[GSoC] Ways to make the x86_64 port work with UEFI

Tue May 29 23:00:24 UTC 2018

On Tue, May 29, 2018 at 11:26 AM, Amaan Cheval <amaan.cheval at gmail.com>
wrote:

> On Tue, May 29, 2018 at 9:28 PM, Amaan Cheval <amaan.cheval at gmail.com>
> wrote:
> > Noted, thanks a ton for the details! Unrelated to the topic at hand,
> > but out of interest, is this the only reading material for further
> > details? http://ceur-ws.org/Vol-1697/EWiLi16_12.pdf
> >
> > In brief: My tests for keeping a libfake.a (compiled without -fpic)
> > and a loader.so with a user-appXYZ.c have been successful, but I'm not
> > sure if my assumptions hold for all cases. See the details below for
> > more.
> >
> > Next actions:
> > - Read more of Linkers and Loaders since it seems to be the only
> > detailed resource I've found at this point
> > - Experiment with actually using the existing librtems*.a I've got and
> > making them boot as a PE UEFI application image
> >
> > ------------------------------------------------------------
> --------------------
> >
> > In more detail:
> >
> > The problem:
> > That UEFI needs a relocatable PE file, i.e. one that can function
> > regardless of the physical address it's loaded at (no virtual
> > addresses that early).
> > To build an ELF of that kind, the resources I've seen all build their
> > source with -fpic, and then use objcopy to convert the ELF into a
> > relocatable PE, with an embedded runtime self-relocator (akin to
> > load-time relocation, if I'm understanding correctly).
> >
> > What Joel suggested seems to be the simplest option - see if not using
> > -fpic for _all_ of RTEMS' build system is fine. I think it might be
> > from some testing, but I'm not sure if this is conclusive since I need
> > to understand the specifics of the entire development process better.
> >
> > So here's my understanding of the situation at the moment:
> >
> > - librtems*.a is made up of object files, compiled without -fpic, and
> > that should be fine because I believe object files will use RIP
> > relative addressing code by default on x64 where it can, and leave the
> > rest for link-time relocations to handle. IF this is true, this works
> > perfectly for us because all memory accesses and jumps/calls are
> > relative.
>

Just to be clear. For Deos, I am compiling all code with -fPIC. This
includes all librtemscpu.a and librtemsbsp.a. When I accidentally
missed an adapter file, that caused an issue.

> >
> > - We can have a loader.c which acts as the core with the efi_main
> > function - compile it with -fpic into loader.so, and then link
> > loader.so, librtems*.a, and user-appXYZ.c together to form a
> > relocatable ELF, then convert it into a PE using objcopy. Note that
> > from what I can tell, the ELF generated from this still has type EXEC,
> > not DYN, according to readelf.
>
> Correction: This was a leftover file that I'd forgotten to take out
> after renaming a target. Sorry about the confusion. Just using the
> "-shared" flag does cause the resulting ELF to be of type DYN.
>
> >
> > The concerns I have are about my assumptions; if GCC generates any
> > code that uses absolute addressing and that is resolved as a link-time
> > relocation, that could be problematic because the final relocatable PE
> > may not match up with the resolved absolute address.
>

The Deos kernel guys had me check readelf on a known good executable
with the ones I was producing. The loadable sections should match up.
For example, on one architecture I missed an alignment in the linkcmds
and on another, an argument hidden in bsp_specs made a section writable
which should have been read-only. Just check what you can with readelf
and objdump section headers.

> >
> > My tests with a fake static archive library, and creating a PE have
> > been successful, but I'm unsure of how to trigger the relocation
> > behavior by the UEFI firmware (i.e. loading the UEFI image at an
> > address other than it's preferred one). One idea is to have a UEFI
> > application image that loads this test UEFI application image through
> > the "LoadImage" function UEFI provides as a service and then to use
> > QEMU's monitor / gdb inspection capabilities to see if the address the
> > image is loaded at genuinely changes.
>

That's the question. Does the executable end up at a reliably known address?

