> If the process being ptraced is running, PTRACE_KILL will have no
> effect. I've seen this in 2.5.44, and the code in 2.4.18 and 2.5.66
> seems to be equivalent.
> According to the ptrace(2) man page (as of man-pages-1.56),
> PTRACE_KILL doesn't require the process to be stopped for this to
> Who is right ?
First of all, it is dangerous to depend on subtle properties
of obscure calls like ptrace. Only the part that is common to
all Unix implementations of ptrace is somewhat reliable.
A random non-Linux man page says
The ptrace() function allows a parent process to control the
execution of a child process. Its primary use is for the
implementation of breakpoint debugging. The child process
behaves normally until it encounters a signal (see
signal(3HEAD)), at which time it enters a stopped state and
its parent is notified via the wait(2) function. When the
child is in the stopped state, its parent can examine and
modify its "core image" using ptrace(). Also, the parent can
cause the child either to terminate or continue, with the
possibility of ignoring the signal that caused it to stop.
8 This request causes the child to terminate with the
same consequences as exit(2).
so, this suggests that ptrace only does something when the child
The Linux man page says
While being traced, the child will stop each time a signal
is delivered, even if the signal is being ignored. (The
exception is SIGKILL, which has its usual effect.) The
parent will be notified at its next wait(2) and may
inspect and modify the child process while it is stopped.
The parent then causes the child to continue, optionally
ignoring the delivered signal (or even delivering a dif-
ferent signal instead).
When the parent is finished tracing, it can terminate the
child with PTRACE_KILL or cause it to continue executing
in a normal, untraced mode via PTRACE_DETACH.
Here is is not completely clear whether the start of a new paragraph
means that PTRACE_KILL functions also when the child is not stopped.
For Linux, inspection of the kernel source suggests that the intention
of the author was to make PTRACE_KILL work also when the child is not
stopped, but indeed, as you say, it doesnt work that way.
Since it is not clear what the right behaviour is, it is not clear
whether there is something to fix. Maybe the man page could use an
additional sentence, or removal of whitespace.
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