This is still not intended for review, I just demonstrate the (IMO) full branch where the end is the ability to run truss in cap mode.
I am posting individual commits from the branch for review still.

Update comment.

Edit comments and the man page update.

Use atomic to load p_flag.

Fix PTCHLD_TRACED_BY_ME.
Check uap->data < 0.
Use more correct type for rsrv[] to avoid compat32 conversions.
Man page editing.

In D58315#1338064, @markj wrote:

Why not implement this with procctl()? That seems more natural to me.

please re-upload the patch with the full context.

Add PTCHLD_TRACED_BY_ME.
Add space for future ABI expansion.

Please upload patches with the full context.

In D58321#1337618, @netchild wrote:

So you disagree with kern.features.exterror.
Do you also disagree with kern.features.exterr_strings which covers the optional behavior of verbose exterror and is useful in unit tests to differentiate between "fails because something is wrong -> error" and "fails because it does not exist -> intendet -> ok".

In D58191#1337590, @pho wrote:

I ran all of the stress2 tests without seeing any issues.

This adds two syscalls to each dynamic syscall failure path. I understand that calling the sysctl is the easiest way to handle dynamically loaded modules. But I already got some negative comments due to exterrors adding some nanoseconds to the failing syscalls timings.
Although I do not object in principle, can we have some smarter way to avoid the sysctl if we already queried the specific module? E.g. we can make each module to register 'range' for dynamic errors' pathes, and memoize the range with the values in libc.
This is esp. sorrow because the display of the source line is really a niche feature.

Could somebody provide an isolated test for the situation where the patch is needed?

As I said it elsewhere, there is no point in kern.features.exterror. You cannot run 15 userspace on <= 14 kernel.

Restore P2_ZOMBIEREF_PARENT when reparenting to reaper, to allow reaper do it deeds.

Add comment for the placement of NOTE_FORK activation.
Edit the man page.

In D58292#1337369, @markj wrote:

In D58292#1337327, @kib wrote:

Report NOTE_FORK for procdesc after the child is attached to the debugger.

I suggest adding a test for this, or at least a comment explaining why it is important.

Fix inverted assertion.

Report NOTE_FORK for procdesc after the child is attached to the debugger.

In D58302#1337168, @aokblast wrote:

In D58302#1337163, @kib wrote:

Could f6ee1d20d36813e990691 fixed your issue?

Anyway, the useful tool is gdb' pinfo /o type.

It should be the correct reason. Thanks! I don't have ia32 machine so I planned to check the dwarf info.

Could f6ee1d20d36813e990691 fixed your issue?

Why? p_asig and p_tree_refcount are at the end of the struct proc.

Upload the right patch

The current commit from the topic branch.

In D58264#1337050, @markj wrote:

In D58264#1336261, @kib wrote:

Note that the patch breaks existing tests, but IMO the current behavior is not logical. For instance, when I truss -f /bin/sh using process descriptors, shell is missing the zombie exit status. In other words, pdopenpid() breaks correct code.

Maybe pdwait() shouldn't let you reap a zombie that isn't your child?

Update the man page with the review comments.

For the discussion, I am still somewhat surprised that so much of the FreeBSD symbols are leaked into linuxkpi headers, Most amusing and dangerous is PAGE_MASK (AFAIR).

In D58056#1336834, @ngie wrote:

In D58056#1336833, @kib wrote:

In D58056#1336831, @ngie wrote:

This change broke the build on armv7 (32-bit ARM). I addressed the definition/declaration mismatch in rG5cafd6213f14.

This is the supposed fix, I am waiting for tinderbox to finish.

diff --git a/sys/kern/sys_generic.c b/sys/kern/sys_generic.c
index c867867f2618..c16e806e0bd7 100644
--- a/sys/kern/sys_generic.c
+++ b/sys/kern/sys_generic.c
@@ -2353,8 +2353,8 @@ kern_exterrctl(struct thread *td, u_int op, u_int flags, void *ptr)
 }
 
 int
-exterr_set(int eerror, int category, const char *mmsg, uintptr_t pp1,
-    uintptr_t pp2, int line)
+exterr_set(int eerror, int category, const char *mmsg, uint64ptr_t pp1,
+    uint64ptr_t pp2, int line)
 {
   struct thread *td;
 

I *just* committed that 😅...

In D58056#1336831, @ngie wrote:

This change broke the build on armv7 (32-bit ARM). I addressed the definition/declaration mismatch in rG5cafd6213f14.

Validate uap->data to be the size of the structure.
Edit man page.

Style

Note that the patch breaks existing tests, but IMO the current behavior is not logical. For instance, when I truss -f /bin/sh using process descriptors, shell is missing the zombie exit status. In other words, pdopenpid() breaks correct code.

Real latest diff.

Document PT_SET_SC_RET