[llvm-dev] Linking Linux kernel with LLD

Sean Silva via llvm-dev llvm-dev at lists.llvm.org
Wed Feb 1 14:01:26 PST 2017

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
> ,
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