this is possible with the new build system that allows src/*/$(ARCH)/*
files which do not shadow a file in the parent directory, and yields a
more logical organization. eventually it will be possible to remove
arch/*/src from the build system.
switch to ll/sc model so that new atomic.h can provide optimized
versions of all the atomic primitives without needing an ll/sc loop
written in asm for each one.
all isa levels which use ldrex/strex now use the inline ll/sc model
even if the type of barrier to use is not known until runtime (v6).
the cas model is only used for arm v5 and earlier, and it has been
optimized to make the call via inline asm with custom constraints
rather than as a C function call.
rather than having each arch provide its own atomic.h, there is a new
shared atomic.h in src/internal which pulls arch-specific definitions
from arc/$(ARCH)/atomic_arch.h. the latter can be extremely minimal,
defining only a_cas or new ll/sc type primitives which the shared
atomic.h will use to construct everything else.
this commit avoids making heavy changes to the individual archs'
atomic implementations. definitions which are identical or
near-identical to what the new shared atomic.h would produce have been
removed, but otherwise the changes made are just hooking up the
arch-specific files to the new infrastructure. major changes to take
advantage of the new system will come in subsequent commits.
commit 2f853dd6b9 failed to replicate
the old makefile logic that caused arch/arm/src/arm/atomics.s to be
built. since this was the only .s file under arch/*/src, rather than
trying to reproduce the old logic, I'm just moving it up a level and
adjusting the glob pattern in the makefile to catch it. eventually
arch/*/src will probably be removed in favor of moving all these files
to appropriate src/*/$(ARCH) locations.
the __SOFTFP__ macro which was wrongly being used does not reflect the
ABI (arm vs armhf) but just the availability of floating point
instructions/registers, so -mfloat-abi=softfp was wrongly being
treated as armhf. __ARM_PCS_VFP is the correct predefined macro to
check for the armhf EABI variant. this macro usage was corrected for
the build process in commit 4918c2bb20
but reloc.h was apparently overlooked at the time.
these files are all accepted as legacy arm syntax when producing arm
code, but legacy syntax cannot be used for producing thumb2 with
access to the full ISA. even after switching to UAL, some asm source
files contain instructions which are not valid in thumb mode, so these
will need to be addressed separately.
the idea of the three-instruction sequence being removed was to be
able to return to thumb code when used on armv4t+ from a thumb caller,
but also to be able to run on armv4 without the bx instruction
available (in which case the low bit of lr would always be 0).
however, without compiler support for generating such a sequence from
C code, which does not exist and which there is unlikely to be
interest in implementing, there is little point in having it in the
asm, and it would likely be easier to add pre-armv4t support via
enhanced linker handling of R_ARM_V4BX than at the compiler level.
removing this code simplifies adding support for building libc in
thumb2-only form (for cortex-m).
using the actual mcontext_t definition rather than an overlaid pointer
array both improves correctness/readability and eliminates some ugly
hacks for archs with 64-bit registers bit 32-bit program counter.
also fix UB due to comparison of pointers not in a common array
object.
this builds on commits a603a75a72 and
0ba35d69c0 to ensure that a compiler
cannot conclude that it's valid to reorder the asm to a point before
the thread pointer is set up, or to treat the inline function as if it
were declared with attribute((const)).
other archs already use volatile asm for thread pointer loading.
these functions are part of the ARM EABI, meaning compilers may
generate references to them. known versions of gcc do not use them,
but llvm does. they are not provided by libgcc, and the de facto
standard seems to be that libc provides them.
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.
compilers targeting armv7 may be configured to produce thumb2 code
instead of arm code by default, and in the future we may wish to
support targets where only the thumb instruction set is available.
the instructions this patch omits in thumb mode are needed only for
non-thumb versions of armv4 or earlier, which are not supported by any
current compilers/toolchains and thus rather pointless to have. at
some point these compatibility return sequences may be removed from
all asm source files, and in that case it would make sense to remove
them here too and remove the ifdef.
compilers targeting armv7 may be configured to produce thumb2 code
instead of arm code by default, and in the future we may wish to
support targets where only the thumb instruction set is available.
the changes made here avoid operating directly on the sp register,
which is not possible in thumb code, and address an issue with the way
the address of _DYNAMIC is computed.
previously, the relative address of _DYNAMIC was stored with an
additional offset of -8 versus the pc-relative add instruction, since
on arm the pc register evaluates to ".+8". in thumb code, it instead
evaluates to ".+4". both are two (normal-size) instructions beyond "."
in the current execution mode, so the numbered label 2 used in the
relative address expression is simply moved two instructions ahead to
be compatible with both instruction sets.
