linux-kernel - Re: [PATCH] Fix get ERESTARTSYS with m32 in x86

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Date:	Wed, 30 Apr 2014 15:35:33 +0200 (CEST)
From:	Mark Kettenis <mark.kettenis@...all.nl>
To:	hpa@...or.com
CC:	pinskia@...il.com, teawater@...il.com, tglx@...utronix.de,
	mingo@...hat.com, x86@...nel.org, eparis@...hat.com,
	ak@...ux.intel.com, linux-kernel@...r.kernel.org,
	gdb@...rceware.org
Subject: Re: [PATCH] Fix get ERESTARTSYS with m32 in x86_64 when debug by GDB

> Date: Tue, 29 Apr 2014 22:10:15 -0700
> From: "H. Peter Anvin" <hpa@...or.com>
> 
> On 04/29/2014 10:08 PM, Andrew Pinski wrote:
> > 
> > restoring the values is hard since even the ptrace interface does not
> > allow for that.
> > 
> 
> So that begs the ultimate question, which is: given the fact that there
> is *state missing* from the state vector (this is the core of the
> problem), is there a way we can add that state so that gdb will be able
> to save and restore it?

Carrying around additional state in GDB is complicated; I'd rather
avoid it.

arch/x86/kernel/ptrace.c:putreg32() has this bit of code:

        case offsetof(struct user32, regs.orig_eax):
                /*
                 * A 32-bit debugger setting orig_eax means to restore
                 * the state of the task restarting a 32-bit syscall.
                 * Make sure we interpret the -ERESTART* codes correctly
                 * in case the task is not actually still sitting at the
                 * exit from a 32-bit syscall with TS_COMPAT still set.
                 */
                regs->orig_ax = value;
                if (syscall_get_nr(child, regs) >= 0)
                        task_thread_info(child)->status |= TS_COMPAT;
                break;

which gets used for 32-bit compat ptrace(2).  Perhaps the same logic
should be added to putreg() if the child is a 32-bit process?

If (and only if) the goal of that TS_COMPAT flag solely is to trigger
the error code sign-extension in arch/x86/asm/syscall.h:syscall_get_error(),
we could work around to problem in GDB by checking "orig_ax" to see if
we're continuing an interrupted system call and sign extend the error
code in the real "eax" register if we are.
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