Change Details

Currently RTLD is linked against libc_nossp_pic which means that any libc symbol used in rtld can pull in a lot of depedencies. This was causing symbol such as __libc_interposing and all the pthread stubs to be included in RTLD even though they are not required. It turns out most of these dependencies can easily be avoided by providing overrides inside of rtld. This change is motivated by CHERI, where we have an experimental ABI that requires additional relocation processing to allow the use of function pointers inside of rtld. Instead of adding this self-relocation code to RTLD I attempted to remove most function pointers from RTLD and discovered that most of them came from the libc dependencies instead of being actually used inside rtld. A nice side-effect of this change is that rtld is now 18% smaller on amd64. text data bss dec hex filename 0x21eb6 0xce0 0xe60 145910 239f6 /home/alr48/ld-elf-x86.before.so.1 0x1be36 0x728 0xdd8 119606 1d336 /home/alr48/ld-elf-x86.after.so.1 The number of R_X86_64_RELATIVE relocations that need to be processed on startup has also gone down from 368 to 187 (almost 50% less).

Currently RTLD is linked against libc_nossp_pic which means that any libc symbol used in rtld can pull in a lot of depedencies. This was causing symbol such as __libc_interposing and all the pthread stubs to be included in RTLD even though they are not required. It turns out most of these dependencies can easily be avoided by providing overrides inside of rtld. This change is motivated by CHERI, where we have an experimental ABI that requires additional relocation processing to allow the use of function pointers inside of rtld. Instead of adding this self-relocation code to RTLD I attempted to remove most function pointers from RTLD and discovered that most of them came from the libc dependencies instead of being actually used inside rtld. A nice side-effect of this change is that rtld is now 18% smaller on amd64. text data bss dec hex filename 0x21eb6 0xce0 0xe60 145910 239f6 /home/alr48/ld-elf-x86.before.so.1 0x1a6ed 0x728 0xdd8 113645 1bbed /home/alr48/ld-elf-x86.after.so.1 The number of R_X86_64_RELATIVE relocations that need to be processed on startup has also gone down from 368 to 187 (almost 50% less).

Currently RTLD is linked against libc_nossp_pic which means that any libc symbol used in rtld can pull in a lot of depedencies. This was causing symbol such as __libc_interposing and all the pthread stubs to be included in RTLD even though they are not required. It turns out most of these dependencies can easily be avoided by providing overrides inside of rtld. This change is motivated by CHERI, where we have an experimental ABI that requires additional relocation processing to allow the use of function pointers inside of rtld. Instead of adding this self-relocation code to RTLD I attempted to remove most function pointers from RTLD and discovered that most of them came from the libc dependencies instead of being actually used inside rtld. A nice side-effect of this change is that rtld is now 18% smaller on amd64. text data bss dec hex filename 0x21eb6 0xce0 0xe60 145910 239f6 /home/alr48/ld-elf-x86.before.so.1 0x1be360x1a6ed 0x728 0xdd8 1196063645 1d336bbed /home/alr48/ld-elf-x86.after.so.1 The number of R_X86_64_RELATIVE relocations that need to be processed on startup has also gone down from 368 to 187 (almost 50% less).