the file-level crt_arch.h asm fragments generally make direct (non-PLT) calls from _start to _start_c, which is only valid when there is a local, non-interposable definition for _start_c. generally, the linker is expected to know that local definitions in a main executable (as opposed to shared library) output are non-interposable, making this work, but historically there have been linker bugs in this area, and microblaze is reportedly still broken, flagging the relocation for the call as a textrel. the equivalent _dlstart_c, called from the same crt_arch.h asm fragments, has always used hidden visibility without problem, and semantically it should be hidden, so make it hidden. this ensures the direct call is always valid regardless of whether the linker properly special-cases main executable output.
after commit a48ccc159a5fa061a18419296100ee48a1cd6cc9 removed the use of _Noreturn on the stage3_func type (which only worked due to it being defined to the "GNU C" attribute in C99 mode), GCC could no longer assume that the ends of __dls2 and __dls2b are unreachable, and produced a warning that a function marked _Noreturn returns. also, since commit 4390383b32250a941ec616e8bff6f568a801b1c0, the _Noreturn declaration for __libc_start_main in crt1/rcrt1 has been not only inconsistent with the definition, but wrong. formally, __libc_start_main does return, via a (hopefully) tail call to a helper function after the barrier. incorrect usage of _Noreturn in the declaration was probably formal UB. the _Noreturn specifiers were not useful in any of these places, so remove them all. now, the only remaining usage of _Noreturn is in public interfaces where _Noreturn is part of their contract.
this cleans up what had become widespread direct inline use of "GNU C" style attributes directly in the source, and lowers the barrier to increased use of hidden visibility, which will be useful to recovering some of the efficiency lost when the protected visibility hack was dropped in commit dc2f368e565c37728b0d620380b849c3a1ddd78f, especially on archs where the PLT ABI is costly.
this overhaul further reduces the amount of arch-specific code needed by the dynamic linker and removes a number of assumptions, including: - that symbolic function references inside libc are bound at link time via the linker option -Bsymbolic-functions. - that libc functions used by the dynamic linker do not require access to data symbols. - that static/internal function calls and data accesses can be made without performing any relocations, or that arch-specific startup code handled any such relocations needed. removing these assumptions paves the way for allowing libc.so itself to be built with stack protector (among other things), and is achieved by a three-stage bootstrap process: 1. relative relocations are processed with a flat function. 2. symbolic relocations are processed with no external calls/data. 3. main program and dependency libs are processed with a fully-functional libc/ldso. reduction in arch-specific code is achived through the following: - crt_arch.h, used for generating crt1.o, now provides the entry point for the dynamic linker too. - asm is no longer responsible for skipping the beginning of argv[] when ldso is invoked as a command. - the functionality previously provided by __reloc_self for heavily GOT-dependent RISC archs is now the arch-agnostic stage-1. - arch-specific relocation type codes are mapped directly as macros rather than via an inline translation function/switch statement.
the only immediate effect of this commit is enabling PIE support on some archs that did not previously have any Scrt1.s, since the existing asm files for crt1 override this C code. so some of the crt_arch.h files committed are only there for the sake of documenting what their archs "would do" if they used the new C-based crt1. the expectation is that new archs should use this new system rather than using heavy asm for crt1. aside from being easier and less error-prone, it also ensures that PIE support is available immediately (since Scrt1.o is generated from the same C source, using -fPIC) rather than having to be added as an afterthought in the porting process.