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cdc905d16b
A timer sigqueue may find itself already pending when it is tried to
be enqueued. This situation can happen if the timer sigqueue is enqueued
but then the timer is reset afterwards and fires before the pending
signal managed to be delivered.
However when such a double enqueue occurs while the corresponding signal
is ignored, the sigqueue is expected to be found either on the dedicated
ignored list if the timer was periodic or dropped if the timer was
one-shot. In any case it is not supposed to be queued on the real signal
queue.
An assertion verifies the latter expectation on top of the return value
of prepare_signal(), assuming "false" means that the signal is being
ignored. But prepare_signal() may also fail if the target is exiting as
the last task of its group. In this case the double enqueue observes the
sigqueue queued, as in such a situation:
TASK A (same group as B) TASK B (same group as A)
------------------------ ------------------------
// timer event
// queue signal to TASK B
posix_timer_queue_signal()
// reset timer through syscall
do_timer_settime()
// exit, leaving task B alone
do_exit()
do_exit()
synchronize_group_exit()
signal->flags = SIGNAL_GROUP_EXIT
// ========> <IRQ> timer event
posix_timer_queue_signal()
// return false due to SIGNAL_GROUP_EXIT
if (!prepare_signal())
WARN_ON_ONCE(!list_empty(&q->list))
And this spuriously triggers this warning:
WARNING: CPU: 0 PID: 5854 at kernel/signal.c:2008 posixtimer_send_sigqueue
CPU: 0 UID: 0 PID: 5854 Comm: syz-executor139 Not tainted 6.12.0-rc6-next-20241108-syzkaller #0
RIP: 0010:posixtimer_send_sigqueue+0x9da/0xbc0 kernel/signal.c:2008
Call Trace:
<IRQ>
alarm_handle_timer
alarmtimer_fired
__run_hrtimer
__hrtimer_run_queues
hrtimer_interrupt
local_apic_timer_interrupt
__sysvec_apic_timer_interrupt
instr_sysvec_apic_timer_interrupt
sysvec_apic_timer_interrupt
</IRQ>
Fortunately the recovery code in that case already does the right thing:
just exit from posixtimer_send_sigqueue() and wait for __exit_signal()
to flush the pending signal. Just make sure to warn only the case when
the sigqueue is queued and the signal is really ignored.
Fixes:
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.