commit de2b67f8 introduced a
regression by adding a -include option to CFLAGS_AUTO which did not
work without additional -I options. this broke subsequent trycppif
tests and caused x86_64 to be misdetected as x32, among other issues.
simply using the full relative pathname to vis.h rather than -I is the
cleanest way to fix the problem.

this is implemented via the build system and does not affect source
files. the idea is to use protected or hidden visibility to prevent
the compiler from pessimizing function calls within a shared (or
position-independent static) libc in the form of overhead setting up
for a call through the PLT. the ld-time symbol binding via the
-Bsymbolic-functions option already optimized out the PLT itself, but
not the code in the caller needed to support a call through the PLT.
on some archs this overhead can be substantial; on others it's
trivial.

analogous to commit 646cb9a4 for sh.

these are perfectly fine with ld-time symbol binding, but otherwise
result in textrels. they cannot be replaced with @PLT jump targets
because the PLT thunks require a GOT register to be setup, so use a
hidden alias instead.

analogous to commit 646cb9a4 for sh.

these are perfectly fine with ld-time symbol binding, but if the calls
go through a PLT thunk, they are invalid because the caller does not
setup a GOT register. use a hidden alias to bypass the issue.

analogous to commit 8ed66ecb for i386.

none of these are actual textrels because of ld-time binding performed
by -Bsymbolic-functions, but I'm changing them with the goal of making
ld-time binding purely an optimization rather than relying on it for
semantic purposes.

in the case of memmove's call to memcpy, making it explicit that the
memmove asm is assuming the forward-copying behavior of the memcpy asm
is desirable anyway; in case memcpy is ever changed, the semantic
mismatch would be apparent while editing memmcpy.s.

this fixes truncation of error messages containing long pathnames or
symbol names.

the dlerror state was previously required by POSIX to be global. the
resolution of bug 97 relaxed the requirements to allow thread-safe
implementations of dlerror with thread-local state and message buffer.

this reverts the commit f29fea00
which was based on a bug in C99 and POSIX and did not match IEEE-754
http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1515.pdf

these functions are never called directly; only their addresses are
used, so PLT indirections should never happen unless a broken
application tries to redefine them, but it's still best to make them
hidden.

this change is made in preparation to support linking without
-Bsymbolic-functions.

new in linux v4.0, commit 9791554b45a2acc28247f66a5fd5bbc212a6b8c8
used to work around a floating-point abi issue on mips

new in linux v3.19, commit fe3d197f84319d3bce379a9c0dc17b1f48ad358c
used for on-demand kernel allocation of bounds tables for mpx on x86

new in linux v4.0, commit ad6f939ab193750cc94a265f58e007fb598c97b7

syscall number was reserved in linux v4.0, kernel commit
add4b1b02da7e7ec35c34dd04d351ac53f3f0dd8

the casts of the argument to unsigned int suppressed diagnosis of
errors like passing a pointer instead of a character. putting the
actual function call in an unreachable branch restores any diagnostics
that would be present if the macros didn't exist and functions were
used.

commit 5fc14878 attempted to fix it,
but neglected the fact that mips has branch delay slots.

the braf instruction's destination register is an offset from the
address of the braf instruction plus 4 (or equivalently, the address
of the next instruction after the delay slot). the code for dlsym was
incorrectly computing the offset to pass using the address of the
delay slot itself. in other places, a label was placed after the delay
slot, but I find this confusing. putting the label on the branch
instruction itself, and manually adding 4, makes it more clear which
branch the offset in the constant pool goes with.

even hidden functions need @PLT symbol references; otherwise an
absolute address is produced instead of a PC-relative one.

this caused the dynamic linker/startup code to abort when r0 happened
to contain a negative value.

the conventional way to implement sigsetjmp is to save the signal mask
then tail-call to setjmp; siglongjmp then restores the signal mask and
calls longjmp. the problem with this approach is that a signal already
pending, or arriving between unmasking of signals and restoration of
the saved stack pointer, will have its signal handler run on the stack
that was active before siglongjmp was called. this can lead to
unbounded stack usage when siglongjmp is used to leave a signal
handler.

in the new design, sigsetjmp saves its own return address inside the
extended part of the sigjmp_buf (outside the __jmp_buf part used by
setjmp) then calls setjmp to save a jmp_buf inside its own execution.
it then tail-calls to __sigsetjmp_tail, which uses the return value of
setjmp to determine whether to save the current signal mask or restore
a previously-saved mask.