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.
while it's the same for all presently supported archs, it differs at
least on sparc, and conceptually it's no less arch-specific than the
other O_* macros. O_SEARCH and O_EXEC are still defined in terms of
O_PATH in the main fcntl.h.
the previous values (2k min and 8k default) were too small for some
archs. aarch64 reserves 4k in the signal context for future extensions
and requires about 4.5k total, and powerpc reportedly uses over 2k.
the new minimums are chosen to fit the saved context and also allow a
minimal signal handler to run.
since the default (SIGSTKSZ) has always been 6k larger than the
minimum, it is also increased to maintain the 6k usable by the signal
handler. this happens to be able to store one pathname buffer and
should be sufficient for calling any function in libc that doesn't
involve conversion between floating point and decimal representations.
x86 (both 32-bit and 64-bit variants) may also need a larger minimum
(around 2.5k) in the future to support avx-512, but the values on
these archs are left alone for now pending further analysis.
the value for PTHREAD_STACK_MIN is not increased to match MINSIGSTKSZ
at this time. this is so as not to preclude applications from using
extremely small thread stacks when they know they will not be handling
signals. unfortunately cancellation and multi-threaded set*id() use
signals as an implementation detail and therefore require a stack
large enough for a signal context, so applications which use extremely
small thread stacks may still need to avoid using these features.
these macros have the same distinct definition on blackfin, frv, m68k,
mips, sparc and xtensa kernels. POLLMSG and POLLRDHUP additionally
differ on sparc.
the memory model we use internally for atomics permits plain loads of
values which may be subject to concurrent modification without
requiring that a special load function be used. since a compiler is
free to make transformations that alter the number of loads or the way
in which loads are performed, the compiler is theoretically free to
break this usage. the most obvious concern is with atomic cas
constructs: something of the form tmp=*p;a_cas(p,tmp,f(tmp)); could be
transformed to a_cas(p,*p,f(*p)); where the latter is intended to show
multiple loads of *p whose resulting values might fail to be equal;
this would break the atomicity of the whole operation. but even more
fundamental breakage is possible.
with the changes being made now, objects that may be modified by
atomics are modeled as volatile, and the atomic operations performed
on them by other threads are modeled as asynchronous stores by
hardware which happens to be acting on the request of another thread.
such modeling of course does not itself address memory synchronization
between cores/cpus, but that aspect was already handled. this all
seems less than ideal, but it's the best we can do without mandating a
C11 compiler and using the C11 model for atomics.
in the case of pthread_once_t, the ABI type of the underlying object
is not volatile-qualified. so we are assuming that accessing the
object through a volatile-qualified lvalue via casts yields volatile
access semantics. the language of the C standard is somewhat unclear
on this matter, but this is an assumption the linux kernel also makes,
and seems to be the correct interpretation of the standard.
this syscall allows fexecve to be implemented without /proc, it is new
in linux v3.19, added in commit 51f39a1f0cea1cacf8c787f652f26dfee9611874
(sh and microblaze do not have allocated syscall numbers yet)
added a x32 fix as well: the io_setup and io_submit syscalls are no
longer common with x86_64, so use the x32 specific numbers.
the definitions are generic for all kernel archs. exposure of these
macros now only occurs on the same feature test as for the function
accepting them, which is believed to be more correct.
these syscalls are new in linux v3.18, bpf is present on all
supported archs except sh, kexec_file_load is only allocted for
x86_64 and x32 yet.
bpf was added in linux commit 99c55f7d47c0dc6fc64729f37bf435abf43f4c60
kexec_file_load syscall number was allocated in commit
f0895685c7fd8c938c91a9d8a6f7c11f22df58d2
the register constraints in the non-clang case were tested to work on
clang back to 3.2, and earlier versions of clang have known bugs that
preclude building musl.
there may be other reasons to prefer not to use inline syscalls, but
if so the function-call-based implementations should be added back in
a unified way for all archs.
