main. However gcc
> resolves main() to whatever is available while linking the
> libukboot.ld.o. Which is the weak one.
>
> In the upcoming gcc v9 release a better support of incremental link
> optimization is announced. Which hopefully will fix this issue. But
> for now, let's fix it with a workaround.
>
> Let's create a sublib under the ukboot, containing only the weak
> main. This way we will leave no chance for the first "ld -r" to link
> with the wrong main. The next "ld -r" will have all the objects, so it
> will be able to make correct decision.
>
> Signed-off-by: Yuri Volchkov <yuri.volchkov@xxxxxxxxx>
> ---
>   lib/ukboot/Makefile.uk | 12 ++++++++++++
>   lib/ukboot/boot.c      |  7 -------
>   lib/ukboot/weak_main.c |  9 +++++++++
>   3 files changed, 21 insertions(+), 7 deletions(-)
>   create mode 100644 lib/ukboot/weak_main.c
>
> diff --git a/lib/ukboot/Makefile.uk b/lib/ukboot/Makefile.uk
> index 2b7f69f..55f205d 100644
> --- a/lib/ukboot/Makefile.uk
> +++ b/lib/ukboot/Makefile.uk
> @@ -4,3 +4,15 @@ CINCLUDES-$(CONFIG_LIBUKBOOT)          += 
> -I$(LIBUKBOOT_BASE)/include
>   CXXINCLUDES-$(CONFIG_LIBUKBOOT)       += -I$(LIBUKBOOT_BASE)/include
>   
>   LIBUKBOOT_SRCS-y += $(LIBUKBOOT_BASE)/boot.c
> +
> +# The main() is in the separate library to fool the LTO. Which is
> +# trying to resolve the main() function call to whatever is available
> +# during liking the library object file. And this will be the weak
> +# main. So even if the proper main() is provided later, the weak one
> +# will be called, because the decision is already made.
> +#
> +# This does not sound right, but LTO does not work well with
> +# incremental linking (ld -r). This possibly will be fixed in gcc
> +# v9. But we have to deal with it now.
> +$(eval $(call addlib_s,libukboot_main,$(CONFIG_LIBUKBOOT)))
> +LIBUKBOOT_MAIN_SRCS-y += $(LIBUKBOOT_BASE)/weak_main.c
> diff --git a/lib/ukboot/boot.c b/lib/ukboot/boot.c
> index 174dd5b..4846782 100644
> --- a/lib/ukboot/boot.c
> +++ b/lib/ukboot/boot.c
> @@ -266,10 +266,3 @@ void ukplat_entry(int argc, char *argv[])
>         main_thread_func(&tma);
>   #endif
>   }
> -
> -/* Internal main */
> -int main(int argc __unused, char *argv[] __unused)
> -{
> -       printf("weak main() called. Symbol was not replaced!\n");
> -       return -EINVAL;
> -}
> diff --git a/lib/ukboot/weak_main.c b/lib/ukboot/weak_main.c
> new file mode 100644
> index 0000000..697720c
> --- /dev/null
> +++ b/lib/ukboot/weak_main.c
> @@ -0,0 +1,9 @@
> +#include <stdio.h>
> +#include <errno.h>
> +
> +/* Internal main */
> +int __weak main(int argc __unused, char *argv[] __unused)
> +{
> +       printf("weak main() called. Symbol was not replaced!\n");
> +       return -EINVAL;
> +}
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