the old test was broken in that it would never fail on a toolchains built
without dynamic linking support, leading to the wrapper script possibly being
installed on compilers that do not support it. in addition, the new test is
portable across compilers: the old test only worked on GCC.

the new test works by testing whether the toolchain libc defines __GLIBC__:
most non-musl Linux libc's do define this for compatibility even when they
are not glibc, so this is a safe bet to check for musl. in addition, the
compiler runtime would need to have a somewhat glibc-compatible ABI in the
first place, so any non-glibc compatible libc's compiler runtime might not
work. it is safer to disable these cases by default and have the user enable
the wrappers manually there using --enable-wrapper if they certain it works.

this overhauls part of the build system in order to support multiple
toolchain wrapper scripts, as opposed to solely the musl-gcc wrapper as
before. it thereby replaces --enable-gcc-wrapper with --enable-wrapper=...,
which has the options 'auto' (the default, detect whether to use wrappers),
'all' (build and install all wrappers), 'no' (don't build any) and finally
the options named after the individual compiler scripts (currently only
'gcc' is available) to build and install only that wrapper.
the old --enable-gcc-wrapper is removed from --help, but still available.

it also modifies the wrappers to use the C compiler specified to the build
system as 'inner' compiler, when applicable. as wrapper detection works by
probing this compiler, it may not work with any other.

this improves compatibility with the behavior of other systems and
with some applications which set an empty TZ var to disable use of
local time by mktime, etc.

The callers need to check the value of the pointer anyway, so make
them pass the pointer to gnu_lookup instead of reloading it there.

Reorder gnu_lookup arguments so that always-used ones are listed
first. GCC can choose a calling convention with arguments in registers
(e.g. up to 3 arguments in eax, ecx, edx on x86), but cannot reorder
the arguments for static functions.

Do not reference dso->syms and dso->strings until point of use.
Check 'h1 == (h2|1)', the simplest condition, before the others.

Introduce gnu_lookup_filtered and use it to speed up symbol lookups in
find_sym (do_dlsym is left as is, based on an expectation that
frequently dlsym queries will use a dlopen handle rather than
RTLD_NEXT or RTLD_DEFAULT, and will not need to look at more than one
DSO).

With -Os, GCC uses a multiply rather than a shift and addition for 'h*33'.
Use a more efficient expression explicitely.

the TLS ABI spec for mips, powerpc, and some other (presently
unsupported) RISC archs has the return value of __tls_get_addr offset
by +0x8000 and the result of DTPOFF relocations offset by -0x8000. I
had previously assumed this part of the ABI was actually just an
implementation detail, since the adjustments cancel out. however, when
the local dynamic model is used for accessing TLS that's known to be
in the same DSO, either of the following may happen:

1. the -0x8000 offset may already be applied to the argument structure
passed to __tls_get_addr at ld time, without any opportunity for
runtime relocations.

2. __tls_get_addr may be used with a zero offset argument to obtain a
base address for the module's TLS, to which the caller then applies
immediate offsets for individual objects accessed using the local
dynamic model. since the immediate offsets have the -0x8000 adjustment
applied to them, the base address they use needs to include the
+0x8000 offset.

it would be possible, but more complex, to store the pointers in the
dtv[] array with the +0x8000 offset pre-applied, to avoid the runtime
cost of adding 0x8000 on each call to __tls_get_addr. this change
could be made later if measurements show that it would help.

previously, loading of additional libraries beyond libc/ldso did not
work on nommu kernels, nor did loading programs via invocation of the
dynamic linker as a command.

this interface is non-standardized and is a GNU invention, and as
such, our implementation should match the behavior of the GNU
function. one peculiarity the old implementation got wrong was the
handling of all-zero digit sequences: they are supposed to compare
greater than digit sequences of which they are a proper prefix, as in
009 < 00.

in addition, high bytes were treated with char signedness rather than
as unsigned. this was wrong regardless of what the GNU function does
since the resulting order relation varied by arch.

the new strverscmp implementation makes explicit the cases where the
order differs from what strcmp would produce, of which there are only
two.

commit ba819787 introduced this
regression. since the __malloc0 weak alias was not properly provided
by __simple_malloc, use of calloc forced the full malloc to be linked.

previously, calloc's implementation encoded assumptions about the
implementation of malloc, accessing a size_t word just prior to the
allocated memory to determine if it was obtained by mmap to optimize
out the zero-filling. when __simple_malloc is used (static linking a
program with no realloc/free), it doesn't matter if the result of this
check is wrong, since all allocations are zero-initialized anyway. but
the access could be invalid if it crosses a page boundary or if the
pointer is not sufficiently aligned, which can happen for very small
allocations.

this patch fixes the issue by moving the zero-fill logic into malloc.c
with the full malloc, as a new function named __malloc0, which is
provided by a weak alias to __simple_malloc (which always gives
zero-filled memory) when the full malloc is not in use.

this symbol is needed only on archs where the PLT call ABI is klunky,
and only for position-independent code compiled with stack protector.
thus references usually only appear in shared libraries or PIE
executables, but they can also appear when linking statically if some
of the object files being linked were built as PIC/PIE.