as an added bonus, this design makes it so that siglongjmp and longjmp
are identical. this is useful because the __longjmp_chk function we
need to add for ABI-compatibility assumes siglongjmp and longjmp are
the same, but for different reasons -- it was designed assuming either
can access a flag just past the __jmp_buf indicating whether the
signal masked was saved, and act on that flag. however, early versions
of musl did not have space past the __jmp_buf for the non-sigjmp_buf
version of jmp_buf, so our setjmp cannot store such a flag without
risking clobbering memory on (very) old binaries.

previously, the dynamic tlsdesc lookup functions and the i386
special-ABI ___tls_get_addr (3 underscores) function called
__tls_get_addr when the slot they wanted was not already setup;
__tls_get_addr would then in turn also see that it's not setup and
call __tls_get_new.

calling __tls_get_new directly is both more efficient and avoids the
issue of calling a non-hidden (public API/ABI) function from asm.

for the special i386 function, a weak reference to __tls_get_new is
used since this function is not defined when static linking (the code
path that needs it is unreachable in static-linked programs).

applying the attribute to a weak_alias macro was a hack. instead use a
separate declaration to apply the visibility, and consolidate
declarations together to avoid having visibility mess all over the
file.

in a few places, non-hidden symbols were referenced from asm in ways
that assumed ld-time binding. while these is no semantic reason these
symbols need to be hidden, fixing the references without making them
hidden was going to be ugly, and hidden reduces some bloat anyway.

in the asm files, .global/.hidden directives have been moved to the
top to unclutter the actual code.

at the point of call it was declared hidden, but the definition was
not hidden. for some toolchains this inconsistency produced textrels
without ld-time binding.

otherwise the call instruction in the inline syscall asm results in
textrels without ld-time binding.

otherwise the call/jump from the crt_arch.h asm may not resolve
correctly without -Bsymbolic-functions.

the zero initialization is redundant since decode_vec does its own
clearing, and it increases the risk that buggy compilers will generate
calls to memset. as long as symbols are bound at ld time, such a call
will not break anything, but it may be desirable to turn off ld-time
binding in the future.

this was already essentially possible as a result of the previous
commits changing the dynamic linker/thread pointer bootstrap process.
this commit mainly adds build system infrastructure:

configure no longer attempts to disable stack protector. instead it
simply determines how so the makefile can disable stack protector for
a few translation units used during early startup.

stack protector is also disabled for memcpy and memset since compilers
(incorrectly) generate calls to them on some archs to implement
struct initialization and assignment, and such calls may creep into
early initialization.

no explicit attempt to enable stack protector is made by configure at
this time; any stack protector option supported by the compiler can be
passed to configure in CFLAGS, and if the compiler uses stack
protector by default, this default is respected.

since 1.1.0, musl has nominally required a thread pointer to be setup.
most of the remaining code that was checking for its availability was
doing so for the sake of being usable by the dynamic linker. as of
commit 71f099cb, this is no longer
necessary; the thread pointer is now valid before any libc code
(outside of dynamic linker bootstrap functions) runs.

this commit essentially concludes "phase 3" of the "transition path
for removing lazy init of thread pointer" project that began during
the 1.1.0 release cycle.

this allows the dynamic linker itself to run with a valid thread
pointer, which is a prerequisite for stack protector on archs where
the ssp canary is stored in TLS. it will also allow us to remove some
remaining runtime checks for whether the thread pointer is valid.

as long as the application and its libraries do not require additional
size or alignment, this early thread pointer will be kept and reused
at runtime. otherwise, a new static TLS block is allocated after
library loading has finished and the thread pointer is switched over.

previously, the layout of the static TLS block was perturbed by the
size of the dtv; dtv size increasing from 0 to 1 perturbed both TLS
arch types, and the TLS-above-TP type's layout was perturbed by the
specific number of dtv slots (libraries with TLS). this behavior made
it virtually impossible to setup a tentative thread pointer address
before loading libraries and keep it unchanged as long as the
libraries' TLS size/alignment requirements fit.

the new code fixes the location of the dtv and pthread structure at
opposite ends of the static TLS block so that they will not move
unless size or alignment changes.

previously a new GDT slot was requested, even if one had already been
obtained by a previous call. instead extract the old slot number from
GS and reuse it if it was already set. the formula (GS-3)/8 for the
slot number automatically yields -1 (request for new slot) if GS is
zero (unset).

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.