Re: [PATCH] kbuild: Add environment variables for userprogs flags

[Nick Desaulniers <ndesaulniers@google.com>]

On Wed, Jan 12, 2022 at 2:44 PM Elliot Berman <quic_eberman@quicinc.com> wrote:
>
> Allow additional arguments be passed to userprogs compilation.
> Reproducible clang builds need to provide a sysroot and gcc path to
> ensure same toolchain is used across hosts. KCFLAGS is not currently
> used for any user programs compilation, so add new USERCFLAGS and
> USERLDFLAGS which serves similar purpose as HOSTCFLAGS/HOSTLDFLAGS.
>
> Specifically, I'm trying to force CC_CAN_LINK to consistently fail in
> an environment where a user sysroot is not specifically available.
> Currently, Clang might automatically detect GCC installation on hosts
> which have it installed to a default location in /. With addition of
> these environment variables, our build environment can do like
> "--sysroot=/dev/null" to force sysroot detection to fail.

Hi Elliot,
Thanks for the patch! Sorry for the delay in reviewing; I didn't quite
get around to it then went on vacation for a week.  Things get buried
in my inbox quickly; feel free to ping me if a week goes by with no
response on whichever channel works best for you.

I'm happy with the intent of this patch; GNU binutils has been removed
from Android, so supporting CC_CAN_LINK for Android kernel builds has
been a question I've been thinking about (though, not with higher
priority with some of our other issues), since we'll need to either
incorporate musl or bionic libc into our kernel build.  I was thinking
of adding a SYSROOT command line variable for that, but I see your
approach is more flexible.

One minor nit below, a typo, a few questions, and in the commit
message, but this generally LGTM.

For the commit message, I think it would be good to expand `can do
like "--sysroot=/dev/null"` fully into
```
can specify
$ make USERCFLAGS=--sysroot=/dev/null USERLDFLAGS=-Wl,--sysroot=/dev/null
```

>
> Signed-off-by: Elliot Berman <quic_eberman@quicinc.com>
> ---
>  Documentation/kbuild/kbuild.rst | 8 ++++++++
>  Makefile                        | 9 ++++++---
>  init/Kconfig                    | 8 ++++----
>  usr/include/Makefile            | 3 +++
>  4 files changed, 21 insertions(+), 7 deletions(-)
>
> diff --git a/Documentation/kbuild/kbuild.rst b/Documentation/kbuild/kbuild.rst
> index 2d1fc03d346e..16e90a3ae01b 100644
> --- a/Documentation/kbuild/kbuild.rst
> +++ b/Documentation/kbuild/kbuild.rst
> @@ -77,6 +77,14 @@ HOSTLDLIBS
>  ----------
>  Additional libraries to link against when building host programs.
>
> +USERCFLAGS
> +----------
> +Additional options used for $(CC) when compiling userprogs.
> +
> +USERLDFLAGS
> +----------
> +Additional options used for $(LD) when linking userprogs.

Probably should note the necessity of `-Wl,` prefixes here.

Is `userprogs` cannonical? Yeah, I guess (reading
Documentation/kbuild/makefiles.rst). I wonder if we should mention
these in Documentation/kbuild/makefiles.rst as well? Under `5.3
Controlling compiler options for userspace programs`.

> +
>  KBUILD_KCONFIG
>  --------------
>  Set the top-level Kconfig file to the value of this environment
> diff --git a/Makefile b/Makefile
> index 45278d508d81..4a55537c8ca0 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -431,15 +431,17 @@ HOSTCC    = gcc
>  HOSTCXX        = g++
>  endif
>
> -export KBUILD_USERCFLAGS := -Wall -Wmissing-prototypes -Wstrict-prototypes \
> -                             -O2 -fomit-frame-pointer -std=gnu89
> -export KBUILD_USERLDFLAGS :=
> +KBUILD_USERCFLAGS := -Wall -Wmissing-prototypes -Wstrict-prototypes \
> +                    -O2 -fomit-frame-pointer -std=gnu89
> +KBUILD_USERLDFLAGS := $(USERLDFLAGGS)

^ I think there's an extra G in USERLDFLAGS above.