normally libssp_nonshared.a from the compiler toolchain should provide
__stack_chk_fail_local, but reportedly it appears prior to -lc in the
link order, thus failing to satisfy references from libc itself (which
arise only if libc.a was built as PIC/PIE with stack protector
enabled).

linux kernel commit 46e12c07b3b9603c60fc1d421ff18618241cb081 caused
the mips syscall mechanism to fail with EFAULT when the userspace
stack pointer is invalid, breaking __unmapself used for detached
thread exit. the workaround is to set $sp to a known-valid, readable
address, and the simplest one to obtain is the address of the current
function, which is available (per o32 calling convention) in $25.

this error simply indicated a system without memory protection (NOMMU)
and should not cause failure in the caller.

nominally the low bits of the trap number on sh are the number of
syscall arguments, but they have never been used by the kernel, and
some code making syscalls does not even know the number of arguments
and needs to pass an arbitrary high number anyway.

sh3/sh4 traditionally used the trap range 16-31 for syscalls, but part
of this range overlapped with hardware exceptions/interrupts on sh2
hardware, so an incompatible range 32-47 was chosen for sh2.

using trap number 31 everywhere, since it's in the existing sh3/sh4
range and does not conflict with sh2 hardware, is a proposed
unification of the kernel syscall convention that will allow binaries
to be shared between sh2 and sh3/sh4. if this is not accepted into the
kernel, we can refit the sh2 target with runtime selection mechanisms
for the trap number, but doing so would be invasive and would entail
non-trivial overhead.

due to the way the interrupt and syscall trap mechanism works,
userspace on sh2 must never set the stack pointer to an invalid value.
thus, the approach used on most archs, where __unmapself executes with
no stack for the interval between SYS_munmap and SYS_exit, is not
viable on sh2.

in order not to pessimize sh3/sh4, the sh asm version of __unmapself
is not removed. instead it's renamed and redirected through code that
calls either the generic (safe) __unmapself or the sh3/sh4 asm,
depending on compile-time and run-time conditions.

the sh2 target is being considered an ISA subset of sh3/sh4, in the
sense that binaries built for sh2 are intended to be usable on later
cpu models/kernels with mmu support. so rather than hard-coding
sh2-specific atomics, the runtime atomic selection mechanisms that was
already in place has been extended to add sh2 atomics.

at this time, the sh2 atomics are not SMP-compatible; since the ISA
lacks actual atomic operations, the new code instead masks interrupts
for the duration of the atomic operation, producing an atomic result
on single-core. this is only possible because the kernel/hardware does
not impose protections against userspace doing so. additional changes
will be needed to support future SMP systems.

care has been taken to avoid producing significant additional code
size in the case where it's known at compile-time that the target is
not sh2 and does not need sh2-specific code.

functions which open in-memory FILE stream variants all shared a tail
with __fdopen, adding the FILE structure to stdio's open file list.
replacing this common tail with a function call reduces code size and
duplication of logic. the list is also partially encapsulated now.

function signatures were chosen to facilitate tail call optimization
and reduce the need for additional accessor functions.

with these changes, static linked programs that do not use stdio no
longer have an open file list at all.

this patch activates the new byte-based C locale (high bytes treated
as abstract code unit "characters" rather than decoded as multibyte
characters) by making the value of MB_CUR_MAX depend on the active
locale. for the C locale, the LC_CTYPE category pointer is null,
yielding a value of 1. all other locales yield a value of 4.

this patch adjusts libc components which use the multibyte functions
internally, and which depend on them operating in a particular
encoding, to make the appropriate locale changes before calling them
and restore the calling thread's locale afterwards. activating the
byte-based C locale without these changes would cause regressions in
stdio and iconv.

in the case of iconv, the current implementation was simply using the
multibyte functions as UTF-8 conversions. setting a multibyte UTF-8
locale for the duration of the iconv operation allows the code to
continue working.

in the case of stdio, POSIX requires that FILE streams have an
encoding rule bound at the time of setting wide orientation. as long
as all locales, including the C locale, used the same encoding,
treating high bytes as UTF-8, there was no need to store an encoding
rule as part of the stream's state.

a new locale field in the FILE structure points to the locale that
should be made active during fgetwc/fputwc/ungetwc on the stream. it
cannot point to the locale active at the time the stream becomes
oriented, because this locale could be mutable (the global locale) or
could be destroyed (locale_t objects produced by newlocale) before the
stream is closed. instead, a pointer to the static C or C.UTF-8 locale
object added in commit commit aeeac9ca
is used. this is valid since categories other than LC_CTYPE will not
affect these functions.

this patch makes the functions which work directly on multibyte
characters treat the high bytes as individual abstract code units
rather than as multibyte sequences when MB_CUR_MAX is 1. since
MB_CUR_MAX is presently defined as a constant 4, all of the new code
added is dead code, and optimizing compilers' code generation should
not be affected at all. a future commit will activate the new code.

