this assumption is borderline-unsafe to begin with, and fails badly
with -ffunction-sections since the linker can move the callee
arbitrarily far away when it lies in a different section.
the entry point code supports being loaded by a loader which is not
fdpic-aware (in practice, either kernel with mmu or qemu without fdpic
support). this mostly just works, but signal handling will wrongly use
a function descriptor address as a code address if the personality is
not adjusted to fdpic.
ideally this code could be placed with sigaction so that it's not
needed except if/when a signal handler is installed. however,
personality is incorrectly maintained per-thread by the kernel, rather
than per-process, so it's necessary to correct the personality before
any threads are started. also, in order to skip the personality
syscall when an fdpic-aware loader is used, we need to be able to
detect how the program was loaded, and this information is only
readily available at the entry point.
the __fdpic_fixup code is not needed for ET_DYN executables, which
instead use reloctions, so we can omit it from the dynamic linker and
static-pie entry point and save some code size.
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.
this version of the entry point is only suitable for static linking in
ET_EXEC form. neither dynamic linking nor pie is supported yet. at
some point in the future the fdpic and non-fdpic versions of this code
may be unified but for now it's easiest to work with them separately.
i386 and x86_64 versions already had the .text directive; other archs
did not. normally, top-level (file scope) __asm__ starts in the .text
section anyway, but problems were reported with some versions of
clang, and it seems preferable to set it explicitly anyway, at least
for the sake of consistency between archs.
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.
Applications ended up with copy relocations for this array, which
resulted in libc's references to this array pointing to the
application's copy. The dynamic linker, however, can require this array
before the application is relocated, and therefore before the
application's copy of this array is initialized. This resulted in
garbage being loaded into FPSCR before executing main, which violated
the ABI.
We fix this by putting the array in crt1 and making the libc copy
private. This prevents libc's reference to the array from pointing to
an uninitialized copy in the application.
linux, gcc, etc. all use "sh" as the name for the superh arch. there
was already some inconsistency internally in musl: the dynamic linker
was searching for "ld-musl-sh.path" as its path file despite its own
name being "ld-musl-superh.so.1". there was some sentiment in both
directions as to how to resolve the inconsistency, but overall "sh"
was favored.
Rich Felker
Bobby Bingham