Differential D12773 Diff 65724 lib/libc/sys/fast_sigblock.2

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				emasteUnsubmitted Done Inline Actions we can drop the `All rights reserved.` emaste: we can drop the `All rights reserved.`
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				.Dd December 13, 2019
				.Dt FAST_SIGBLOCK 2
				.Os
				.Sh NAME
				.Nm fast_sigblock
				.Nd controls signals blocking with a simple memory write
				.Sh LIBRARY
				brooksUnsubmitted Done Inline Actions This description doesn't seem right. brooks: This description doesn't seem right.
				.Lb libc
				.Sh SYNOPSIS
				.In sys/signalvar.h
				.Ft int
				.Fn fast_sigblock "int cmd" "void *ptr"
				.Sh DESCRIPTION
				.Bf -symbolic
				This function is not intended for a direct usage by applications.
				The functionality is provided for implementing some optimizations in
				.Xr ld-elf.so.1 8
				and
				.Lb libthr .
				.Ef
				.Pp
				The function configures the kernel facility that allows a thread to
				block asynchronous signals delivery with a single write to userspace
				memory, avoiding overhead of system calls like
				.Xr sigprocmask 2
				for establishing critical sections.
				The C runtime uses it to optimize implementation of async-signal-safe
				functionality.
				.Pp
				A thread might register a
				.Dv sigblock
				variable of type
				.Vt int
				as a location which is consulted by kernel when calculating the
				blocked signal mask for delivery of asynchronous signals.
				If the variable contains non-zero count of blocks (see below),
				rpokalaUnsubmitted Not Done Inline Actions Did you mean `count of blockers`, or `count of blocked threads`, or something else? `count of blocks` doesn't scan as correct English to me. rpokala: Did you mean `count of blockers`, or `count of blocked threads`, or something else? `count of…
				kibAuthorUnsubmitted Done Inline Actions This thing only works for current thread. Block is one increment of the variable. I have to make this possible because both rtld and libthr might need to recurse into critical sections. kib: This thing only works for current thread. Block is one increment of the variable. I have to…
				rpokalaUnsubmitted Not Done Inline Actions This thing only works for current thread. Block is one increment of the variable. But what does that mean? re-reads Okay, it's incremented every time something says "block signals until I'm done", right? And its possible for multiple things to say that recursively, leading to a count greater than one? If that's correct, then "blocks" means "requests to block signals"; in that case, it could be worded more clearly like this:: `If the variable contains a non-zero count of requests to block signals (see below)` rpokala: > This thing only works for current thread. Block is one increment of the variable. But what…
				cemUnsubmitted Not Done Inline Actions @rpokala It's just a recursion count, yeah. I might phrase the sentence as: "If the variable indicates that blocking is requested, then the kernel ..." (and just leave out the recursion detail, which isn't important to the description of the on/off behavior). The recursion is already described later (sentence ending in l. 80). cem: @rpokala It's just a recursion count, yeah. I might phrase the sentence as: "If the variable…
				then kernel effectively operates as if the mask contained all
				cemUnsubmitted Not Done Inline Actions nit: s/contained/containing/ cem: nit: s/contained/containing/
				blockable signals was supplied to
				.Xr sigprocmask 2 .
				.Pp
				The variable is supposed to be modified only from the owning thread,
				there is no way to guarantee visibility of update from other thread
				to kernel when signals are delivered.
				jillesUnsubmitted Not Done Inline Actions This implies that fast signal blocking should not be in effect for an indefinite time in a multi-threaded process, since fast signal blocking is not taken into account by the mechanism that directs a process-wide signal to a thread that does not have it masked (taking it into account would require access to other process memory when sending a signal). jilles: This implies that fast signal blocking should not be in effect for an indefinite time in a…
				kibAuthorUnsubmitted Done Inline Actions If used properly (in other words, if there is no memory corruption), then it is. But if application wrote a garbage into the fast_sigblock word, it breaks of course. I understand your point that this significantly increases the possible non-compliance for misbehaving programs, and this was one of the reasons why I did not rushed with the patch, which was written several years ago. But in the end, I think that it is not too bad, because the kernel integrity is maintained, and stuff like SIGKILL/SIGSTOP is not affected. kib: If used properly (in other words, if there is no memory corruption), then it is. But if…
				jillesUnsubmitted Not Done Inline Actions Hmm, my point was that in this scenario: The process does not ignore SIGUSR1. Thread 1 has fast signal blocking in effect. Thread 2 does not have fast signal blocking in effect, and has SIGUSR1 unmasked. Something sends SIGUSR1 to the process. FOREACH_THREAD_IN_PROC visits thread 1 before thread 2. the signal will not be handled until thread 1 disables fast signal blocking, or thread 2 does something that causes the process's signal queue to be checked. The same thing happens with libthr's deferred signals (but in that case only for signals that have a handler, not ones that use the default action). When thread 1 masks SIGUSR1 normally, sigtd() will always pick thread 2, and the signal is handled without delay. As a result, fast signal blocking is only suitable for short critical sections, not as a generic replacement for sigprocmask()/pthread_sigmask(). To @cem, making this work as a generic replacement for sigprocmask()/pthread_sigmask() would require a rather different design since accessing a different process's pageable memory while sending a signal (in sigtd()) seems a bad idea. jilles: Hmm, my point was that in this scenario: - The process does not ignore SIGUSR1. - Thread 1…
				.Pp
				Lower bits of the sigblock variable are reserved as flags,
				which might be set or cleared by kernel at arbitrary moments.