> >
> > If any of you have any resources, that'd be highly appreciated. Some
> > resources I'm using so far are:
> > - https://eli.thegreenplace.net/2011/11/03/position-
> independent-code-pic-in-shared-libraries/
> > - https://eli.thegreenplace.net/2011/08/25/load-time-
> relocation-of-shared-libraries/
> > - https://eli.thegreenplace.net/2012/01/03/understanding-the-
> x64-code-models
> >
> > Sorry about the length of the email!
> >
> > On Fri, May 25, 2018 at 10:51 PM, Joel Sherrill <joel at rtems.org> wrote:
> >>
> >>
> >> On Fri, May 25, 2018, 12:11 PM Amaan Cheval <amaan.cheval at gmail.com>
> wrote:
> >>>
> >>> Hey! Could you link me to some code that you used for the Deos setup
> >>> you mentioned?
> >>> My understanding is that the -shared option can link static archives
> >>> to create a "shared" library in the sense that it doesn't include the
> >>> usual crt0 runtime environment and whatnot, but the code within is
> >>> still position-dependent. Given that the PE image that EFI needs is
> >>> one that needs to be truly relocatable, this may not work - BUT, I've
> >>> only just noticed the ./gnuefi/reloc_x86_64.c file which seems to
> >>> handle some kinds of runtime relocations encoded within the converted
> >>> PE file, so maybe this will work after all. I'll continue to
> >>> investigate and let you know how it goes!
> >>
> >>
> >> Deos isn't a good example except that you can compile with -fPIC and put
> >> that code into a static library. Deos is a closed source Level A (man
> rated
> >> flight) ARINC 653 RTOS. It's boot process reads configuration
> information
> >> about each partition and associates .so's with each address space per
> the
> >> configuration. It can't change after that.
> >>
> >> The RTEMS exe is mostly linked as normal except to use some arguments
> to say
> >> some symbols are from a shared library.
> >>
> >> The base address of the exe is that of the provided virtual address
> space
> >> with .data and .bss in their respective spaces.
> >>
> >> And our entry point is in C so there is no asm before that. Great
> >> simplification.
> >>
> >>>
> >>> Regarding how TLS differs with PIC - could you elaborate? Is it
> >>> something we'll need to solve for if we go with the -fPIC option, or
> >>> is it something we need to keep in mind as a limitation, but isn't
> >>> really a blocker?
> >>
> >>
> >> I don't think PIC changes the TLS mechanism on arm or PowerPC but on
> i386,
> >> when not PIC the TLS base is in %gs and it's a subroutine call when PIC.
> >> Just as well for Deos since they assume an application won't change the
> >> segment register values.
> >>
> >> Other than this one TLS difference, it is a normal exe to me. They just
> >> magically provide their .so's before we run.
> >>>
> >>>
> >>> On Fri, May 25, 2018 at 10:13 PM, Joel Sherrill <joel at rtems.org>
> wrote:
> >>> >
> >>> >
> >>> > On Fri, May 25, 2018, 11:15 AM Amaan Cheval <amaan.cheval at gmail.com>
> >>> > wrote:
> >>> >>
> >>> >> Hey!
> >>> >>
> >>> >> Skippable details about how FreeBSD handles the UEFI boot process!
> >>> >>
> >>> >>
> >>> >> ------------------------------------------------------------
> --------------------
> >>> >>
> >>> >> Having looked into it a bit more, my understanding of how FreeBSD
> >>> >> handles this process is:
> >>> >> - They build a two-stage bootloader for EFI, called boot1.efi and
