The addresses of NOPs are collected in a section named __patchable_function_entries. A __patchable_function_entries entry is relocated by a symbolic relocation (e.g. R_X86_64_64, R_AARCH64_ABS64, R_PPC64_ADDR64). In -shared or -pie mode, the linker will create a dynamic relocation (non-preemptible: relative relocation (e.g. R_X86_64_RELATIVE); preemptible: symbolic relocation (e.g. R_X86_64_64)).
In either case, the section contents will be modified at runtime. Thus, the section should have the SHF_WRITE flag to avoid text relocations (DF_TEXTREL). When -ffunction-sections is used, ideally GCC should emit one __patchable_function_entries (SHF_LINK_ORDER) per .text.foo . If the corresponding .text.foo is discarded (--gc-sections, COMDAT, /DISCARD/), the linker can discard the associated __patchable_function_entries. This can be seen as a lightweight COMDAT section group. (A section group adds an extra section and costs 3 words) Currently lld (LLVM linker) has implemented such SHF_LINK_ORDER collecting features. GNU ld and gold don't have the features. I have summarized the feature requests in this post https://sourceware.org/ml/binutils/2019-11/msg00266.html gcc -fpatchable-function-entry=2 -ffunction-sections -c a.c [ 4] .text.f0 PROGBITS 0000000000000000 000040 000009 00 AX 0 0 1 ### No W flag ### One __patchable_function_entries instead of 3. [ 5] __patchable_function_entries PROGBITS 0000000000000000 000049 000018 00 A 0 0 1 [ 6] .rela__patchable_function_entries RELA 0000000000000000 000280 000048 18 I 13 5 8 [ 7] .text.f1 PROGBITS 0000000000000000 000061 000009 00 AX 0 0 1 [ 8] .text.f2 PROGBITS 0000000000000000 00006a 000009 00 AX 0 0 1
