Re: ptrace PT_SETREGS issue

> On 22 Sep 2025, at 13:39, Paul Floyd <paulf2718@gmail.com> wrote:
> 
> 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;
> }
> 

I did some more work on this. I added a check with ptrace PT_GETREGS and then did a memcmp of the two and they are the same.

So it looks like vgdb and ptrace both think everything is OK.

Now I have to see how I can debug Valgrind from the moment the vgdb does a ptrace detach of its process.

A+
Paul