Arguments

Description

This is how a tracing engine asks a traced thread to do something. This call is controlled by the action argument, which has the same meaning as the enum utrace_resume_action value returned by event reporting callbacks.

If target is already dead (target->exit_state nonzero), all actions except UTRACE_DETACH fail with -ESRCH.

The following sections describe each option for the action argument.

UTRACE_DETACH

After this, the engine data structure is no longer accessible, and the thread might be reaped. The thread will start running again if it was stopped and no longer has any attached engines that want it stopped.

If the report_reap callback may already have begun, this fails with -ESRCH. If the report_death callback may already have begun, this fails with -EALREADY.

If target is not already stopped, then a callback to this engine might be in progress or about to start on another CPU. If so, then this returns -EINPROGRESS; the detach happens as soon as the pending callback is finished. To synchronize after an -EINPROGRESS return, see utrace_barrier.

If target is properly stopped before utrace_control is called, then after successful return it's guaranteed that no more callbacks to the engine->ops vector will be made.

The only exception is SIGKILL (and exec or group-exit by another thread in the group), which can cause asynchronous report_death and/or report_reap callbacks even when UTRACE_STOP was used. (In that event, this fails with -ESRCH or -EALREADY, see above.)

UTRACE_STOP

This asks that target stop running. This returns 0 only if target is already stopped, either for tracing or for job control. Then target will remain stopped until another utrace_control call is made on engine; target can be woken only by SIGKILL (or equivalent, such as exec or termination by another thread in the same thread group).

This returns -EINPROGRESS if target is not already stopped. Then the effect is like UTRACE_REPORT. A report_quiesce or report_signal callback will be made soon. Your callback can then return UTRACE_STOP to keep target stopped.

This does not interrupt system calls in progress, including ones that sleep for a long time. For that, use UTRACE_INTERRUPT. To interrupt system calls and then keep target stopped, your report_signal callback can return UTRACE_STOP.

UTRACE_RESUME

Just let target continue running normally, reversing the effect of a previous UTRACE_STOP. If another engine is keeping target stopped, then it remains stopped until all engines let it resume. If target was not stopped, this has no effect.

UTRACE_REPORT

This is like UTRACE_RESUME, but also ensures that there will be a report_quiesce or report_signal callback made soon. If target had been stopped, then there will be a callback before it resumes running normally. If another engine is keeping target stopped, then there might be no callbacks until all engines let it resume.

Since this is meaningless unless report_quiesce callbacks will be made, it returns -EINVAL if engine lacks UTRACE_EVENT(QUIESCE).

UTRACE_INTERRUPT

This is like UTRACE_REPORT, but ensures that target will make a report_signal callback before it resumes or delivers signals. If target was in a system call or about to enter one, work in progress will be interrupted as if by SIGSTOP. If another engine is keeping target stopped, then there might be no callbacks until all engines let it resume.

This gives engine an opportunity to introduce a forced signal disposition via its report_signal callback.

UTRACE_SINGLESTEP

It's invalid to use this unless arch_has_single_step returned true. This is like UTRACE_RESUME, but resumes for one user instruction only.

Note that passing UTRACE_SINGLESTEP or UTRACE_BLOCKSTEP to utrace_control or returning it from an event callback alone does not necessarily ensure that stepping will be enabled. If there are more callbacks made to any engine before returning to user mode, then the resume action is chosen only by the last set of callbacks. To be sure, enable UTRACE_EVENT(QUIESCE) and look for the report_quiesce callback with a zero event mask, or the report_signal callback with UTRACE_SIGNAL_REPORT.

Since this is not robust unless report_quiesce callbacks will be made, it returns -EINVAL if engine lacks UTRACE_EVENT(QUIESCE).

UTRACE_BLOCKSTEP

It's invalid to use this unless arch_has_block_step returned true. This is like UTRACE_SINGLESTEP, but resumes for one whole basic block of user instructions.

Since this is not robust unless report_quiesce callbacks will be made, it returns -EINVAL if engine lacks UTRACE_EVENT(QUIESCE).

UTRACE_BLOCKSTEP devolves to UTRACE_SINGLESTEP when another tracing engine is using UTRACE_SINGLESTEP at the same time.