struct_utrace_engine_ops.9 - Online Linux Manual Page

STRUCT UTRACE_ENGINE - Online Linux Manual PageSection : 9
Updated : June 2018
Source : Kernel Hackers Manual 2.6.
Note : utrace core API

NAMEstruct_utrace_engine_ops − tracing engine callbacks

SYNOPSISstruct utrace_engine_ops { u32 (* report_quiesce) (u32 action, struct utrace_engine *engine,unsigned long event); u32 (* report_signal) (u32 action, struct utrace_engine *engine,struct pt_regs *regs,siginfo_t *info,const struct k_sigaction *orig_ka,struct k_sigaction *return_ka); u32 (* report_clone) (u32 action, struct utrace_engine *engine,unsigned long clone_flags,struct task_struct *child); u32 (* report_jctl) (u32 action, struct utrace_engine *engine,int type, int notify); u32 (* report_exec) (u32 action, struct utrace_engine *engine,const struct linux_binfmt *fmt,const struct linux_binprm *bprm,struct pt_regs *regs); u32 (* report_syscall_entry) (u32 action, struct utrace_engine *engine,struct pt_regs *regs); u32 (* report_syscall_exit) (u32 action, struct utrace_engine *engine,struct pt_regs *regs); u32 (* report_exit) (u32 action, struct utrace_engine *engine,long orig_code, long *code); u32 (* report_death) (struct utrace_engine *engine,bool group_dead, int signal); void (* report_reap) (struct utrace_engine *engine,struct task_struct *task); void (* release) (void *data); };

MEMBERSreport_quiesce Requested by UTRACE_EVENT(QUIESCE). This does not indicate any event, but just that current is in a safe place for examination. This call is made before each specific event callback, except for report_reap. The event argument gives the UTRACE_EVENT(which) value for the event occurring. This callback might be made for events engine has not requested, if some other engine is tracing the event; calling utrace_set_events call here can request the immediate callback for this occurrence of event. event is zero when there is no other event, current is now ready to check for signals and return to user mode, and some engine has used UTRACE_REPORT or UTRACE_INTERRUPT to request this callback. For this case, if report_signal is not NULL, the report_quiesce callback may be replaced with a report_signal callback passing UTRACE_SIGNAL_REPORT in its action argument, whenever current is entering the signal−check path anyway. report_signal Requested by UTRACE_EVENT(SIGNAL_*) or UTRACE_EVENT(QUIESCE). Use utrace_signal_action and utrace_resume_action on action. The signal action is UTRACE_SIGNAL_REPORT when some engine has used UTRACE_REPORT or UTRACE_INTERRUPT; the callback can choose to stop or to deliver an artificial signal, before pending signals. It´s UTRACE_SIGNAL_HANDLER instead when signal handler setup just finished (after a previous UTRACE_SIGNAL_DELIVER return); this serves in lieu of any UTRACE_SIGNAL_REPORT callback requested by UTRACE_REPORT or UTRACE_INTERRUPT, and is also implicitly requested by UTRACE_SINGLESTEP or UTRACE_BLOCKSTEP into the signal delivery. The other signal actions indicate a signal about to be delivered; the previous engine´s return value sets the signal action seen by the the following engine´s callback. The info data can be changed at will, including info−>si_signo. The settings in return_ka determines what UTRACE_SIGNAL_DELIVER does. orig_ka is what was in force before other tracing engines intervened, and it´s NULL when this report began as UTRACE_SIGNAL_REPORT or UTRACE_SIGNAL_HANDLER. For a report without a new signal, info is left uninitialized and must be set completely by an engine that chooses to deliver a signal; if there was a previous report_signal callback ending in UTRACE_STOP and it was just resumed using UTRACE_REPORT or UTRACE_INTERRUPT, then info is left unchanged from the previous callback. In this way, the original signal can be left in info while returning UTRACE_STOP|UTRACE_SIGNAL_IGN and then found again when resuming with UTRACE_INTERRUPT. The UTRACE_SIGNAL_HOLD flag bit can be OR´d into the return value, and might be in action if the previous engine returned it. This flag asks that the signal in info be pushed back on current´s queue so that it will be seen again after whatever action is taken now. report_clone Requested by UTRACE_EVENT(CLONE). Event reported for parent, before the new task child might run. clone_flags gives the flags used in the clone system call, or equivalent flags for a fork or vfork system call. This function can use utrace_attach_task on child. Then passing UTRACE_STOP to utrace_control on child here keeps the child stopped before it ever runs in user mode, UTRACE_REPORT or UTRACE_INTERRUPT ensures a callback from child before it starts in user mode. report_jctl Requested by UTRACE_EVENT(JCTL). Job control event; type is CLD_STOPPED or CLD_CONTINUED, indicating whether we are stopping or resuming now. If notify is nonzero, current is the last thread to stop and so will send SIGCHLD to its parent after this callback; notify reflects what the parent´s SIGCHLD has in si_code, which can sometimes be CLD_STOPPED even when type is CLD_CONTINUED. report_exec Requested by UTRACE_EVENT(EXEC). An execve system call has succeeded and the new program is about to start running. The initial user register state is handy to be tweaked directly in regs. fmt and bprm gives the details of this exec. report_syscall_entry Requested by UTRACE_EVENT(SYSCALL_ENTRY). Thread has entered the kernel to request a system call. The user register state is handy to be tweaked directly in regs. The action argument contains an enum utrace_syscall_action, use utrace_syscall_action to extract it. The return value overrides the last engine´s action for the system call. If the final action is UTRACE_SYSCALL_ABORT, no system call is made. The details of the system call being attempted can be fetched here with syscall_get_nr and syscall_get_arguments. The parameter registers can be changed with syscall_set_arguments. See above about the UTRACE_SYSCALL_RESUMED flag in action. Use UTRACE_REPORT in the return value to guarantee you get another callback (with UTRACE_SYSCALL_RESUMED flag) in case current stops with UTRACE_STOP before attempting the system call. report_syscall_exit Requested by UTRACE_EVENT(SYSCALL_EXIT). Thread is about to leave the kernel after a system call request. The user register state is handy to be tweaked directly in regs. The results of the system call attempt can be examined here using syscall_get_error and syscall_get_return_value. It is safe here to call syscall_set_return_value or syscall_rollback. report_exit Requested by UTRACE_EVENT(EXIT). Thread is exiting and cannot be prevented from doing so, but all its state is still live. The code value will be the wait result seen by the parent, and can be changed by this engine or others. The orig_code value is the real status, not changed by any tracing engine. Returning UTRACE_STOP here keeps current stopped before it cleans up its state and dies, so it can be examined by other processes. When current is allowed to run, it will die and get to the report_death callback. report_death Requested by UTRACE_EVENT(DEATH). Thread is really dead now. It might be reaped by its parent at any time, or self−reap immediately. Though the actual reaping may happen in parallel, a report_reap callback will always be ordered after a report_death callback. report_reap Requested by UTRACE_EVENT(REAP). Called when someone reaps the dead task (parent, init, or self). This means the parent called wait, or else this was a detached thread or a process whose parent ignores SIGCHLD. No more callbacks are made after this one. The engine is always detached. There is nothing more a tracing engine can do about this thread. After this callback, the engine pointer will become invalid. The task pointer may become invalid if get_task_struct hasn´t been used to keep it alive. An engine should always request this callback if it stores the engine pointer or stores any pointer in engine−>data, so it can clean up its data structures. Unlike other callbacks, this can be called from the parent´s context rather than from the traced thread itself−−it must not delay the parent by blocking. release If not NULL, this is called after the last utrace_engine_put call for a struct utrace_engine, which could be implicit after a UTRACE_DETACH return from another callback. Its argument is the engine´s data member.