> >>> >> loader.efi[1]
> >>> >> - loader.efi is an interactive prompt which may autoboot, or a "boot
> >>> >> kernelImg" command can be used to load the actual kernel
> >>> >> - The kernel is loaded as an ELF through helper functions. The
> >>> >> command_boot[2] function drives this:
> >>> >>   - In brief, through calls go through:
> >>> >>     command_boot -> mod_loadkld -> file_load ->
> >>> >> file_formats[i]->l_load (actually the loadfile function in
> >>> >> load_elf.c[3])
> >>> >>   - The loadfile function parses the program and section headers of
> >>> >> the ELF file (through more function detours that are not really
> >>> >> important)
> >>> >>   - Once the ELF has been loaded at the correct entry_addr that it
> >>> >> expects to be loaded at in memory, the l_exec[4] function is called,
> >>> >> which is actually elf64_exec in elf64_freebsd.c[5], at which
> hopefully
> >>> >> through trampolining magic, the control flow will transfer to the
> >>> >> kernel or ELF module
> >>> >>
> >>> >>
> >>> >>
> >>> >> ------------------------------------------------------------
> --------------------
> >>> >>
> >>> >> What this means for RTEMS if we go with gnu-efi is essentially 2
> >>> >> options, given that the objcopy method of converting from ELF->PE
> >>> >> requires the ELF to be a position-independent shared library:
> >>> >>
> >>> >> - Using -fPIC to compile all of RTEMS, including the RTEMS user's
> >>> >> application code. This way we'd have librtemsbso.so, librtemscpu.so,
> >>> >> etc. which would then be linked into user_app.c through -fPIC and
> >>> >> -shared flags still, creating one singular hello.so, which can then
> >>> >> finally be converted into hello.efi and put on a FAT filesystem and
> >>> >> booted. This seems doable, but I'm fairly concerned about it further
> >>> >> complicating our build system and likely being quite singular in its
> >>> >> focus on EFI.
> >>> >
> >>> >
> >>> > I'm using PIC on the Deos BSP. RTEMS is still a .a and exes are
> linked
> >>> > with
> >>> > our static libraries and Deos .so.
> >>> >
> >>> > Unless the loader forces something, you can use PIC with no build
> system
> >>> > changes.
> >>> >
> >>> > note that thread local storage is different on i386 with and without
> >>> > PIC.
> >>> >>
> >>> >>
> >>> >> - The FreeBSD way of a (loader.efi) and a hello.exe (ELF64) put on
> >>> >> possibly the same partition on the FAT filesystem required for UEFI
> >>> >> application images anyway. The loader.efi can find the hello.exe
> file
> >>> >> through perhaps a config file it can read or by having a magic-name
> >>> >> like rtems.exe or something. This effectively means we need an ELF
> >>> >> dynamic linker / loader (akin to ld.so) within RTEMS' source. I
> think
> >>> >> using FreeBSD's code for this should be fine. One added benefit of
> >>> >> this method is that librtems* and user applications remain as
> ELF64s,
> >>> >> which in the future could also be used with Multiboot with a
> slightly
> >>> >> modified "loader" (i.e. one which generates the apt Multiboot magic
> >>> >> header, and boots the PC from 32-bit protected mode to 64-bit long
> >>> >> mode).
> >>> >>
> >>> >> I prefer the latter approach personally. If both of these seem too
> >>> >> complicated, we can of course go back to considering generating the
> PE
> >>> >> header format in ASM the way Linux distros use EFISTUB and the code
