[bug #64812] Support for relocation type in grub2

[Leung Wing Chung]

URL:
  <https://savannah.gnu.org/bugs/?64812>

                 Summary: Support for relocation type in grub2
                   Group: GNU GRUB
               Submitter: l00862805
               Submitted: Wed 25 Oct 2023 01:48:46 AM UTC
                Category: Compilation
                Severity: Major
                Priority: 5 - Normal
              Item Group: Feature Request
                  Status: None
                 Privacy: Public
             Assigned to: None
         Originator Name: Ricky Leung
        Originator Email: 
             Open/Closed: Open
                 Release: 2.00
                 Release: 
         Discussion Lock: Any
         Reproducibility: Every Time
         Planned Release: None


    _______________________________________________________

Follow-up Comments:


-------------------------------------------------------
Date: Wed 25 Oct 2023 01:48:46 AM UTC By: Leung Wing Chung <l00862805>
I am a compiler engineer currently working on a project to run Grub2 with
Clang + LLVM instead of GCC. I wanted to reach out to the community to discuss
an issue and inquire about your plans to support relocation type.

When I am building Grub2 with `time osc build --root=$(pwd)/build `, I
received the following error message: "unsupported relocation 0x108".

Upon further investigation, I discovered that this error is due to the
different approaches used by GCC and LLVM when handling relocations.
Specifically, when compiling the code with GCC using the `-mcmodel=large`
flag, the relocation types `R_AARCH64_ADR_PRE`, `R_AARCH64_ADD_ABS`, and
`R_AARCH64_ABS64` were used. However, when using LLVM with the same flag, the
relocation type `R_AARCH64_MOVW_UABS_G0_NC` was encountered. Here's the
example:

test.c:

```c
const char *test(void){
        return "xx";
}
```

Execution with GCC:

```cmd
$ gcc -o test.o -c test.c -mcmodel=large
$ readelf -r test.o

Relocation section '.rela.text' at offset 0x200 contains 3 entries:
  Offset          Info           Type           Sym. Value    Sym. Name +
Addend
000000000000  000200000113 R_AARCH64_ADR_PRE 0000000000000000 .text + 10
000000000004  000200000115 R_AARCH64_ADD_ABS 0000000000000000 .text + 10
000000000010  000500000101 R_AARCH64_ABS64   0000000000000000 .rodata + 0

Relocation section '.rela.eh_frame' at offset 0x248 contains 1 entry:
  Offset          Info           Type           Sym. Value    Sym. Name +
Addend
00000000001c  000200000105 R_AARCH64_PREL32  0000000000000000 .text + 0
```

Execution with Clang:

```cmd
$ clang -o test.o -c test.c -mcmodel=large
$ readelf -r test.o

Relocation section '.rela.text' at offset 0x1c0 contains 4 entries:
  Offset          Info           Type           Sym. Value    Sym. Name +
Addend
000000000000  000700000108 R_AARCH64_MOVW_UA 0000000000000000 .rodata.str1.1 +
0
000000000004  00070000010a R_AARCH64_MOVW_UA 0000000000000000 .rodata.str1.1 +
0
000000000008  00070000010c R_AARCH64_MOVW_UA 0000000000000000 .rodata.str1.1 +
0
00000000000c  00070000010d R_AARCH64_MOVW_UA 0000000000000000 .rodata.str1.1 +
0

Relocation section '.rela.eh_frame' at offset 0x220 contains 1 entry:
  Offset          Info           Type           Sym. Value    Sym. Name +
Addend
00000000001c  000600000104 R_AARCH64_PREL64  0000000000000000 .text + 0
```

To address this issue and ensure compatibility across different compilers, I
would like to know if there are any plans to support relocation type in Grub2.
his would enable seamless compilation and execution of Grub2 using both GCC
and LLVM with the `-mcmodel=large` flag.

Thank you for your time and contributions.







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