calls to __aeabi_read_tp may be generated by the compiler to access
TLS on pre-v6 targets. previously, this function was hard-coded to
call the kuser helper, which would crash on kernels with kuser helper
removed.
to fix the problem most efficiently, the definition of __aeabi_read_tp
is moved so that it's an alias for the new __a_gettp. however, on v7+
targets, code to initialize the runtime choice of thread-pointer
loading code is not even compiled, meaning that defining
__aeabi_read_tp would have caused an immediate crash due to using the
default implementation of __a_gettp with a HCF instruction.
fortunately there is an elegant solution which reduces overall code
size: putting the native thread-pointer loading instruction in the
default code path for __a_gettp, so that separate default/native code
paths are not needed. this function should never be called before
__set_thread_area anyway, and if it is called early on pre-v6
hardware, the old behavior (crashing) is maintained.
ideally __aeabi_read_tp would not be called at all on v7+ targets
anyway -- in fact, prior to the overhaul, the same problem existed,
but it was never caught by users building for v7+ with kuser disabled.
however, it's possible for calls to __aeabi_read_tp to end up in a v7+
binary if some of the object files were built for pre-v7 targets, e.g.
in the case of static libraries that were built separately, so this
case needs to be handled.
previously, builds for pre-armv6 targets hard-coded use of the "kuser
helper" system for atomics and thread-pointer access, resulting in
binaries that fail to run (crash) on systems where this functionality
has been disabled (as a security/hardening measure) in the kernel.
additionally, builds for armv6 hard-coded an outdated/deprecated
memory barrier instruction which may require emulation (extremely
slow) on future models.
this overhaul replaces the behavior for all pre-armv7 builds (both of
the above cases) to perform runtime detection of the appropriate
mechanisms for barrier, atomic compare-and-swap, and thread pointer
access. detection is based on information provided by the kernel in
auxv: presence of the HWCAP_TLS bit for AT_HWCAP and the architecture
version encoded in AT_PLATFORM. direct use of the instructions is
preferred when possible, since probing for the existence of the kuser
helper page would be difficult and would incur runtime cost.
for builds targeting armv7 or later, the runtime detection code is not
compiled at all, and much more efficient versions of the non-cas
atomic operations are provided by using ldrex/strex directly rather
than wrapping cas.
these syscalls are new in linux v3.17 and present on all supported
archs except sh.
seccomp was added in commit 48dc92b9fc3926844257316e75ba11eb5c742b2c
it has operation, flags and pointer arguments (if flags==0 then it is
the same as prctl(PR_SET_SECCOMP,...)), the uapi header for flag
definitions is linux/seccomp.h
getrandom was added in commit c6e9d6f38894798696f23c8084ca7edbf16ee895
it provides an entropy source when open("/dev/urandom",..) would fail,
the uapi header for flags is linux/random.h
memfd_create was added in commit 9183df25fe7b194563db3fec6dc3202a5855839c
it allows anon mmap to have an fd, that can be shared, sealed and needs no
mount point, the uapi header for flags is linux/memfd.h
based on patch by Jens Gustedt.
mtx_t and cnd_t are defined in such a way that they are formally
"compatible types" with pthread_mutex_t and pthread_cond_t,
respectively, when accessed from a different translation unit. this
makes it possible to implement the C11 functions using the pthread
functions (which will dereference them with the pthread types) without
having to use the same types, which would necessitate either namespace
violations (exposing pthread type names in threads.h) or incompatible
changes to the C++ name mangling ABI for the pthread types.
for the rest of the types, things are much simpler; using identical
types is possible without any namespace considerations.
conceptually, a_spin needs to be at least a compiler barrier, so the
compiler will not optimize out loops (and the load on each iteration)
while spinning. it should also be a memory barrier, or the spinning
thread might keep spinning without noticing stores from other threads,
thus delaying for longer than it should.
ideally, an optimal a_spin implementation that avoids unnecessary
cache/memory contention should be chosen for each arch, but for now,
the easiest thing is to perform a useless a_cas on the calling
thread's stack.
unfortunately this needs to be able to vary by arch, because of a huge
mess GCC made: the GCC definition, which became the ABI, depends on
quirks in GCC's definition of __alignof__, which does not match the
formal alignment of the type.
