Linker error on relocating a program above 2GB in x86_64 linux?

Question

I have a user program which normally compiles to have an entry point at `0x400460` which I have to relocate to have an entry point starting at within `2GB` of the shared libraries loaded in Linux. e.g ``` linux-vdso.so.1 => (0x00007fff109cd000) libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007fcd195e6000) lib64/ld-linux-x86-64.so.2 (0x00007fcd199af000) ``` I am using `gcc` command line argument `-Wl,-Ttext=0x80000000` to specify the start address for the `.text` segemnt. The issue is that when I am giving an address above `2GB` in this argument I am getting a linker error which is: ``` gcc test.c -ggdb -Wl,-Ttext=0x80000000 -o test1 /usr/bin/ld: /usr/lib/debug/usr/lib/x86_64-linux-gnu/crt1.o(.debug_info): relocation 0 has invalid symbol index 10 /usr/bin/ld: /usr/lib/debug/usr/lib/x86_64-linux-gnu/crt1.o(.debug_info): relocation 1 has invalid symbol index 11 /usr/bin/ld: /usr/lib/debug/usr/lib/x86_64-linux-gnu/crt1.o(.debug_info): relocation 2 has invalid symbol index 2 /usr/bin/l…

Accepted Answer

You should first understand that the ABI for x86_64 has several different "models": small, kernel, medium, and large. These are described under the GCC `-mcmodel` option here: http://gcc.gnu.org/onlinedocs/gcc/i386-and-x86-64-Options.html

What you're experiencing is that `crt1.o`, the startup code that gets linked into every program that's responsible for taking the initial ELF register/stack state and passing them into the libc startup code which eventually calls `main`, seems to be using the small model. You can see here:

```
/usr/lib/x86_64-linux-gnu/crt1.o: In function `_start':
(.text+0x20): relocation truncated to fit: R_X86_64_32S against symbol `main' defined in    .text section in /tmp/ccFshK69.o
```

What's happening is that `crt1.o` has a relocation for the address of `main` that only allows for a 32-bit address to be filled in. (Note: Even if `main` were defined in a shared library rather than the executable, there would be a PLT entry in the executable, and the address of this PLT entry would be the official address of `main` to which the relocation would be resolved.)

To solve this problem, you would need a `crt1.o` that can handle full 64-bit addresses. One way to get this might be by using `Scrt1.o`, which is normally only used for PIE executables, instead of `crt1.o`. You could probably achieve this with `-nostartfiles` and manually specifying all the start files on the link command line. It might be worth filing a bug report against glibc requesting that the x86_64 `crt1.o` be converted to "large model" so that it works with main programs not linked in the 32-bit range.

Note that you'll probably also need `-mcmodel=large` (or perhaps `-fPIE` would work) for all of your own code to make it link and work correctly at high addresses. This could make it considerably larger and slower. You might want to rethink why you're doing this and whether you really need to.

Linker error on relocating a program above 2GB in x86_64 linux?

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