Hi,
I finally did figure out where in the host kernel the ptrace syscall
fails with -EFAULT.
In arch/x86/kernel/fpu/regset.c:130:
114 int xstateregs_set(struct task_struct *target, const struct user_regset *regset,
115 unsigned int pos, unsigned int count,
116 const void *kbuf, const void __user *ubuf)
117 {
118 struct fpu *fpu = &target->thread.fpu;
119 struct xregs_state *xsave;
120 int ret;
121
122 if (!boot_cpu_has(X86_FEATURE_XSAVE))
123 return -ENODEV;
124
125 pr_info("in xstateregs_set");
126
127 /*
128 * A whole standard-format XSAVE buffer is needed:
129 */
130 if ((pos != 0) || (count < fpu_user_xstate_size)) {
131 pr_info("EFAULT from xstateregs_set");
132-> pr_info("pos = %i, count = %i, fpu_user_xstate_size= %i\n", pos, count, fpu_user_xstate_size);
133 return -EFAULT;
134 }
Sadly I had to fallback to debugging by printk because kgdb/qemu
gdbstub, all didn't work for some unknown reason :-(
output is:
[ 69.598349] EFAULT from xstateregs_set
[ 69.598350] pos = 0, count = 832, fpu_user_xstate_size= 1088
calling code is in arch/x86/um/os-Linux/registers.c:
49 int restore_fp_registers(int pid, unsigned long *fp_regs)
50 {
51 struct iovec iov;
52
53 if (have_xstate_support) {
54 iov.iov_base = fp_regs;
55 iov.iov_len = sizeof(struct _xstate);
56 if (ptrace(PTRACE_SETREGSET, pid, NT_X86_XSTATE, &iov) < 0)
57 -> return -errno;
58 return 0;
59 } else {
60 return restore_i387_registers(pid, fp_regs);
61 }
62 }
it looks like _xstate is too short for above operation, I wonder why
PTRACE_GETREGSET works without a warning of too short size.
with kind regards
thomas
Thomas,
Am 20.06.2017 um 03:56 schrieb Thomas Meyer:
> Hi,
>
> I finally did figure out where in the host kernel the ptrace syscall
> fails with -EFAULT.
Nice! Thanks a lot for digging into this. I still had no chance to setup
Ipv6 to connect to your host and figure myself. ;-\
> In arch/x86/kernel/fpu/regset.c:130:
>
> 114 int xstateregs_set(struct task_struct *target, const struct user_regset *regset,
> 115 unsigned int pos, unsigned int count,
> 116 const void *kbuf, const void __user *ubuf)
> 117 {
> 118 struct fpu *fpu = &target->thread.fpu;
> 119 struct xregs_state *xsave;
> 120 int ret;
> 121
> 122 if (!boot_cpu_has(X86_FEATURE_XSAVE))
> 123 return -ENODEV;
> 124
> 125 pr_info("in xstateregs_set");
> 126
> 127 /*
> 128 * A whole standard-format XSAVE buffer is needed:
> 129 */
> 130 if ((pos != 0) || (count < fpu_user_xstate_size)) {
> 131 pr_info("EFAULT from xstateregs_set");
> 132-> pr_info("pos = %i, count = %i, fpu_user_xstate_size= %i\n", pos, count, fpu_user_xstate_size);
> 133 return -EFAULT;
> 134 }
>
> Sadly I had to fallback to debugging by printk because kgdb/qemu
> gdbstub, all didn't work for some unknown reason :-(
As always. printk is best debugger ever. ;-)
> output is:
> [ 69.598349] EFAULT from xstateregs_set
> [ 69.598350] pos = 0, count = 832, fpu_user_xstate_size= 1088
>
> calling code is in arch/x86/um/os-Linux/registers.c:
>
> 49 int restore_fp_registers(int pid, unsigned long *fp_regs)
> 50 {
> 51 struct iovec iov;
> 52
> 53 if (have_xstate_support) {
> 54 iov.iov_base = fp_regs;
> 55 iov.iov_len = sizeof(struct _xstate);
> 56 if (ptrace(PTRACE_SETREGSET, pid, NT_X86_XSTATE, &iov) < 0)
> 57 -> return -errno;
> 58 return 0;
> 59 } else {
> 60 return restore_i387_registers(pid, fp_regs);
> 61 }
> 62 }
>
> it looks like _xstate is too short for above operation, I wonder why
> PTRACE_GETREGSET works without a warning of too short size.