				Userspace code should use
				.Xr atomic 9
				operations of incrementing and decrementing by
				.Dv FAST_SIGBLOCK_INC
				quantity to recursively block or unblock signals delivery.
				jillesUnsubmitted Not Done Inline Actions If the kernel only sets and clears the flags from the same thread, then on x86 read-modify-write operations without LOCK prefix would suffice (counter(9) uses this). However, it may not be worth the trouble. jilles: If the kernel only sets and clears the flags from the same thread, then on x86 read-modify…
				.Pp
				If a signal would be delivered when unmasked, kernel might set the
				.Dv FAST_SIGBLOCK_PEND
				.Dq pending signal
				flag in the sigblock variable.
				Userspace should perform
				.Dv FAST_SIGBLOCK_UNBLOCK
				operation when clearing the variable if it notes the pending signal
				bit is set, which would deliver the pending signals immediately.
				Otherwise, signals delivery might be postponed.
				.Pp
				The
				.Fa cmd
				argument specifies one of the following operations:
				.Bl -tag -width FAST_SIGBLOCK_UNSETPTR
				.It Dv FAST_SIGBLOCK_SETPTR
				Register the variable of type
				.Vt int
				at location pointed to by the
				.Fa ptr
				argument as sigblock variable for the calling thread.
				.It Dv FAST_SIGBLOCK_UNSETPTR
				Unregister the currently registered sigblock location.
				Kernel stops inferring the blocked mask from non-zero value of its
				blocked count.
				New location can be registered after previous one is deregistered.
				.It Dv FAST_SIGBLOCK_UNBLOCK
				If there are pending signals which should be delivered to the calling
				thread, they are delivered before returning from the call.
				The sigblock variable should have zero blocking count, and indicate
				that the pending signal exists.
				Effectively this means that the variable should have the value
				.Dv FAST_SIGBLOCK_PEND .
				.El
				.Sh RETURN VALUES
				.Rv -std
				.Sh ERRORS
				The operation may fail with the following errors:
				.Bl -tag -width Er
				.It Bq Er EBUSY
				The
				.Dv FAST_SIGBLOCK_SETPTR
				attempted while the sigblock address was already registered.
				The
				.Dv FAST_SIGBLOCK_UNBLOCK
				was called while sigblock variable value is not equal to
				.Dv FAST_SIGBLOCK_PEND .
				.It Bq Er EINVAL
				The variable address passed to
				.Dv FAST_SIGBLOCK_SETPTR
				is not aligned naturally.
				The
				.Dv FAST_SIGBLOCK_UNSETPTR
				operation was attempted without prior successfull call to
				.Dv FAST_SIGBLOCK_SETPTR .
				.It Bq Er EFAULT
				Attempt to read or write to the sigblock variable failed.
				Note that kernel generates the
				.Dv SIGSEGV
				signal if an attempt to read from the sigblock variable faulted
				during implicit accesses from syscall entry.
				.El
				.Sh SEE ALSO
				.Xr kill 2 ,
				.Xr signal 2 ,
				.Xr sigprocmask 2 ,
				.Xr libthr 3 ,
				.Xr ld-elf.so.1 8
				.Sh STANDARDS
				The
				.Nm
				function is non-standard, although a similar functionality is a common
				optimization provided by several other systems.
				.Sh HISTORY
				The
				.Nm
				function was introduced in
				.Fx 13.0 .
				.Sh BUGS
				The
				.Nm
				symbol is currently not exported by libc, on purpose.
				Consumers should either use the
				.Dv __sys_fast_sigblock
				symbol from the private libc namespace, or utilize
				.Xr syscall 2 .