as abstract code units, bytes 0x80 to 0xff are represented by wchar_t
values 0xdf80 to 0xdfff, at the end of the surrogates range. this
ensures that they will never be misinterpreted as Unicode characters,
and that all wctype functions return false for these "characters"
without needing locale-specific logic. a high range outside of Unicode
such as 0x7fffff80 to 0x7fffffff was also considered, but since C11's
char16_t also needs to be able to represent conversions of these
bytes, the surrogate range was the natural choice.

btowc is required to interpret its argument by conversion to unsigned
char, unless the argument is equal to EOF. since the conversion to
produces a non-character value anyway, we can just unconditionally
convert, for now.

vdso will be available on arm in linux v4.2, the user-space code
for it is in kernel commit 8512287a8165592466cb9cb347ba94892e9c56a5

this extends the brk/stack collision protection added to full malloc
in commit 276904c2 to also protect the
__simple_malloc function used in static-linked programs that don't
reference the free function.

it also extends support for using mmap when brk fails, which full
malloc got in commit 54463033, to
__simple_malloc.

since __simple_malloc may expand the heap by arbitrarily large
increments, the stack collision detection is enhanced to detect
interval overlap rather than just proximity of a single address to the
stack. code size is increased a bit, but this is partly offset by the
sharing of code between the two malloc implementations, which due to
linking semantics, both get linked in a program that needs the full
malloc with realloc/free support.

commit 58165923 added these optional
cancellation points on the basis that cancellable stdio could be
useful, to unblock threads stuck on stdio operations that will never
complete. however, the only way to ensure that cancellation can
achieve this is to violate the rules for side effects when
cancellation is acted upon, discarding knowledge of any partial data
transfer already completed. our implementation exhibited this behavior
and was thus non-conforming.

in addition to improving correctness, removing these cancellation
points moderately reduces code size, and should significantly improve
performance on i386, where sysenter/syscall instructions can be used
instead of "int $128" for non-cancellable syscalls.

the old idiom, f->mode |= f->mode+1, was adapted from the idiom for
setting byte orientation, f->mode |= f->mode-1, but the adaptation was
incorrect. unless the stream was alreasdy set byte-oriented, this code
incremented f->mode each time it was executed, which would eventually
lead to overflow. it could be fixed by changing it to f->mode |= 1,
but upcoming changes will require slightly more work at the time of
wide orientation, so it makes sense to just call fwide. as an
optimization in the single-character functions, fwide is only called
if the stream is not already wide-oriented.

this is undefined, but supported in our implementation of the normal
printf, so for consistency the wide variant should support it too.

this can be used to put off writing an asm version of __unmapself for
new archs, or as a permanent solution on archs where it's not
practical or even possible to run momentarily with no stack.

the concept here is simple: the caller takes a lock on a global shared
stack and uses it to make the munmap and exit syscalls. the only trick
is unlocking, which must be done after the thread exits, and this is
achieved by using the set_tid_address syscall to have the kernel zero
and futex-wake the lock word as part of the exit syscall.

the linux/nommu fdpic ELF loader sets up the brk range to overlap
entirely with the main thread's stack (but growing from opposite
ends), so that the resulting failure mode for malloc is not to return
a null pointer but to start returning pointers to memory that overlaps
with the caller's stack. needless to say this extremely dangerous and
makes brk unusable.

since it's non-trivial to detect execution environments that might be
affected by this kernel bug, and since the severity of the bug makes
any sort of detection that might yield false-negatives unsafe, we
instead check the proximity of the brk to the stack pointer each time
the brk is to be expanded. both the main thread's stack (where the
real known risk lies) and the calling thread's stack are checked. an
arbitrary gap distance of 8 MB is imposed, chosen to be larger than
linux default main-thread stack reservation sizes and larger than any
reasonable stack configuration on nommu.

the effeciveness of this patch relies on an assumption that the amount
by which the brk is being grown is smaller than the gap limit, which
is always true for malloc's use of brk. reliance on this assumption is
why the check is being done in malloc-specific code and not in __brk.

for several pwd/grp functions, the only way the caller can distinguish
between a successful negative result ("no such user/group") and an
internal error is by clearing errno before the call and checking errno
afterwards. the nscd backend support code correctly simulated a
not-found response on systems where such a backend is not running, but
failed to restore errno.

this commit also fixed an outdated/incorrect comment.

the arm atomics/TLS runtime selection code is called from
__set_thread_area and depends on having libc.auxv and __hwcap
available. commit 71f099cb moved the
first call to __set_thread_area to the top of dynamic linking stage 3,
before this data is made available, causing the runtime detection code
to always see __hwcap as zero and thereby select the atomics/TLS
implementations based on kuser helper.

upcoming work on superh will use similar runtime detection.

ideally this early-init code should be cleanly refactored and shared
between the dynamic linker and static-linked startup.

unless/until the byte-based C locale is implemented, defining
MB_CUR_MAX to 1 in the C locale is wrong. no internal code currently
uses the MB_CUR_MAX macro, but having it defined inconsistently is
error-prone. applications get the value from stdlib.h and were
unaffected.

presumably internal code (ungetwc and fputwc) was written assuming a
macro implementation existed; otherwise use of isascii is just a
pessimization.

aside from being invalid, the early check only optimized the error
case, and likely pessimized the common case by separating the
two branches on isascii(c) at opposite ends of the function.