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commit 167390f05564e0a4d3fcb4329377fd7743267560 seems to have
overlooked the presence of a lock here, probably because it was one of
the exceptions not using LOCK() but a rwlock.
as such, it can't be added to the generic table of locks to take, so
add an explicit atfork function for the pthread keys table. the order
it is called does not particularly matter since nothing else in libc
but pthread_exit interacts with keys.
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as reported by Alexey Izbyshev, there is a lock order inversion
deadlock between the malloc lock and aio maplock at MT-fork time:
_Fork attempts to take the aio maplock while fork already has the
malloc lock, but a concurrent aio operation holding the maplock may
attempt to allocate memory.
move the __aio_atfork calls in the parent from _Fork to fork, and
reorder the lock before most other locks, since nothing else depends
on aio(*). this leaves us with the possibility that the child will not
be able to obtain the read lock, if _Fork is used directly and happens
concurrent with an aio operation. however, in that case, the child
context is an async signal context that cannot call any further aio
functions, so all we need is to ensure that close does not attempt to
perform any aio cancellation. this can be achieved just by nulling out
the map pointer.
(*) even if other functions call close, they will only need a read
lock, not a write lock, and read locks being recursive ensures they
can obtain it. moreover, the number of read references held is bounded
by something like twice the number of live threads, meaning that the
read lock count cannot saturate.
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ever since commit 8f11e6127fe93093f81a52b15bb1537edc3fc8af introduced
the thread list lock, this has been wrong. initially, it was wrong via
calling free from the context with the thread list lock held. commit
aa5a9d15e09851f7b4a1668e9dbde0f6234abada deferred the unsafe free but
added a lock, which was also unsafe. in particular, it could deadlock
if code holding freebuf_queue_lock was interrupted by a signal handler
that takes the thread list lock.
commit 4d5aa20a94a2d3fae3e69289dc23ecafbd0c16c4 observed that there
was a lock here but failed to notice that it's invalid.
there is no easy solution to this problem with locks; any attempt at
solving it while still using locks would require the lock to be an
AS-safe one (blocking signals on each access to the dlerror buffer
list to check if there's deferred free work to be done) which would be
excessively costly, and there are also lock order considerations with
respect to how the lock would be handled at fork.
instead, just use an atomic list.
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as the outcome of Austin Group tracker issue #62, future editions of
POSIX have dropped the requirement that fork be AS-safe. this allows
but does not require implementations to synchronize fork with internal
locks and give forked children of multithreaded parents a partly or
fully unrestricted execution environment where they can continue to
use the standard library (per POSIX, they can only portably use
AS-safe functions).
up until recently, taking this allowance did not seem desirable.
however, commit 8ed2bd8bfcb4ea6448afb55a941f4b5b2b0398c0 exposed the
extent to which applications and libraries are depending on the
ability to use malloc and other non-AS-safe interfaces in MT-forked
children, by converting latent very-low-probability catastrophic state
corruption into predictable deadlock. dealing with the fallout has
been a huge burden for users/distros.
while it looks like most of the non-portable usage in applications
could be fixed given sufficient effort, at least some of it seems to
occur in language runtimes which are exposing the ability to run
unrestricted code in the child as part of the contract with the
programmer. any attempt at fixing such contracts is not just a
technical problem but a social one, and is probably not tractable.
this patch extends the fork function to take locks for all libc
singletons in the parent, and release or reset those locks in the
child, so that when the underlying fork operation takes place, the
state protected by these locks is consistent and ready for the child
to use. locking is skipped in the case where the parent is
single-threaded so as not to interfere with legacy AS-safety property
of fork in single-threaded programs. lock order is mostly arbitrary,
but the malloc locks (including bump allocator in case it's used) must
be taken after the locks on any subsystems that might use malloc, and
non-AS-safe locks cannot be taken while the thread list lock is held,
imposing a requirement that it be taken last.
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commit bd153422f28634bb6e53f13f80beb8289d405267 reintroduced the bug
fixed in c21051e90cd27a0b26be0ac66950b7396a156ba1 by refactoring the
__syscall_ret into _Fork where it once again runs before the atfork
handlers are called. since _Fork is a public interface that sets
errno, this can't be fixed the way it was fixed last time without
making new internal interfaces. instead, just save errno, and restore
it only on error to ensure that a value of 0 is never restored.
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the _Fork interface is defined for future issue of POSIX as the
outcome of Austin Group issue 62, which drops the AS-safety
requirement for fork, and provides an AS-safe replacement that does
not run the registered atfork handlers.
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this is in preparation for implementing _Fork from POSIX-future,
factored as a separate commit to improve readability of history.