> >>> >> Chris shared (as I mentioned in my original blog post) for wimboot.
> >>> >> Those approaches may be significantly simpler in a sense, but may
> >>> >> limit how we use UEFI Services - I'm not sure about the details of
> >>> >> this yet - I can investigate if y'all aren't fond of the option I
> laid
> >>> >> down above.
> >>> >>
> >>> >> Let me know!
> >>> >>
> >>> >> [1]
> >>> >>
> >>> >> https://www.freebsd.org/cgi/man.cgi?query=loader&apropos=
> 0&sektion=8&manpath=FreeBSD+11.1-RELEASE+and+Ports&arch=
> default&format=html
> >>> >> [2]
> >>> >>
> >>> >> https://github.com/freebsd/freebsd/blob/
> 433bd38e3a0349f9f89f9d54594172c75b002b74/stand/common/boot.c#L53
> >>> >> [3]
> >>> >>
> >>> >> https://github.com/freebsd/freebsd/blob/
> d8596f6f687a64b994b065f3058155405dfc39db/stand/common/load_elf.c#L150
> >>> >> [4]
> >>> >>
> >>> >> https://github.com/freebsd/freebsd/blob/
> 433bd38e3a0349f9f89f9d54594172c75b002b74/stand/common/boot.c#L107
> >>> >> [5]
> >>> >>
> >>> >> https://github.com/freebsd/freebsd/blob/
> d8596f6f687a64b994b065f3058155405dfc39db/stand/efi/loader/
> arch/amd64/elf64_freebsd.c#L93
> >>> >>
> >>> >> On Sun, May 20, 2018 at 10:52 PM, Joel Sherrill <joel at rtems.org>
> wrote:
> >>> >> >
> >>> >> >
> >>> >> > On Sun, May 20, 2018, 12:10 PM Amaan Cheval <
> amaan.cheval at gmail.com>
> >>> >> > wrote:
> >>> >> >>
> >>> >> >> On Sat, May 19, 2018 at 6:51 PM, Gedare Bloom <gedare at rtems.org>
> >>> >> >> wrote:
> >>> >> >> > On Fri, May 18, 2018 at 5:53 PM, Joel Sherrill <joel at rtems.org
> >
> >>> >> >> > wrote:
> >>> >> >> >>
> >>> >> >> >>
> >>> >> >> >> On Fri, May 18, 2018 at 3:24 PM, Amaan Cheval
> >>> >> >> >> <amaan.cheval at gmail.com>
> >>> >> >> >> wrote:
> >>> >> >> >>>
> >>> >> >> >>> Hi everyone!
> >>> >> >> >>>
> >>> >> >> >>> I've written a quick blog post summarizing the options I've
> >>> >> >> >>> considered
> >>> >> >> >>> to make the x86_64 port work with UEFI firmware - the primary
> >>> >> >> >>> winner
> >>> >> >> >>> seems to be in my eyes to use "gnu-efi" and to add support
> for
> >>> >> >> >>> the
> >>> >> >> >>> target "pei-x86-64" (aliased to "efi-app-x86_64") to
> >>> >> >> >>> "x86_64-rtems5-objcopy" in binutils. I've submitted a patch
> for
> >>> >> >> >>> this
> >>> >> >> >>> here[1].
> >>> >> >> >>
> >>> >> >> >>
> >>> >> >> >> That patch is quite simple so shouldn't be a problem if this
> is
> >>> >> >> >> the
> >>> >> >> >> direction
> >>> >> >> >> that gets consensus.
> >>> >> >> >>>
> >>> >> >> >>>
> >>> >> >> >>> The blog post is here:
> >>> >> >> >>> https://blog.whatthedude.com/post/uefi-app-options/
> >>> >> >> >>>
> >>> >> >> >>> I'd appreciate all feedback (and please do let me know if I
> >>> >> >> >>> haven't
> >>> >> >> >>> provided enough context)!
> >>> >> >> >>>
> >>> >> >> >>> Specifically, some concerns I'd like to discuss are:
> >>> >> >> >>>
> >>> >> >> >>> - Does everyone agree with me on choosing gnu-efi + objcopy
> as
> >>> >> >> >>> our
> >>> >> >> >>> method of choice?
> >>> >> >> >>
> >>> >> >> >>
> >>> >> >> >> Does using gnu-efi add code that runs on the target? Can you
> >>> >> >> >> point
> >>> >> >> >> us to the files, if so.
> >>> >> >>
> >>> >> >> Sure. The files would run on the target, yes. These are the ones
> >>> >> >> listed here (as linked to in my blog post, perhaps without
> >>> >> >> sufficient
> >>> >> >> emphasis):
