Re: [bpf-next PATCH v2 2/4] kbuild: avoid too many execution of scripts/pahole-flags.sh

[Martin Rodriguez Reboredo]

On 10/18/23 12:19, Masahiro Yamada wrote:
> scripts/pahole-flags.sh is executed so many times.
>
> You can check how many times it is invoked during the build, as follows:
>
>    $ cat <<EOF >> scripts/pahole-flags.sh
>    > echo "scripts/pahole-flags.sh was executed" >&2
>    > EOF
>
>    $ make -s
>    scripts/pahole-flags.sh was executed
>    scripts/pahole-flags.sh was executed
>    scripts/pahole-flags.sh was executed
>    scripts/pahole-flags.sh was executed
>    scripts/pahole-flags.sh was executed
>      [ lots of repeated lines suppressed... ]
>
> This scripts is executed more than 20 times during the kernel build
> because PAHOLE_FLAGS is a recursively expanded variable and exported
> to sub-processes.
>
> With the GNU Make >= 4.4, it is executed more than 60 times because
> exported variables are also passed to other $(shell ) invocations.
> Without careful coding, it is known to cause an exponential fork
> explosion. [1]
>
> The use of $(shell ) in an exported recursive variable is likely wrong
> because $(shell ) is always evaluated due to the 'export' keyword, and
> the evaluation can occur multiple times by the nature of recursive
> variables.
>
> Convert the shell script to a Makefile, which is included only when
> CONFIG_DEBUG_INFO_BTF=y.
>
> [1]: https://savannah.gnu.org/bugs/index.php?64746
>
> Signed-off-by: Masahiro Yamada <masahiroy@xxxxxxxxxx>
> Reviewed-by: Alan Maguire <alan.maguire@xxxxxxxxxx>
> Tested-by: Alan Maguire <alan.maguire@xxxxxxxxxx>
> Reviewed-by: Nicolas Schier <n.schier@xxxxxx>
> Tested-by: Miguel Ojeda <ojeda@xxxxxxxxxx>
> Acked-by: Miguel Ojeda <ojeda@xxxxxxxxxx>
> ---
> [...]
> @@ -1002,6 +999,7 @@ KBUILD_CPPFLAGS += $(call cc-option,-fmacro-prefix-map=$(srctree)/=)
>   # include additional Makefiles when needed
>   include-y			:= scripts/Makefile.extrawarn
>   include-$(CONFIG_DEBUG_INFO)	+= scripts/Makefile.debug
> +include-$(CONFIG_DEBUG_INFO_BTF)+= scripts/Makefile.btf

Would've used a tab, for legibility sake.

>   include-$(CONFIG_KASAN)		+= scripts/Makefile.kasan
>   include-$(CONFIG_KCSAN)		+= scripts/Makefile.kcsan
>   include-$(CONFIG_KMSAN)		+= scripts/Makefile.kmsan
> [...]
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@xxxxxxxxx>