GCC's __alignof__ unexpectedly exposes the an implementation detail,
its "preferred alignment" for the type, rather than the formal/ABI
alignment of the type, which it only actually uses in structures. on
most archs the two values are the same, but on some (at least i386)
the preferred alignment is greater than the ABI alignment.
I considered using _Alignas(8) unconditionally, but on at least one
arch (or1k), the alignment of max_align_t with GCC's definition is
only 4 (even the "preferred alignment" for these types is only 4).
when manipulating the robust list, the order of stores matters,
because the code may be asynchronously interrupted by a fatal signal
and the kernel will then access the robust list in what is essentially
an async-signal context.
previously, aliasing considerations made it seem unlikely that a
compiler could reorder the stores, but proving that they could not be
reordered incorrectly would have been extremely difficult. instead
I've opted to make all the pointers used as part of the robust list,
including those in the robust list head and in the individual mutexes,
volatile.
in addition, the format of the robust list has been changed to point
back to the head at the end, rather than ending with a null pointer.
this is to match the documented kernel robust list ABI. the null
pointer, which was previously used, only worked because faults during
access terminate the robust list processing.
the a_cas_l, a_swap_l, a_swap_p, and a_store_l operations were
probably used a long time ago when only i386 and x86_64 were
supported. as other archs were added, support for them was
inconsistent, and they are obviously not in use at present. having
them around potentially confuses readers working on new ports, and the
type-punning hacks and inconsistent use of types in their definitions
is not a style I wish to perpetuate in the source tree, so removing
them seems appropriate.
this was one of the main instances of ugly code duplication: all archs
use basically the same types of relocations, but roughly equivalent
logic was duplicated for each arch to account for the different naming
and numbering of relocation types and variation in whether REL or RELA
records are used.
as an added bonus, both REL and RELA are now supported on all archs,
regardless of which is used by the standard toolchain.
the immediate motivation is supporting TLSDESC relocations which
require allocation and thus may fail (unless we pre-allocate), but
this mechanism should also be used for throwing an error on
unsupported or invalid relocation types, and perhaps in certain cases,
for reporting when a relocation is not satisfiable.
linux 3.14 introduced sched_getattr and sched_setattr syscalls in
commit d50dde5a10f305253cbc3855307f608f8a3c5f73
and the related SCHED_DEADLINE scheduling policy in
commit aab03e05e8f7e26f51dee792beddcb5cca9215a5
but struct sched_attr "extended scheduling parameters data structure"
is not yet exported to userspace (necessary for using the syscalls)
so related uapi definitions are not added yet.
armv7/thumb2 provides a way to do atomics in thumb mode, but for armv6
we need a call to arm mode.
this commit is based on a patch by Stephen Thomas which fixed the
armv7 cases but not the armv6 ones.
all of this should be revisited if/when runtime selection of thread
pointer access and atomics are added.
The mips arch is special in that it uses different RLIMIT_
numbers than other archs, so allow bits/resource.h to override
the default RLIMIT_ numbers (empty on all archs except mips).
Reported by orc.
aside from potentially offering better performance, this change is
needed since the old coprocessor-based approach to barriers is
deprecated in arm v7, and some compilers/assemblers issue errors when
using the deprecated instruction for v7 targets.
the "m" constraint could give a memory reference with an offset that's
not compatible with ldrex/strex, so the arm-specific "Q" constraint is
needed instead.
this is perhaps not the optimal implementation; a_cas still compiles
to nested loops due to the different interface contracts of the kuser
helper cas function (whose contract this patch implements) and the
a_cas function (whose contract mimics the x86 cmpxchg). fixing this
may be possible, but it's more complicated and thus deferred until a
later time.
aside from improving performance and code size, this patch also
provides a means of producing binaries which can run on hardened
kernels where the kuser helpers have been disabled. however, at
present this requires producing binaries for armv6k or later, which
will not run on older cpus. a real solution to the problem of kernels
that omit the kuser helpers would be runtime detection, so that
universal binaries which run on all arm cpu models can also be
compatible with all kernel hardening profiles. robust detection
however is a much harder problem, and will be addressed at a later
time.
Rich Felker
Timo Teräs
Szabolcs Nagy
Trutz Behn