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SubjectRe: [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@zytor.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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