Fix a regression in r303423.

[markj]

Summary

I noticed the bug fixed in r304440 when running
"truss -f make -C /usr/src/bin/cat". The fix exposed another bug: PT_TRACE_ME
shouldn't set P2_PTRACE_FSTP, since the first signal to stop a traced
process after an exec is SIGTRAP. This fix modifies proc_set_traced() to
allow the caller to indicate whether the process will be initially
stopped using SIGSTOP.

[kib]

Shouldn't, instead of not setting the P2_PTRACE_FSTP flag, _ME code treat SIGTRAP same as SIGSTOP ? I.e. P2_PTRACE_FSTP causing issignal() to prefer SIGTRAP for the next selection ?

If yes, this can be done with either additional flag, or a new field in struct proc indicating which signal is FSTP.

[jhb]

In D7576#157227, @kib wrote:

Shouldn't, instead of not setting the P2_PTRACE_FSTP flag, _ME code treat SIGTRAP same as SIGSTOP ? I.e. P2_PTRACE_FSTP causing issignal() to prefer SIGTRAP for the next selection ?

If yes, this can be done with either additional flag, or a new field in struct proc indicating which signal is FSTP.

(BTW, the inconsistency in SIGTRAP/SIGSTOP is definitely a confusing thing about ptrace()). I think it's probably safest to do this. I think a new field to indicate the expected signal number (can be passed as the second arg to proc_set_traced()) would work in terms of the API.

[markj]

In D7576#157227, @kib wrote:

Shouldn't, instead of not setting the P2_PTRACE_FSTP flag, _ME code treat SIGTRAP same as SIGSTOP ? I.e. P2_PTRACE_FSTP causing issignal() to prefer SIGTRAP for the next selection ?

If yes, this can be done with either additional flag, or a new field in struct proc indicating which signal is FSTP.

In this case the SIGTRAP is not intercepted from issignal(). Rather, syscallret() calls ptracestop(SIGTRAP) directly, and thus we're not really sending a signal to the traced process but rather just using ptracestop() to notify the tracing process about the exec of the tracee. So I don't think PT_TRACE_ME is susceptible to the same problem that r303423 attempted to fix. I'm not sure if this contradicts the intent behind your suggestion.

[kib]

I see.