Hayden Livingston via llvm-dev
2019-Jul-01 04:07 UTC
[llvm-dev] Generating completely position agnostic code
I'm on a mission to generate code that can be loaded from disk without any modifications. This means no relocations can occur. Trying to see if this can be done for C++ code that uses STL but has no global variables, and a single function, but of course Clang will generate more functions for STL code. I want to provide an array of function pointers so that for all interactions STL needs to do with LIBC that I'm able to just provide it via indirect calls. Has anyone had success with such a thing in LLVM?
Jorg Brown via llvm-dev
2019-Jul-01 07:56 UTC
[llvm-dev] Generating completely position agnostic code
Qs for you: The code that is being loaded from disk... is it wholly self-contained, or is your executable potentially made up of several pieces that each need to be loaded from disk? What does it mean to use the STL but not have global variables? std::cout is a global variable, so you can't even do Hello World without globals. = = Architectures such as 68K and PowerPC and RISC-V have a dedicated register for accessing global variables, rather than the PC-relative globals used in other architectures. This makes them inherently more amenable to what you describe, since you can put the "array of function pointers" into global space, as part of setting up global space in general, and then load the code from disk, and go. There is no relocation needed since all access to globals is done via the global register, not relative to wherever the program was loaded. Of course, access to something like libc might normally need post-loading relocation, but if you do what you're talking about and use an "array of function pointers" to get to libc, no relocation would be needed. For what it's worth, the original 68K-based Macintosh used a scheme quite similar to this. The big difference for the Mac was that to get to the OS (the equivalent of libc), it didn't use an array of function pointers, per se; it used a certain range of illegal instructions, which generated exceptions when used, and the (highly optimized) exception handlers would recover from the exception by dispatching to an OS routine determined by the specific bits in the illegal instruction. On Sun, Jun 30, 2019 at 9:07 PM Hayden Livingston via llvm-dev < llvm-dev at lists.llvm.org> wrote:> I'm on a mission to generate code that can be loaded from disk without > any modifications. This means no relocations can occur. > > Trying to see if this can be done for C++ code that uses STL but has > no global variables, and a single function, but of course Clang will > generate more functions for STL code. > > I want to provide an array of function pointers so that for all > interactions STL needs to do with LIBC that I'm able to just provide > it via indirect calls. > > Has anyone had success with such a thing in LLVM? > _______________________________________________ > LLVM Developers mailing list > llvm-dev at lists.llvm.org > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev >-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20190701/17e97979/attachment.html>
Hayden Livingston via llvm-dev
2019-Jul-01 08:47 UTC
[llvm-dev] Generating completely position agnostic code
It is wholly self-contained. It's code that has no references to anything beyond a set of pointers passed in as arguments to the function. This piece of code doesn't do any OS work at all. It is purely calling function pointers, doing math and allocating memory. On Mon, Jul 1, 2019 at 12:57 AM Jorg Brown <jorg.brown at gmail.com> wrote:> > Qs for you: > > The code that is being loaded from disk... is it wholly self-contained, or is your executable potentially made up of several pieces that each need to be loaded from disk? > > What does it mean to use the STL but not have global variables? std::cout is a global variable, so you can't even do Hello World without globals. > > = = > > Architectures such as 68K and PowerPC and RISC-V have a dedicated register for accessing global variables, rather than the PC-relative globals used in other architectures. This makes them inherently more amenable to what you describe, since you can put the "array of function pointers" into global space, as part of setting up global space in general, and then load the code from disk, and go. There is no relocation needed since all access to globals is done via the global register, not relative to wherever the program was loaded. Of course, access to something like libc might normally need post-loading relocation, but if you do what you're talking about and use an "array of function pointers" to get to libc, no relocation would be needed. > > For what it's worth, the original 68K-based Macintosh used a scheme quite similar to this. The big difference for the Mac was that to get to the OS (the equivalent of libc), it didn't use an array of function pointers, per se; it used a certain range of illegal instructions, which generated exceptions when used, and the (highly optimized) exception handlers would recover from the exception by dispatching to an OS routine determined by the specific bits in the illegal instruction. > > On Sun, Jun 30, 2019 at 9:07 PM Hayden Livingston via llvm-dev <llvm-dev at lists.llvm.org> wrote: >> >> I'm on a mission to generate code that can be loaded from disk without >> any modifications. This means no relocations can occur. >> >> Trying to see if this can be done for C++ code that uses STL but has >> no global variables, and a single function, but of course Clang will >> generate more functions for STL code. >> >> I want to provide an array of function pointers so that for all >> interactions STL needs to do with LIBC that I'm able to just provide >> it via indirect calls. >> >> Has anyone had success with such a thing in LLVM? >> _______________________________________________ >> LLVM Developers mailing list >> llvm-dev at lists.llvm.org >> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
Jonathan Smith via llvm-dev
2019-Jul-01 10:45 UTC
[llvm-dev] Generating completely position agnostic code
I'd definitely like to throw my vote behind this. I've managed to write what are effectively bare-metal BIOS bootloaders for dedicated tasks on x86 using LLVM, but I did have to modify the x86 backend to achieve it, and it isn't complete. Most of the work came down to modifying the functions that handle reference classification. Combining this with building via lld and code extraction via objcopy allows me to do most of what I need. That said, this is from C code, not C++; I'd be really interested to see it working with C++, but complex static initialization seems to be a difficult problem in that regard. On Mon, Jul 1, 2019 at 12:08 AM Hayden Livingston via llvm-dev <llvm-dev at lists.llvm.org> wrote:> > I'm on a mission to generate code that can be loaded from disk without > any modifications. This means no relocations can occur. > > Trying to see if this can be done for C++ code that uses STL but has > no global variables, and a single function, but of course Clang will > generate more functions for STL code. > > I want to provide an array of function pointers so that for all > interactions STL needs to do with LIBC that I'm able to just provide > it via indirect calls. > > Has anyone had success with such a thing in LLVM? > _______________________________________________ > LLVM Developers mailing list > llvm-dev at lists.llvm.org > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
Joerg Sonnenberger via llvm-dev
2019-Jul-01 23:25 UTC
[llvm-dev] Generating completely position agnostic code
On Sun, Jun 30, 2019 at 09:07:21PM -0700, Hayden Livingston via llvm-dev wrote:> I'm on a mission to generate code that can be loaded from disk without > any modifications. This means no relocations can occur.Why? Implementing self relocation is not that difficult really. It's also quite difficult to avoid unless you can make sure that no pointer variables are statically initialized. That means no arrays of strings, no vtables etc. Joerg
Hayden Livingston via llvm-dev
2019-Jul-08 02:17 UTC
[llvm-dev] Generating completely position agnostic code
Because I don't think it is required. All I want to do is generate machine code for a single function that has all its references coming in as pointers on the stack and it's just calling functions and doing math. I'm literally asking LLVM convert text in the form of C to x86 ASM. Maybe I'll paste LLVM IR to make it clear what I'm asking for. On Mon, Jul 1, 2019 at 4:25 PM Joerg Sonnenberger via llvm-dev <llvm-dev at lists.llvm.org> wrote:> > On Sun, Jun 30, 2019 at 09:07:21PM -0700, Hayden Livingston via llvm-dev wrote: > > I'm on a mission to generate code that can be loaded from disk without > > any modifications. This means no relocations can occur. > > Why? Implementing self relocation is not that difficult really. It's > also quite difficult to avoid unless you can make sure that no pointer > variables are statically initialized. That means no arrays of strings, > no vtables etc. > > Joerg > _______________________________________________ > LLVM Developers mailing list > llvm-dev at lists.llvm.org > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev