ptrace PT_SETREGS issue

Hi

The second issue that I'm seeing with FreeBSD 15 is with ALPHA2 on aarch64.

The problem is with ptrace PT_SETREGS.

The context is when Valgrind is running with a connection to vgdb/gdb. 
In this configuration, if the user wants to interrupt the running 
process with control C the following sequence of events happens

1. vgdb uses ptrace to attach to the Valgrind process

2. It brute-force modifies the PC so that Valgrind will execute a 
function that will interrupt the guest process.

This has been working for a bit over a year since aarch64 support in 
Valgrind was added. With FreeBSD there is an issue. As an additional 
check the ptrace code writes a special value, 0x8BADF00D in x0 (first 
function call argument). On the receiving end there's an assert. That 
assert is now firing (I'm seeing a value of 4 in the first argument). If 
I uncomment the assert then the tests all work. That means that PC is 
getting set OK but not X0.

The code that does this is

static
Bool setregs (pid_t pid, struct reg *regs)
{
     if (ptrace(PT_SETREGS, pid, (caddr_t)regs, 0) < 0) {
         return False;
     }
     return True;
}


This is going to be fairly tricky to debug. I can't run Valgrind under 
gdb since that will prevent vgdb using ptrace to attach to it. I'll see 
if I can put in a pause after vgdb has detached and then attach gdb to 
Valgrind.


In the meantime, any ideas what could have changed with ptrace?

A+

Paul