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if the multithreaded parent forked while another thread was calling
sigaction for SIGABRT or calling abort, the child could inherit a lock
state in which future calls to abort will deadlock, or in which the
disposition for SIGABRT has already been reset to SIG_DFL. this is
nonconforming since abort is AS-safe and permitted to be called
concurrently with fork or in the MT-forked child.
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previously, if a file descriptor had aio operations pending in the
parent before fork, attempting to close it in the child would attempt
to cancel a thread belonging to the parent. this could deadlock, fail,
or crash the whole process of the cancellation signal handler was not
yet installed in the parent. in addition, further use of aio from the
child could malfunction or deadlock.
POSIX specifies that async io operations are not inherited by the
child on fork, so clear the entire aio fd map in the child, and take
the aio map lock (with signals blocked) across the fork so that the
lock is kept in a consistent state.
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the child is single-threaded, but may still need to synchronize with
last changes made to memory by another thread in the parent, so set
need_locks to -1 whereby the next lock-taker will drop to 0 and
prevent further barriers/locking.
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synccall may be called by AS-safe functions such as setuid/setgid after
fork. although fork() resets libc.threads_minus_one, causing synccall to
take the single-threaded path, synccall still takes the thread list
lock. This lock may be held by another thread if for example fork()
races with pthread_create(). After fork(), the value of the lock is
meaningless, so clear it.
maintainer's note: commit 8f11e6127fe93093f81a52b15bb1537edc3fc8af and
e4235d70672d9751d7718ddc2b52d0b426430768 introduced this regression.
the state protected by this lock is the linked list, which is entirely
replaced in the child path of fork (next=prev=self), so resetting it
is semantically sound.
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the hard problem here is unlinking threads from a list when they exit
without creating a window of inconsistency where the kernel task for a
thread still exists and is still executing instructions in userspace,
but is not reflected in the list. the magic solution here is getting
rid of per-thread exit futex addresses (set_tid_address), and instead
using the exit futex to unlock the global thread list.
since pthread_join can no longer see the thread enter a detach_state
of EXITED (which depended on the exit futex address pointing to the
detach_state), it must now observe the unlocking of the thread list
lock before it can unmap the joined thread and return. it doesn't
actually have to take the lock. for this, a __tl_sync primitive is
offered, with a signature that will allow it to be enhanced for quick
return even under contention on the lock, if needed. for now, the
exiting thread always performs a futex wake on its detach_state. a
future change could optimize this out except when there is already a
joiner waiting.
initial/dynamic variants of detached state no longer need to be
tracked separately, since the futex address is always set to the
global list lock, not a thread-local address that could become invalid
on detached thread exit. all detached threads, however, must perform a
second sigprocmask syscall to block implementation-internal signals,
since locking the thread list with them already blocked is not
permissible.
the arch-independent C version of __unmapself no longer needs to take
a lock or setup its own futex address to release the lock, since it
must necessarily be called with the thread list lock already held,
guaranteeing exclusive access to the temporary stack.
changes to libc.threads_minus_1 no longer need to be atomic, since
they are guarded by the thread list lock. it is largely vestigial at
this point, and can be replaced with a cheaper boolean indicating
whether the process is multithreaded at some point in the future.
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If the syscall fails, errno must be set correctly for the caller.
There's no guarantee that the handlers registered with pthread_atfork
won't clobber errno, so we need to ensure it gets set after they are
called.
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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 71f099cb7db821c51d8f39dfac622c61e54d794c, 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.
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as a result of commit 12e1e324683a1d381b7f15dd36c99b37dd44d940, kernel
processing of the robust list is only needed for process-shared
mutexes. previously the first attempt to lock any owner-tracked mutex
resulted in robust list initialization and a set_robust_list syscall.
this is no longer necessary, and since the kernel's record of the
robust list must now be cleared at thread exit time for detached
threads, optimizing it out is more worthwhile than before too.
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the main motivation for this change is to remove the assumption that
the tid of the main thread is also the pid of the process. (the value
returned by the set_tid_address syscall was used to fill both fields
despite it semantically being the tid.) this is historically and
presently true on linux and unlikely to change, but it conceivably
could be false on other systems that otherwise reproduce the linux
syscall api/abi.
only a few parts of the code were actually still using the cached pid.
in a couple places (aio and synccall) it was a minor optimization to
avoid a syscall. caching could be reintroduced, but lazily as part of
the public getpid function rather than at program startup, if it's
deemed important for performance later. in other places (cancellation
and pthread_kill) the pid was completely unnecessary; the tkill
syscall can be used instead of tgkill. this is actually a rather
subtle issue, since tgkill is supposedly a solution to race conditions
that can affect use of tkill. however, as documented in the commit
message for commit 7779dbd2663269b465951189b4f43e70839bc073, tgkill
does not actually solve this race; it just limits it to happening
within one process rather than between processes. we use a lock that
avoids the race in pthread_kill, and the use in the cancellation
signal handler is self-targeted and thus not subject to tid reuse
races, so both are safe regardless of which syscall (tgkill or tkill)
is used.
