Re: [PATCH] Fix get ERESTARTSYS with m32 in x86_64 when debug by GDB

[Hui Zhu]

On 05/01/14 00:28, Hui Zhu wrote:
> On Wed, Apr 30, 2014 at 9:35  PM, Mark Kettenis <mark.kettenis@xxxxxxxxx> wrote:
>>> Date: Tue, 29 Apr 2014 22:10:15 -0700
>>> From: "H. Peter Anvin" <hpa@xxxxxxxxx>
>>>
>>> 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?
>
> Make a new patch that add following code to putreg():
> case offsetof(struct user_regs_struct, orig_ax):
> /*
> * A 64-bit debugger setting orig_ax of a 32-bit inferior
> * 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.
> */
> if (test_ti_thread_flag(task_thread_info(child), TIF_IA32)) {
> struct pt_regs *regs = task_pt_regs(child);
> regs->orig_ax = value;
> if (syscall_get_nr(child, regs) >= 0)
> task_thread_info(child)->status |= TS_COMPAT;
> return 0;
> }
>
>>
>> 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.
>
> I will update patch in
> https://sourceware.org/ml/gdb-patches/2014-04/msg00429.html
> according to this comments.
>
> Thanks,
> Hui
>

I sorry that previous patch has some format issue, post a new one.

Signed-off-by: Hui Zhu <hui@xxxxxxxxxxxxxxxx>
---
--- a/arch/x86/kernel/ptrace.c
+++ b/arch/x86/kernel/ptrace.c
@@ -452,6 +452,23 @@ static int putreg(struct task_struct *ch
         if (child->thread.gs != value)
             return do_arch_prctl(child, ARCH_SET_GS, value);
         return 0;
+    case offsetof(struct user_regs_struct, orig_ax):
+        /*
+         * A 64-bit debugger setting orig_ax of a 32-bit inferior
+         * 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.
+         */
+        if (test_ti_thread_flag(task_thread_info(child), TIF_IA32)) {
+            struct pt_regs *regs = task_pt_regs(child);
+            regs->orig_ax = value;
+            if (syscall_get_nr(child, regs) >= 0)
+                task_thread_info(child)->status |= TS_COMPAT;
+            return 0;
+        }
+        break;
 #endif
     }


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