On Tue, May 29, 2018 at 9:28 PM, Amaan Cheval <amaan.che...@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. > > - 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. > > 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. > > 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 <j...@rtems.org> wrote: >> >> >> On Fri, May 25, 2018, 12:11 PM Amaan Cheval <amaan.che...@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 <j...@rtems.org> wrote: >>> > >>> > >>> > On Fri, May 25, 2018, 11:15 AM Amaan Cheval <amaan.che...@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 <j...@rtems.org> wrote: >>> >> > >>> >> > >>> >> > On Sun, May 20, 2018, 12:10 PM Amaan Cheval <amaan.che...@gmail.com> >>> >> > wrote: >>> >> >> >>> >> >> On Sat, May 19, 2018 at 6:51 PM, Gedare Bloom <ged...@rtems.org> >>> >> >> wrote: >>> >> >> > On Fri, May 18, 2018 at 5:53 PM, Joel Sherrill <j...@rtems.org> >>> >> >> > wrote: >>> >> >> >> >>> >> >> >> >>> >> >> >> On Fri, May 18, 2018 at 3:24 PM, Amaan Cheval >>> >> >> >> <amaan.che...@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@rtems.org >>> >> >> >> http://lists.rtems.org/mailman/listinfo/devel _______________________________________________ devel mailing list devel@rtems.org http://lists.rtems.org/mailman/listinfo/devel
