Re: [User-mode-linux-user] Ptrace broken since 2.4.0-test8pre4?...

[Yuri P. <yu...@as...>]

Hello Jeff,

I tested vanilla test7 with  ptrace() patch.
It breaks uml exactly like I see with any kernel > test7.

Seems like the ORIG_EAX != -1 is needed  to correctly restart syscall
after PTRACE_SYSCALL, but I did not check this codepath thoroughly.

Following what is going with uml, just for the very first exec:

[lots of trap_user.c:228, trap_user.c:149 skipped]

start_syscall():
process.c:181 ptrace(PTRACE_GETREGS, 4901, 0, 501242c0) = 0

finish_exec():
exec_user.c:26 ptrace(PTRACE_POKEUSER, 4901, UESP, bf7fff20) = 0
exec_user.c:27 ptrace(PTRACE_POKEUSER, 4901, EIP, 400019a0) = 0
exec_user.c:28 ptrace(PTRACE_POKEUSER, 4901, EDX, 0) = 0
exec_user.c:29 ptrace(PTRACE_SYSCALL, 4901, 0, 0) = 0

And voila, we got SIGSEGV instead of happy running child:
Child 4901 exited with signal 11

> Have you tried putting that ptrace patch into test7 and seeing if UML stops
> working with that as the host?
>
> test8 is a known bad platform for UML, so I'm going to consider that the
> ptrace patch is OK unless it's possible to turn a good host into a bad one by
> adding that ptrace fix.


--- v2.4.0-test7/linux/arch/i386/kernel/ptrace.c Fri Jun 23 21:55:07 2000
+++ linux/arch/i386/kernel/ptrace.c Sat Sep 2 12:00:02 2000
@@ -99,6 +99,11 @@
                case EFL:
                        value &= FLAG_MASK;
                        value |= get_stack_long(child, EFL_OFFSET) & ~FLAG_MASK;
+                       break;
+               case EIP:
+                       /* Mark us as not being in a system call, so that no restart issues happen
*/
+                       put_stack_long(child, 4*ORIG_EAX - sizeof(struct pt_regs), -1);
+                       break;
        }
        if (regno > GS*4)
                regno -= 2*4;