>
>  KBUILD_HOSTCFLAGS   := $(KBUILD_USERCFLAGS) $(HOST_LFS_CFLAGS) $(HOSTCFLAGS)
>  KBUILD_HOSTCXXFLAGS := -Wall -O2 $(HOST_LFS_CFLAGS) $(HOSTCXXFLAGS)
>  KBUILD_HOSTLDFLAGS  := $(HOST_LFS_LDFLAGS) $(HOSTLDFLAGS)
>  KBUILD_HOSTLDLIBS   := $(HOST_LFS_LIBS) $(HOSTLDLIBS)
>
> +KBUILD_USERCFLAGS   += $(USERCFLAGS)

Since you added USERLDFLAGS to KBUILD_USERLDFLAGS above where it's
defined, why not do so for USERCFLAGS/KBUILD_USERCFLAGS as well?

> +
>  # Make variables (CC, etc...)
>  CPP            = $(CC) -E
>  ifneq ($(LLVM),)
> @@ -530,6 +532,7 @@ export CPP AR NM STRIP OBJCOPY OBJDUMP READELF PAHOLE RESOLVE_BTFIDS LEX YACC AW
>  export PERL PYTHON3 CHECK CHECKFLAGS MAKE UTS_MACHINE HOSTCXX
>  export KGZIP KBZIP2 KLZOP LZMA LZ4 XZ ZSTD
>  export KBUILD_HOSTCXXFLAGS KBUILD_HOSTLDFLAGS KBUILD_HOSTLDLIBS LDFLAGS_MODULE
> +export KBUILD_USERCFLAGS KBUILD_USERLDFLAGS
>
>  export KBUILD_CPPFLAGS NOSTDINC_FLAGS LINUXINCLUDE OBJCOPYFLAGS KBUILD_LDFLAGS
>  export KBUILD_CFLAGS CFLAGS_KERNEL CFLAGS_MODULE
> diff --git a/init/Kconfig b/init/Kconfig
> index f2ae41e6717f..164706c38e8b 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -62,13 +62,13 @@ config LLD_VERSION
>
>  config CC_CAN_LINK
>         bool
> -       default $(success,$(srctree)/scripts/cc-can-link.sh $(CC) $(CLANG_FLAGS) $(m64-flag)) if 64BIT
> -       default $(success,$(srctree)/scripts/cc-can-link.sh $(CC) $(CLANG_FLAGS) $(m32-flag))
> +       default $(success,$(srctree)/scripts/cc-can-link.sh $(CC) $(CLANG_FLAGS) $(USERCFLAGS) $(USERLDFLAGS) $(m64-flag)) if 64BIT
> +       default $(success,$(srctree)/scripts/cc-can-link.sh $(CC) $(CLANG_FLAGS) $(USERCFLAGS) $(USERLDFLAGS) $(m32-flag))
>
>  config CC_CAN_LINK_STATIC
>         bool
> -       default $(success,$(srctree)/scripts/cc-can-link.sh $(CC) $(CLANG_FLAGS) $(m64-flag) -static) if 64BIT
> -       default $(success,$(srctree)/scripts/cc-can-link.sh $(CC) $(CLANG_FLAGS) $(m32-flag) -static)
> +       default $(success,$(srctree)/scripts/cc-can-link.sh $(CC) $(CLANG_FLAGS) $(USERCFLAGS) $(USERLDFLAGS) $(m64-flag) -static) if 64BIT
> +       default $(success,$(srctree)/scripts/cc-can-link.sh $(CC) $(CLANG_FLAGS) $(USERCFLAGS) $(USERLDFLAGS) $(m32-flag) -static)

since USERLDFLAGS get passed to $(CC), they will need `-Wl`, prefixes,
hence the request for expanding the example usage in the commit
message.

>
>  config CC_HAS_ASM_GOTO
>         def_bool $(success,$(srctree)/scripts/gcc-goto.sh $(CC))
> diff --git a/usr/include/Makefile b/usr/include/Makefile
> index 1c2ae1368079..6a8c7dd9ccaf 100644
> --- a/usr/include/Makefile
> +++ b/usr/include/Makefile
> @@ -12,6 +12,9 @@ UAPI_CFLAGS := -std=c90 -Wall -Werror=implicit-function-declaration
>  # It is here just because CONFIG_CC_CAN_LINK is tested with -m32 or -m64.
>  UAPI_CFLAGS += $(filter -m32 -m64, $(KBUILD_CFLAGS))
>
> +# USERCFLAGS might contain sysroot location for CC
> +UAPI_CFLAGS += $(USERCFLAGS)
> +

Do we need to worry about USERLDFLAGS here, too? (or usr/Makefile?)

>  override c_flags = $(UAPI_CFLAGS) -Wp,-MMD,$(depfile) -I$(objtree)/usr/include
>
>  # The following are excluded for now because they fail to build.
> --
> 2.25.1
>


-- 
Thanks,
~Nick Desaulniers