Hello! Currently I develop Dynamic loader for one Nuttx fork for ARM architecture. What do I want? Load ELF files at runtime. Execute code from FLASH (XIP). No relocations into text section. (for sharing .text section between apps) What do I have? 1) .text section is placed into FLASH 2)Function calls are sent to plt: (test_func tries to print "Hello World!") 0000028c <test_func>: ... 290: e59f300c ldr r3, [pc, #12] ; 2a4 <test_func+0x18> # Get GOT-index of "Hello World!" string 294: e79a0003 ldr r0, [sl, r3] # "sl" - addr of GOT. Load "Hello world!" pointer to r0 and pass it to puts() 298: ebfffff0 bl 260 <puts@plt> ... All seems OK. Code is position independent. GOT is placed into RAM and after load-time relocations has correct addresses of all stuff. Important thing: 298: ebfffff0(!!!) bl 260 <puts@plt> Branch to puts@plt is relative. So, PLT must be placed into FLASH too. With PLT into FLASH I have a problem: compiler generates pc-relative code! 00000260 <puts@plt>: 260: e28fc600 add ip, pc, #0, 12 264: e28cca00 add ip, ip, #0, 20 268: e5bcf0f0 ldr pc, [ip, #240]! ; 0xf0 With pc-relative code into PLT, GOT must be placed into particular address into RAM, but MMU-less system can't provide such opportunity. I have found example of code for PLT functions into presentation [ https://www.slideshare.net/linaroorg/sfo15406-arm-fdpic-toolset-kernel-libraries-for-cortexm-cortexr-mmuless-cores ]: plt(foo): ldr ip, .L1 # foo's descriptor offset add ip, ip, r9 # from caller's FDT ldr r9, [ip, #4] # foo's data address ldr pc, [ip] # foo's code address L1. word foo(GOTOFFFUNCDESC) Such code is exactly what I want. It seems, they developed a number of patches, but they were not merged into any compiler's upstream I've tried: - arm-none-eabi-gcc - linaro - clang Used compiler keys: -fpic -mpic-register=r10 -msingle-pic-base -mno-pic-data-is-text-relative How can I obtain PLT code mentiont above? P.S. I desired something like -mno-plt flag to disable PLT and I will have probably not too fast, but at least suitable for XIP code. But such flag is implemented only for MIPS, as I can see. Is there similar flag for ARM? Many thanks, Dmitrii Rodionov
