svn commit: r304928 - in head/lib/libc: amd64/sys i386/sys sys

[Konstantin Belousov]

On Sun, Aug 28, 2016 at 04:09:51PM -0700, John Baldwin wrote:
> OTOH, given that we explicitly documented it as not being true, I suspect
> any applications that are using ptrace() are going off the documentation, not
> the implementation artifact.  Note that Linux's ptrace() documents the same
> requirement as before this change (caller is required to clear errno), so I
> doubt there is any actual software out there that expects the
> FreeBSD-specific behavior.  Given that and the extra maintenance overhead of
> having to dink with errno in assembly on X architectures, I'd rather we keep
> the old language in the manpage and remove the 'errno' frobbing in the system
> call wrappers.  To be honest, my first response to this commit was one of
> surprise that we modify errno directly as that is inconsistent with other
> system calls.  (I haven't looked to see if any other system call wrappers
> modify errno for non-error cases.)

The problematic calls are PT_PEEK_I and PT_PEEK_D, as far as I understand.

I dug into the ptrace(2) consumers, I found a lot of things using
it which I would not expect to use, besides usual suspects of gdb
lldb libunwind reptyr etc.  Most surprising was that even high-profile
consumers including gdb sometimes fail to check errno after PT_PEEK. On
the other hand, I did not found a case in gdb where errno is checked
after PT_PEEK but not zeroed before the syscall.

I almost agreed with you after the reading, but then I decided to look
into glibc just in case.  What I found there is really fascinating.
>From glibc/sysdeps/unix/sysv/linux:
  res = INLINE_SYSCALL (ptrace, 4, request, pid, addr, data);
  if (res >= 0 && request > 0 && request < 4)
    {
      __set_errno (0);
      return ret;
    }
#define PTRACE_PEEKTEXT		   1
#define PTRACE_PEEKDATA		   2
#define PTRACE_PEEKUSR		   3

In the end, I might consider changing the ptrace wrappers into
consolidated C source, it would look like that

int
ptrace(int request, pid_t pid, caddr_t addr, int data)
{

	errno = 0;
	return (__sys_ptrace(request, pid, addr, data));
}