On Wed, Jan 16, 2019 at 10:34 PM Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx> wrote: > > I want to avoid applying this patch because this patch is ugly, and > people are trying to fix llvm-ar. I tried the latest llvm-ar, and I > saw some improvement, but still cannot build the kernel with it. > > The main reason of this post is for the record, and suggest how llvm-ar > should work. > > As you may have noticed, there are two issues: > > [1] llvm-ar does not recognize the 'P' option > [2] llvm-ar does not flatten thin archives (not fixed yet) > > Let me explain each of them. > > [1] Why is 'P' option needed for GNU ar? > > It may not be so clear the reason of commit 9a6cfca4f413 ("kbuild: > thin archives use P option to ar"). > > If two objects from different directories have the same file name, > GNU ar may drop the one when flattening thin archives. > > Here is a simple test case: > > $ gcc -c -x c /dev/null -o foo.o > $ mkdir a > $ gcc -c -x c /dev/null -o a/foo.o > $ ar rcST a/built-in.a a/foo.o > $ ar rcST built-in.a a/built-in.a foo.o > $ cat built-in.a > !<thin> > // 8 ` > foo.o/ > > /0 0 0 0 644 944 ` > > We expect built-in.a contains both a/foo.o and foo.o, but the former > is lost. The 'P' option solves this. > > $ rm -f built-in.a > $ ar rcSTP built-in.a a/built-in.a foo.o > $ cat built-in.a > !<thin> > // 16 ` > a/foo.o/ > foo.o/ > /0 0 0 0 644 944 ` > /9 0 0 0 644 944 ` > > Interestingly, GNU ar works as expected without 'P' when the order of > a/built-in.a and foo.o are opposite: > > $ rm built-in.a > $ ar rcST built-in.a foo.o a/built-in.a > $ cat built-in.a > !<thin> > // 16 ` > foo.o/ > a/foo.o/ > /0 0 0 0 644 944 ` > /7 0 0 0 644 944 ` > > Anyway, even the latest GNU toolchain works like that, so > Kbuild does need the 'P' option. > > The real world example is sh. > > arch/sh/kernel/cpu/fpu.c > arch/sh/kernel/cpu/sh2a/fpu.c > arch/sh/kernel/cpu/sh4/fpu.c > arch/sh/kernel/cpu/sh5/fpu.c > > [2] flattening thin archives > > llvm-ar cannot flatten archives. > > This issue was filed in the following: > https://bugs.llvm.org/show_bug.cgi?id=37530 > > Its status was marked as "RESOLVED FIXED", but actually not fixed > as of writing (Jan 17, 2019). > > The older version of llvm-ar worked like this: > > $ rm -f built-in.a a/built-in.a > $ llvm-ar rcST a/built-in.a a/foo.o > $ llvm-ar rcST built-in.a a/built-in.a > $ cat built-in.a !<thin> > // 14 ` > a/built-in.a/ > /0 0 0 0 644 136 ` > > Recently, two commits were pushed to the trunk. > [llvm-ar] Flatten thin archives. > [llvm-ar] Resubmit recursive thin archive test with fix for full path names and better error messages > > As far as I tested, the latest llvm-ar works as follows: > > $ llvm-ar rcST a/built-in.a a/foo.o > $ llvm-ar rcST built-in.a a/built-in.a > $ cat built-in.a > !<thin> > // 8 ` > foo.o/ > > /0 0 0 0 644 800 ` > > OK, it flattens the thin archive, but the problem is it rips off > the directory path. > > GNU ar works as follows: > > $ ar rcST a/built-in.a a/foo.o > $ ar rcST built-in.a a/built-in.a > $ cat built-in.a > !