Re: [PATCH v8] Build process uses script to add CFI directives to x86 asm

[Alex <alexinbeijing@gmail.com>]

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Thanks for the reply! Comments below:

On Sun, Jun 14, 2015 at 6:37 AM, Rich Felker <dalias@libc.org> wrote:

> On Fri, Jun 05, 2015 at 10:39:18AM +0200, Alex Dowad wrote:
> > diff --git a/Makefile b/Makefile
> > index 2eb7b30..9b55fd8 100644
> > --- a/Makefile
> > +++ b/Makefile
> > @@ -120,7 +120,11 @@ $(foreach s,$(wildcard src/*/$(ARCH)*/*.s),$(eval
> $(call mkasmdep,$(s))))
> >       $(CC) $(CFLAGS_ALL_STATIC) -c -o $@ $(dir $<)$(shell cat $<)
> >
> >  %.o: $(ARCH)/%.s
> > -     $(CC) $(CFLAGS_ALL_STATIC) -c -o $@ $<
> > +ifeq ($(ADD_CFI),yes)
> > +     LC_ALL=C awk -f tools/add-cfi.$(ARCH).awk $< | $(CC) $(ASFLAGS) -x
> assembler -c -o $@ -
> > +else
> > +     $(CC) $(ASFLAGS) -c -o $@ $<
> > +endif
>
> Removing $(CFLAGS_STATIC_ALL) here is a regression. -Wa,--noexecstack
> is necessary to prevent the kernel from giving us an executable stack
> when asm files are linked. We could move it to a separate ASFLAGS, but
> the patch doesn't do this, and unless there's a real need to avoid
> passing CFLAGS, I'd rather not add more vars. (In this case, needing
> the new var would be a silent security regression for anyone building
> without re-running configure.)
>

The reason for not passing CFLAGS is because clang chokes on "-g" when
assembling code with CFI directives. I also thought that ASFLAGS might be a
useful customization point for people who want to edit config.mak to create
a custom build. But you are the judge of that.

Since it seems that CFLAGS is needed, would it be acceptable to bypass the
issue by saying that clang users simply won't be able to do debug builds of
musl until their compiler is fixed? The current state of LLVM's CFI
generation is so bad that debug builds probably won't be useful anyways.

If that is a sticking point, I might put together a patch for LLVM and see
if they want it. Unfortunately, I have already discovered a bunch of other
problems with LLVM which would be nice to fix, but time for developing and
polishing patches is limited...

As an aside, I admire the fact that you picked up on that subtle
regression. The standard of code quality and attention to detail on this
project is very high, as compared to other open-source projects I have
worked on. Kudos to all the contributors!

As for the naming (tools/add-cfi.$(ARCH).awk), I'm not opposed to this
> and the configure test for it is nice, but I wonder if there will be
> significant code duplication between versions of this script for
> different archs that would make it preferable to take the arch as an
> argument. What do you think? Or does awk have an easy #include-like
> mechanism?
>

I'm not an AWKer, but from what I have read, apparently "awk -f script1.awk
-f script2.awk" is the equivalent of concatenating "script1.awk" and
"script2.awk", so shared functions can easily be put in a common file.

It seems that the amount of shared code will be small, however. Actually,
the entire script for x86-32 is already fairly small. I feel that anything
more sophisticated than picking a script based on arch would just be
complicating matters for little benefit.

If it turns out that I am wrong, the commonalities can be abstracted out
later. At that time, with several such preprocessing scripts available to
look at, it will be clearer what and how to abstract.


>
> >  #
> > +# Preprocess asm files to add extra debugging information if debug is
> > +# enabled, our assembler supports the needed directives, and the
> > +# preprocessing script has been written for our architecture.
> > +#
> > +printf "checking whether we should preprocess assembly to add debugging
> information... "
> > +if fnmatch '-g*|*\ -g*' "$CFLAGS_AUTO" &&
> > +   test -f "tools/add-cfi.$ARCH.awk" &&
> > +   echo ".cfi_startproc
> > +.cfi_endproc" | $CC -x assembler -c -o /dev/null -
> > +then
> > +  ADD_CFI=yes
> > +else
> > +  ADD_CFI=no
> > +fi
> > +printf "%s\n" "$ADD_CFI"
> > +
> > +#
>
> This test looks nice and robust. I'd mildly prefer:
>
>   printf '.cfi_startproc\n.cfi_endproc\n'
>
> to avoid the multi-line string with echo, but that's a tiny detail.
>

OK. It was written like this because "echo '.cfi_startproc\n.cfi_endproc'"
didn't work on BusyBox ash. But it seems that printf is fine. Will revise.

Thanks, AD

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