Linux-kernel mailing list archive 1999-23,: When did ptrace become able to modify orig_eax?

# When did ptrace become able to modify orig_eax?

Jeff Dike (jdike@karaya.com)
Sun, 06 Jun 1999 17:32:04 -0300

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I'm trying to get the user-mode kernel to run without needing changes to the
hosting kernel. As part of that, I'm looking at using ptrace to do system
call redirection.

Ideally, I'd like to prevent system calls from happening at all in the hosting
kernel, but that looks impossible. My fallback position is to get the hosting
kernel to execute something innocuous like getpid. That looks like it should
work in entry.S, but in ptrace.c, there is this in the PTRACE_POKEUSR code:

if (addr == ORIG_EAX)
return -EIO;

which kind of gets in the way.

I just looked at the 2.2.7 code, and it looks like that disappeared, plus
there is PTRACE_SETREGS, which is what I want anyway.

So, the question is, does anyone remember when that happened?

I haven't written the code to make this work, but the idea is that it will do
this:
On system call entry, use PTRACE_GETREGS to decode and execute the system
call in user space.
PTRACE_SETREGS and PTRACE_SYSCALL to make the process call getpid
On system call return, PTRACE_SETREGS to make the system call return the
user-space value.

Is there any reason that this wouldn't work?

Are there better ways to do it?

Jeff

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