linux-kernel - ptrace versus setuid changes in 4.14?

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Message-ID: <20171221091811.5bbb47db@tomh>
Date:   Thu, 21 Dec 2017 09:18:11 -0500
From:   Tom Horsley <horsley1953@...il.com>
To:     linux-kernel@...r.kernel.org
Subject: ptrace versus setuid changes in 4.14?

On my fedora 26 box with a 4.13 kernel, when a process
under ptrace control did an exec of a setuid program,
the program lost all of its setuid privileges and
ptrace could operate on it like a normal program.

Experimental evidence seems to indicate that on
fedora 27 with a 4.14 kernel, ptrace cannot
do a PEEKDATA to read anything from the just
execed setuid program. (I get errno 5 - I/O error).

Am I confused somehow, or did something really change
in this vicinity?

It puts a real crimp in my fancy debug feature to
patch code into a setuid program to make it re-exec itself,
then detach from it. (I don't suppose we could get
a setoptions feature to tell the kernel to detach
from setuid programs automagically and let the debugger
know it is no longer in control of the process?)