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2015-11-11fix dynamic loader library mapping for nommu systemsRich Felker-0/+4
on linux/nommu, non-writable private mappings of files may actually use memory shared with other processes or the fs cache. the old nommu loader code (used when mmap with MAP_FIXED fails) simply wrote over top of the original file mapping, possibly clobbering this shared memory. no such breakage was observed in practice, but it should have been possible. the new code starts by mapping anonymous writable memory on archs that might support nommu, then maps load segments over top of it, falling back to read if MAP_FIXED fails. we use an anonymous map rather than a writable file map to avoid reading more data from disk than needed. since pages cannot be loaded lazily on fault, in case of large data/bss, mapping the full file may read a lot of data that will subsequently be thrown away when processing additional LOAD segments. as a result, we cannot skip the first LOAD segment when operating in this mode. these changes affect only non-FDPIC nommu support.
2015-09-22add real fdpic loading of shared librariesRich Felker-0/+4
previously, the normal ELF library loading code was used even for fdpic, so only the kernel-loaded dynamic linker and main app could benefit from separate placement of segments and shared text.
2015-09-22add general fdpic support in dynamic linker and arch support for shRich Felker-3/+14
at this point not all functionality is complete. the dynamic linker itself, and main app if it is also loaded by the kernel, take advantage of fdpic and do not need constant displacement between segments, but additional libraries loaded by the dynamic linker follow normal ELF semantics for mapping still. this fully works, but does not admit shared text on nommu. in terms of actual functional correctness, dlsym's results are presently incorrect for function symbols, RTLD_NEXT fails to identify the caller correctly, and dladdr fails almost entirely. with the dynamic linker entry point working, support for static pie is automatically included, but linking the main application as ET_DYN (pie) probably does not make sense for fdpic anyway. ET_EXEC is equally relocatable but more efficient at representing relocations.
2015-09-17add fdpic structs and reloc types for dynamic linkingRich Felker-0/+16
2015-05-25move call to dynamic linker stage-3 into stage-2 functionRich Felker-1/+1
this move eliminates a duplicate "by-hand" symbol lookup loop from the stage-1 code and replaces it with a call to find_sym, which can be used once we're in stage 2. it reduces the size of the stage 1 code, which is helpful because stage 1 will become the crt start file for static-PIE executables, and it will allow stage 3 to access stage 2's automatic storage, which will be important in an upcoming commit.
2015-04-13dynamic linker bootstrap overhaulRich Felker-0/+57
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.