> >>> >> >> https://wiki.osdev.org/UEFI#Developing_with_GNU-EFI
> >>> >> >>
> >>> >> >> >>
> >>> >> >> >> Can you tell which approach FreeBSD takes?
> >>> >> >>
> >>> >> >> FreeBSD takes the gnu-efi approach I see as the "winner" here
> (also
> >>> >> >> a
> >>> >> >> link in the post):
> >>> >> >>
> >>> >> >>
> >>> >> >>
> >>> >> >> https://github.com/freebsd/freebsd/blob/
> 996b0b6d81cf31cd8d58af5d8b45f0b4945d960d/stand/efi/loader/Makefile#L98-L1
> >>> >> >
> >>> >> >
> >>> >> > This is (no surprise) appropriately licensed and IMO the winning
> >>> >> > solution.
> >>> >> > Knowing it is what FreeBSD does makes it an easy choice.
> >>> >> >
> >>> >> > A comment in the readme mentions there is a i386 version of this
> code
> >>> >> > so
> >>> >> > that could be used to let pc386 boot from UEFI.
> >>> >> >
> >>> >> >>
> >>> >> >>
> >>> >> >> >>
> >>> >> >> >>>
> >>> >> >> >>> - How do we integrate gnu-efi into our build process? A part
> of
> >>> >> >> >>> the
> >>> >> >> >>> RSB, making sure the path to the libraries are in an exported
> >>> >> >> >>> variable? Or perhaps a part of the RTEMS kernel itself if the
> >>> >> >> >>> licenses
> >>> >> >> >>> are compatible (I don't see any on the project[2], only
> >>> >> >> >>> copyright
> >>> >> >> >>> notices within the source files of the release versions).
> >>> >> >> >>
> >>> >> >> >>
> >>> >> >> >> GNU-efi would be built like qemu or the device tree compiler
> >>> >> >> >> would
> >>> >> >> >> be my guess and x86_64-rtems toolset might add that to the
> >>> >> >> >> standard
> >>> >> >> >> set of tools. License on host tools being GPL isn't an issue.
> >>> >> >> >>
> >>> >> >> >
> >>> >> >> > It appears to be a standard 2-clause BSD released by Intel as
> >>> >> >> > specified in the README file of gnu-efi.
> >>> >> >> >
> >>> >> >> >>
> >>> >> >> >>>
> >>> >> >> >>> - Regardless of how we manage UEFI, do we require Multiboot
> >>> >> >> >>> support
> >>> >> >> >>> too? Multiboot drops us in a 32-bit protected mode
> environment,
> >>> >> >> >>> whereas 64-bit UEFI firmware will boot us into 64-bit long
> mode
> >>> >> >> >>> -
> >>> >> >> >>> this
> >>> >> >> >>> would mean the kernel would need to support separate
> code-paths
> >>> >> >> >>> for
> >>> >> >> >>> the 2 if we want to support both methods.
> >>> >> >> >>
> >>> >> >> >>
> >>> >> >> >> That's a good question. For GSoC, I think UEFI is fine and
> >>> >> >> >> perhaps a
> >>> >> >> >> ticket
> >>> >> >> >> under the general "modern PC support" ticket for multiboot
> >>> >> >> >> support.
> >>> >> >> >> Unless
> >>> >> >> >> that eliminates a LOT of PCs.
> >>> >> >> >>
> >>> >> >> >> I don't want you to spend all summer getting an image to boot
> >>> >> >> >> both
> >>> >> >> >> ways. Personally, I want you to have a working BSP one way. :)
> >>> >> >> > +1
> >>> >> >> >
> >>> >> >>
> >>> >> >> Noted, thanks!
> >>> >> >>
> >>> >> >> >>>
> >>> >> >> >>>
> >>> >> >> >>> [1] https://www.sourceware.org/ml/
> binutils/2018-05/msg00197.html
> >>> >> >> >>> [2] https://sourceforge.net/projects/gnu-efi/
> >>> >> >> >>
> >>> >> >> >>
> >>> >> >> >> --joel
> >>> >> >> >>
> >>> >> >> >> _______________________________________________
> >>> >> >> >> devel mailing list
> >>> >> >> >> devel at rtems.org
> >>> >> >> >> http://lists.rtems.org/mailman/listinfo/devel
>
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