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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.
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this addresses alpine linux issue #3692 and brings the syslog message
length limit in alignment with uclibc's implementation.
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based on patch by Dima Krasner, with minor improvements for code size.
connect can fail if there is no listening syslogd, in which case a
useless socket was kept open, preventing subsequent syslog call from
attempting to connect again.
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this was previously a no-op, somewhat intentionally, because I failed
to understand that it only has an effect when sending to the logging
facility fails and thus is not the nuisance that it would be if always
sent output to the console.
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this behavior is no longer valid in general, and was never necessary.
if the LOG_PERROR option is set, output to stderr could still succeed.
also, when the LOG_CONS option is added, it will need syslog to
proceed even if opening the log socket fails.
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this is a nonstandard feature, but easy and inexpensive to add. since
the corresponding macro has always been defined in our syslog.h, it
makes sense to actually support it. applications may reasonably be
using the presence of the macro to assume that the feature is
supported.
the behavior of omitting the 'header' part of the log message does not
seem to be well-documented, but matches other implementations (at
least glibc) which have this option.
based on a patch by Clément Vasseur, but simplified using %n.
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errno must be saved upon vsyslog entry, otherwise its value could be
changed by some libc function before reaching the %m handler in
vsnprintf.
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1. as reported by William Haddon, the value returned by snprintf was
wrongly used as a length passed to sendto, despite it possibly
exceeding the buffer length. this could lead to invalid reads and
leaking additional data to syslog.
2. openlog was storing a pointer to the ident string passed by the
caller, rather than copying it. this bug is shared with (and even
documented in) other implementations like glibc, but such behavior
does not seem to meet the requirements of the standard.
3. extremely long ident provided to openlog, or corrupt ident due to
the above issue, could possibly have resulted in buffer overflows.
despite having the potential for smashing the stack, i believe the
impact is low since ident points to a short string literal in typical
application usage (and per the above bug, other usages will break
horribly on other implementations).
4. when used with LOG_NDELAY, openlog was not connecting the
newly-opened socket; sendto was being used instead. this defeated the
main purpose of LOG_NDELAY: preparing for chroot.
5. the default facility was not being used at all, so all messages
without an explicit facility passed to syslog were getting logged at
the kernel facility.
6. setlogmask was not thread-safe; no synchronization was performed
updating the mask. the fix uses atomics rather than locking to avoid
introducing a lock in the fast path for messages whose priority is not
in the mask.
7. in some code paths, the syslog lock was being unlocked twice; this
could result in releasing a lock that was actually held by a different
thread.
some additional enhancements to syslog such as a default identifier
based on argv[0] or similar may still be desired; at this time, only
the above-listed bugs have been fixed.
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also update syslog to use SOCK_CLOEXEC rather than separate fcntl
step, to make it safe in multithreaded programs that run external
programs.
emulation is not atomic; it could be made atomic by holding a lock on
forking during the operation, but this seems like overkill. my goal is
not to achieve perfect behavior on old kernels (which have plenty of
other imperfect behavior already) but to avoid catastrophic breakage
in (1) syslog, which would give no output on old kernels with the
change to use SOCK_CLOEXEC, and (2) programs built on a new kernel
where configure scripts detected a working SOCK_CLOEXEC, which later
get run on older kernels (they may otherwise fail to work completely).
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i did some testing trying to switch malloc to use the new internal
lock with priority inheritance, and my malloc contention test got
20-100 times slower. if priority inheritance futexes are this slow,
it's simply too high a price to pay for avoiding priority inversion.
maybe we can consider them somewhere down the road once the kernel
folks get their act together on this (and perferably don't link it to
glibc's inefficient lock API)...
as such, i've switch __lock to use malloc's implementation of
lightweight locks, and updated all the users of the code to use an
array with a waiter count for their locks. this should give optimal
performance in the vast majority of cases, and it's simple.
malloc is still using its own internal copy of the lock code because
it seems to yield measurably better performance with -O3 when it's
inlined (20% or more difference in the contention stress test).
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these functions are allowed to be cancellation points, but then we
would have to install cleanup handlers to avoid termination with locks
held.
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with datagram sockets, depending on fprintf not to flush the output
early was very fragile; the new version simply uses a small fixed-size
buffer. it could be updated to dynamic-allocate large buffers if
needed, but i can't envision any admin being happy about finding
64kb-long lines in their syslog...
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per the standard, SIGPIPE is not generated for SOCK_DGRAM.
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it actually appears the hacks to block SIGPIPE are probably not
necessary, and potentially harmful. if i can confirm this, i'll remove
them.
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