Moving the ELF64 relocation table

2
votes

I'm attempting to move the .rela.plt relocation table of a shared object to the end of the file (Because I'm going to add new relocation entries in the future).

First, I read all entries in the .rela.plt section. The original section is at offset 0x528, and .rela.dyn is at 0x600. Then I copy the contents of .rela.dyn at the end of the file; to the offset 0xa528. I update the section header table entry for .rela.dyn by adding 0xa000 to both the sh_addr and sh_offset fields. I update the DT_RELA entry in the dynamic table with the new address; and finally I add a LOAD segment (with permissions RWX) to the end of the program header table.

As a result, R_X86_64_RELATIVE entries in the relocation table are resolved correctly; but the dynamic linker does not update the addresses for external calls in the GOT, therefore causing a segmentation fault. This does not happen in 32-bit shared objects.

What am I missing?

1
shared-librariesldelfrelocation

1 Answers

1
votes

I solved the problem by adding the .rel.plt table right after the .rel.dyn table. That is, there should not be a single redundant byte in between; no alignment padding, no other sections, no whatever. The dynamic loader expects the relocation segments to be continuous for some reason. The JMPREL entry in the dynamic section seems redundant at x64.