DESCRIPTIONEach report_*() callback corresponds to an UTRACE_EVENT(*) bit. utrace_set_events calls on engine choose which callbacks will be made to engine from task. Most callbacks take an action argument, giving the resume action chosen by other tracing engines. All callbacks take an engine argument. The report_reap callback takes a task argument that might or might not be current. All other report_* callbacks report an event in the current task. For some calls, action also includes bits specific to that event and utrace_resume_action is used to extract the resume action. This shows what would happen if engine wasn´t there, or will if the callback´s return value uses UTRACE_RESUME. This always starts as UTRACE_RESUME when no other tracing is being done on this task. All return values contain enum utrace_resume_action bits. For some calls, other bits specific to that kind of event are added to the resume action bits with OR. These are the same bits used in the action argument. The resume action returned by a callback does not override previous engines´ choices, it only says what engine wants done. What current actually does is the action that´s most constrained among the choices made by all attached engines. See utrace_control for more information on the actions. When UTRACE_STOP is used in report_syscall_entry, then current stops before attempting the system call. In this case, another report_syscall_entry callback will follow after current resumes if UTRACE_REPORT or UTRACE_INTERRUPT was returned by some callback or passed to utrace_control. In a second or later callback, UTRACE_SYSCALL_RESUMED is set in the action argument to indicate a repeat callback still waiting to attempt the same system call invocation. This repeat callback gives each engine an opportunity to reexamine registers another engine might have changed while current was held in UTRACE_STOP. In other cases, the resume action does not take effect until current is ready to check for signals and return to user mode. If there are more callbacks to be made, the last round of calls determines the final action. A report_quiesce callback with event zero, or a report_signal callback, will always be the last one made before current resumes. Only UTRACE_STOP is ‟sticky”−−if engine returned UTRACE_STOP then current stays stopped unless engine returns different from a following callback. The report_death and report_reap callbacks do not take action arguments, and only UTRACE_DETACH is meaningful in the return value from a report_death callback. None of the resume actions applies to a dead thread. All report_*() hooks are called with no locks held, in a generally safe environment when we will be returning to user mode soon (or just entered the kernel). It is fine to block for memory allocation and the like, but all hooks are asynchronous and must not block on external events! If you want the thread to block, use UTRACE_STOP in your hook´s return value; then later wake it up with utrace_control. 0

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