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

From: Elliot Berman
Date: Fri Jan 28 2022 - 14:40:19 EST

Hi Fangrui,

On 1/27/2022 7:15 PM, Fangrui Song wrote:
On 2022-01-26, Nick Desaulniers wrote:
On Wed, Jan 12, 2022 at 2:44 PM Elliot Berman <quic_eberman@xxxxxxxxxxx> 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
```

Is -Wl,--sysroot=/dev/null to override a -Wl,--sysroot specified
previously on the command line?


--sysroot isn't specified previously on the commandline for Android kernel builds. Clang is falling back to sysroot detection based on common installation paths, and we want to effectively disable that for reproducible builds.

I believe this was introduced in Clang 13 [1], but it might also be a change in AOSP Clang's prebuilt. I wasn't able to get clear picture of what changed in Clang. Android12-5.10 builds which use Clang 12 are unaffected.

[1]: https://releases.llvm.org/13.0.0/tools/clang/docs/ReleaseNotes.html#modified-compiler-flags

The driver option --sysroot does two things:

* Decide include/library search paths (e.g. $sysroot/usr/include, $sysroot/lib64).
* Pass --sysroot to ld.

In ld, it means: if a linker script is in the sysroot directory, when ld opens an
absolute path file (via INPUT or GROUP), add sysroot before the absolute path.
In ld, --sysroot=/dev/null is not different --sysroot= (empty value).
>>>
Signed-off-by: Elliot Berman <quic_eberman@xxxxxxxxxxx>
---
 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