llvm dev - Feb 2017 - Linking Linux kernel with LLD

On Sat, Jan 28, 2017 at 9:57 AM, Dmitry Golovin <dima at golovin.in>
wrote:
> At this point I'm able to link Linux kernel with LLD and objcopy
doen't
> give me any errors.
>
> The versions are:
>
> Linux 4.10.0-rc5 (+ applied the patch from my previous message)
> LLD 5.0.0 (https://github.com/llvm-mirror/lld
> db83a5cc3968b3aac1dbe3270190bd3282862e74) (+ applied D28612)
> GNU objcopy (GNU Binutils) 2.27
>
> The problem is that the resulting kernel doesn't boot. Does anybody
have
> any suggestions on how to debug it or any guesses what did go wrong while
> linking?
>
Based on our experience getting FreeBSD working, we spent most time getting
the bootloader to accept the kernel.

To debug this, we mostly used two approaches:
- printf debugging in the bootloader (will require rebuilding the
bootloader multiple times)
- using objdump-like tools to look at the differences between a good (BFD
or gold linked) kernel and the failing (LLD-linked) kernel. (e.g. different
program header, different section contents in certain sections that the
bootloader looks at, etc.)

As far as the setup, I would recommend setting up qemu for actually running
the LLD-linked kernel and custom bootloader etc. because then you can have
a single script that rebuilds the bootloader and kernel and copies the
files to the VM. This reduces iteration time significantly.
Davide is the one that set that up and could probably provide more details,
but qemu docs might be good enough that you can set things up without much
effort (not sure though).

-- Sean Silva

>
> Regards,
> Dmitry
>
>
> 28.01.2017, 05:48, "Sean Silva via llvm-dev" <llvm-dev at
lists.llvm.org>:
>
>
>
> On Fri, Jan 27, 2017 at 1:31 PM, Rui Ueyama <ruiu at google.com>
wrote:
>
> Sean,
>
> So as you noticed that linker script tokenization rule is not very trivial
> -- it is context sensitive. The current lexer is extremely simple and
> almost always works well. Improving "almost always" to
"perfect" is not
> high priority because we have many more high priority things, but I'm
fine
> if someone improves it. If you are interested, please take it. Or maybe
> I'll take a look at it. It shouldn't be hard. It's probably
just a half day
> work.
>
>
> Yeah. To be clear, I wasn't saying that this was high priority. Since
I'm
> complaining so much about it maybe I should take a look this weekend :)
>
>
>
> As far as I know, the grammar is LL(1), so it needs only one push-back
> buffer. Handling INCLUDE directive can be a bit tricky though.
>
> Maybe we should rename ScriptParserBase ScriptLexer.
>
>
> That sounds like a good idea.
>
> -- Sean Silva
>
>
>
> On Fri, Jan 27, 2017 at 11:17 AM, Rafael Avila de Espindola <
> rafael.espindola at gmail.com> wrote:
>
> > Hmm..., the crux of not being able to lex arithmetic expressions seems
to
> > be due to lack of context sensitivity. E.g. consider `foo*bar`. Could
be
> a
> > multiplication, or could be a glob pattern.
> >
> > Looking at the code more closely, adding context sensitivity
wouldn't be
> > that hard. In fact, our ScriptParserBase class is actually a lexer
(look
> at
> > the interface; it is a lexer's interface). It shouldn't be
hard to change
> > from an up-front tokenization to a more normal lexer approach of
scanning
> > the text for each call that wants the next token. Roughly speaking,
just
> > take the body of the for loop inside ScriptParserBase::tokenize and
add a
> > helper which does that on the fly and is called by consume/next/etc.
> > Instead of an index into a token vector, just keep a `const char *`
> pointer
> > that we advance.
> >
> > Once that is done, we can easily add a `nextArithmeticToken` or
something
> > like that which just lexes with different rules.
>
> I like that idea. I first thought of always having '*' as a token,
but
> then space has to be a token, which is an incredible pain.
>
> I then thought of having a "setLexMode" method, but the lex mode
can
> always be implicit from where we are in the parser. The parser should
> always know if it should call next or nextArithmetic.
>
> And I agree we should probably implement this. Even if it is not common,
> it looks pretty silly to not be able to handle 2*5.
>
> Cheers,
> Rafael
>
>
>
> ,
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
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>As far as the setup, I would recommend setting up qemu for actually running
the LLD-linked kernel and custom bootloader etc. because then you can have a
single >script that rebuilds the bootloader and kernel and copies the files
to the VM. This reduces iteration time significantly.
>Davide is the one that set that up and could probably provide more details,
but qemu docs might be good enough that you can set things up without much
effort
>(not sure though).
>
>-- Sean Silva
 ?
By the way, yesterday I configured "smallest possible kernel", linked
it with BFD and launched under QEMU.
It is very small and takes a few seconds to build it from scratch for me, used
next article:
http://mgalgs.github.io/2015/05/16/how-to-build-a-custom-linux-kernel-for-qemu-2015-edition.html

Now I am going to link it with LLD and check if it boots or now.
I think that should be fastest way - boot that little core and then enable
features
one by one or group by group and fix other things on the road.

Previously I also worked on a patches for kernel but did not try to minimize it
and used some default configuration,
what probably was good for finding mutliple issues from all sides, but not ideal
way to fix/test startup and things.

George.
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On Thu, Feb 2, 2017 at 12:38 AM, George Rimar <grimar at accesssoftek.com>
wrote:
> >As far as the setup, I would recommend setting up qemu for actually
> running the LLD-linked kernel and custom bootloader etc. because then you
> can have a single >script that rebuilds the bootloader and kernel and
> copies the files to the VM. This reduces iteration time significantly.
> >Davide is the one that set that up and could probably provide more
> details, but qemu docs might be good enough that you can set things up
> without much effort
> >(not sure though).
> >
> >-- Sean Silva
>  ​
> By the way, yesterday I configured "smallest possible kernel",
linked it
> with BFD and launched under QEMU.
> It is very small and takes a few seconds to build it from scratch for me,
> used next article:
> http://mgalgs.github.io/2015/05/16/how-to-build-a-custom-
> linux-kernel-for-qemu-2015-edition.html
>
> Now I am going to link it with LLD and check if it boots or now.
> I think that should be fastest way - boot that little core and then enable
> features
> one by one or group by group and fix other things on the road.
>
My experience with linker bugs is that usually when things are mis-linked,
they are in the "core". E.g. startup code. So linking a small kernel
may
not avoid as many bugs as you expect. For example, for FreeBSD, I don't
think we hit any issues in anything that could have been configured out.

-- Sean Silva

>
> Previously I also worked on a patches for kernel but did not try to
> minimize it and used some default configuration,
> what probably was good for finding mutliple issues from all sides, but not
> ideal way to fix/test startup and things.
>
> George.
>
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