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such archs are expected to omit definitions of the SYS_* macros for
syscalls their kernels lack from arch/$ARCH/bits/syscall.h. the
preprocessor is then able to select the an appropriate implementation
for affected functions. two basic strategies are used on a
case-by-case basis:
where the old syscalls correspond to deprecated library-level
functions, the deprecated functions have been converted to wrappers
for the modern function, and the modern function has fallback code
(omitted at the preprocessor level on new archs) to make use of the
old syscalls if the new syscall fails with ENOSYS. this also improves
functionality on older kernels and eliminates the incentive to program
with deprecated library-level functions for the sake of compatibility
with older kernels.
in other situations where the old syscalls correspond to library-level
functions which are not deprecated but merely lack some new features,
such as the *at functions, the old syscalls are still used on archs
which support them. this may change at some point in the future if or
when fallback code is added to the new functions to make them usable
(possibly with reduced functionality) on old kernels.
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this is the first step in an overhaul aimed at greatly simplifying and
optimizing everything dealing with thread-local state.
previously, the thread pointer was initialized lazily on first access,
or at program startup if stack protector was in use, or at certain
random places where inconsistent state could be reached if it were not
initialized early. while believed to be fully correct, the logic was
fragile and non-obvious.
in the first phase of the thread pointer overhaul, support is retained
(and in some cases improved) for systems/situation where loading the
thread pointer fails, e.g. old kernels.
some notes on specific changes:
- the confusing use of libc.main_thread as an indicator that the
thread pointer is initialized is eliminated in favor of an explicit
has_thread_pointer predicate.
- sigaction no longer needs to ensure that the thread pointer is
initialized before installing a signal handler (this was needed to
prevent a situation where the signal handler caused the thread
pointer to be initialized and the subsequent sigreturn cleared it
again) but it still needs to ensure that implementation-internal
thread-related signals are not blocked.
- pthread tsd initialization for the main thread is deferred in a new
manner to minimize bloat in the static-linked __init_tp code.
- pthread_setcancelstate no longer needs special handling for the
situation before the thread pointer is initialized. it simply fails
on systems that cannot support a thread pointer, which are
non-conforming anyway.
- pthread_cleanup_push/pop now check for missing thread pointer and
nop themselves out in this case, so stdio no longer needs to avoid
the cancellable path when the thread pointer is not available.
a number of cases remain where certain interfaces may crash if the
system does not support a thread pointer. at this point, these should
be limited to pthread interfaces, and the number of such cases should
be fewer than before.
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there are several reasons for this. some of them are related to race
conditions that arise since fork is required to be async-signal-safe:
if fork or pthread_create is called from a signal handler after the
fork syscall has returned but before the subsequent userspace code has
finished, inconsistent state could result. also, there seem to be
kernel and/or strace bugs related to arrival of signals during fork,
at least on some versions, and simply blocking signals eliminates the
possibility of such bugs.
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this mirrors the stdio_impl.h cleanup. one header which is not
strictly needed, errno.h, is left in pthread_impl.h, because since
pthread functions return their error codes rather than using errno,
nearly every single pthread function needs the errno constants.
in a few places, rather than bringing in string.h to use memset, the
memset was replaced by direct assignment. this seems to generate much
better code anyway, and makes many functions which were previously
non-leaf functions into leaf functions (possibly eliminating a great
deal of bloat on some platforms where non-leaf functions require ugly
prologue and/or epilogue).
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some functions that should have been testing whether pthread_self()
had been called and initialized the thread pointer were instead
testing whether pthread_create() had been called and actually made the
program "threaded". while it's unlikely any mismatch would occur in
real-world problems, this could have introduced subtle bugs. now, we
store the address of the main thread's thread descriptor in the libc
structure and use its presence as a flag that the thread register is
initialized. note that after fork, the calling thread (not necessarily
the original main thread) is the new main thread.
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after fork, we have a new process and the pid is equal to the tid of
the new main thread. there is no need to make two separate syscalls to
obtain the same number.
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note that this presently does not handle consistency of the libc's own
global state during forking. as per POSIX 2008, if the parent process
was threaded, the child process may only call async-signal-safe
functions until one of the exec-family functions is called, so the
current behavior is believed to be conformant even if non-ideal. it
may be improved at some later time.
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