<thin> > // 10 ` > a/foo.o/ > > /0 0 0 0 644 800 ` > > If you use the latest llvm-ar for building the kernel, you will see > a mysterious 'llvm-ar: error: No such file or directory.' error. > (I think the error message could be improved because it is unclear > what is missing.) > > [Workaround in this patch] > > Currently, llvm-ar cannot flatten thin archives correctly. So, Kbuild > uses '$(AR) t' to expand thin archives from subdirectories, then repack > all objects into the current directory built-in.a. > > The 'P' for cmd_link_l_target is unneeded because Kbuild does not > support subdir objects for lib-y in the first place. > > [How to use] > > Pass AR=llvm-ar in the configuration and build command: > > $ make AR=llvm-ar defconfig > $ make AR=llvm-ar > > Or, simply in one line: > > $ make AR=llvm-ar defconfig all Thank you Masahiro for the excellent feedback on what's missing with llvm-ar. I will work with Jordan (cc'ed) to sort out these remaining issues. I think we should do our best to fix these in llvm-ar to the extent possible, before modifying Kbuild to support it. > > Signed-off-by: Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx> For the above reason; Nacked-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx> to prevent this from going in before we've had time to address the feedback from this patch in llvm-ar. There's no rush to support llvm-ar, so I'd like to not burden the kernel with these changes. But we will keep this patch in our backpocket should we determine that we cannot meet compatibility with binutils ar. > --- > > This patch can be cleanly applied to Linux 5.0-rc2 + the following: > https://patchwork.kernel.org/patch/10761625/ > https://patchwork.kernel.org/patch/10767193/ > https://patchwork.kernel.org/patch/10767195/ > https://patchwork.kernel.org/patch/10767503/ > https://patchwork.kernel.org/patch/10767509/ > > > init/Kconfig | 3 +++ > scripts/Makefile.build | 8 +++++++- > 2 files changed, 10 insertions(+), 1 deletion(-) > > diff --git a/init/Kconfig b/init/Kconfig > index 513fa54..185274ac 100644 > --- a/init/Kconfig > +++ b/init/Kconfig > @@ -23,6 +23,9 @@ config CLANG_VERSION > int > default $(shell,$(srctree)/scripts/clang-version.sh $(CC)) > > +config AR_IS_LLVM > + def_bool $(success,$(AR) --version | head -n 1 | grep -q LLVM) > + > config CC_HAS_ASM_GOTO > def_bool $(success,$(srctree)/scripts/gcc-goto.sh $(CC)) > > diff --git a/scripts/Makefile.build b/scripts/Makefile.build > index f8e2794..b866bb5 100644 > --- a/scripts/Makefile.build > +++ b/scripts/Makefile.build > @@ -399,7 +399,13 @@ $(sort $(subdir-obj-y)): $(subdir-ym) ; > ifdef builtin-target > > quiet_cmd_ar_builtin = AR $@ > +ifdef CONFIG_AR_IS_LLVM > + cmd_ar_builtin = rm -f $@; $(AR) rcST$(KBUILD_ARFLAGS) $@ \ > + $(foreach f, $(real-prereqs), \ > + $(if $(filter $(subdir-obj-y), $(f)), $(shell $(AR) t $(f)), $(f))) > +else > cmd_ar_builtin = rm -f $@; $(AR) rcSTP$(KBUILD_ARFLAGS) $@ $(real-prereqs) > +endif > > $(builtin-target): $(real-obj-y) FORCE > $(call if_changed,ar_builtin) > @@ -427,7 +433,7 @@ ifdef lib-target > quiet_cmd_link_l_target = AR $@ > > # lib target archives do get a symbol table and index > -cmd_link_l_target = rm -f $@; $(AR) rcsTP$(KBUILD_ARFLAGS) $@ $(real-prereqs) > +cmd_link_l_target = rm -f $@; $(AR) rcsT$(KBUILD_ARFLAGS) $@ $(real-prereqs) > > $(lib-target): $(lib-y) FORCE > $(call if_changed,link_l_target) > -- > 2.7.4 > -- Thanks, ~Nick Desaulniers