Does PTRACE_GETREGSET return a size? Maybe we have to take this into account.
It could be that your host CPU has a smaller set.
Also check whether PTRACE_SETREGSET always fails.
Thanks,
//richard
Am Dienstag, den 20.06.2017, 08:58 +0200 schrieb Richard Weinberger:
> Thomas,
>
> Am 20.06.2017 um 03:56 schrieb Thomas Meyer:
> > Hi,
> >
> > I finally did figure out where in the host kernel the ptrace
> > syscall
> > fails with -EFAULT.
>
> Nice! Thanks a lot for digging into this. I still had no chance to
> setup
> Ipv6 to connect to your host and figure myself. ;-\
>
> > In arch/x86/kernel/fpu/regset.c:130:
> >
> > 114 int xstateregs_set(struct task_struct *target, const struct
> > user_regset *regset,
> > 115 unsigned int pos, unsigned int count,
> > 116 const void *kbuf, const void __user *ubuf)
> > 117 {
> > 118 struct fpu *fpu = &target->thread.fpu;
> > 119 struct xregs_state *xsave;
> > 120 int ret;
> > 121
> > 122 if (!boot_cpu_has(X86_FEATURE_XSAVE))
> > 123 return -ENODEV;
> > 124
> > 125 pr_info("in xstateregs_set");
> > 126
> > 127 /*
> > 128 * A whole standard-format XSAVE buffer is needed:
> > 129 */
> > 130 if ((pos != 0) || (count < fpu_user_xstate_size)) {
> > 131 pr_info("EFAULT from xstateregs_set");
> > 132-> pr_info("pos = %i, count = %i,
> > fpu_user_xstate_size= %i\n", pos, count, fpu_user_xstate_size);
> > 133 return -EFAULT;
> > 134 }
> >
> > Sadly I had to fallback to debugging by printk because kgdb/qemu
> > gdbstub, all didn't work for some unknown reason :-(
>
> As always. printk is best debugger ever. ;-)
>
> > output is:
> > [ 69.598349] EFAULT from xstateregs_set
> > [ 69.598350] pos = 0, count = 832, fpu_user_xstate_size= 1088
> >
> > calling code is in arch/x86/um/os-Linux/registers.c:
> >
> > 49 int restore_fp_registers(int pid, unsigned long *fp_regs)
> > 50 {
> > 51 struct iovec iov;
> > 52
> > 53 if (have_xstate_support) {
> > 54 iov.iov_base = fp_regs;
> > 55 iov.iov_len = sizeof(struct _xstate);
> > 56 if (ptrace(PTRACE_SETREGSET, pid,
> > NT_X86_XSTATE, &iov) < 0)
> > 57 -> return -errno;
> > 58 return 0;
> > 59 } else {
> > 60 return restore_i387_registers(pid, fp_regs);
> > 61 }
> > 62 }
> >
> > it looks like _xstate is too short for above operation, I wonder
> > why
> > PTRACE_GETREGSET works without a warning of too short size.
>
> Does PTRACE_GETREGSET return a size?
Yes, it returns 832. the size of struct _xstate.
> Maybe we have to take this into account.
> It could be that your host CPU has a smaller set.
> Also check whether PTRACE_SETREGSET always fails.
In UML the first userspace ptrace always fails, so init get's killed.
The check "count < fpu_user_xstate_size" was introduced by commit:
commit 91c3dba7dbc199191272f4a9863f86ea3bfd679f
Author: Yu-cheng Yu <[email protected]>
Date: Fri Jun 17 13:07:17 2016 -0700
x86/fpu/xstate: Fix PTRACE frames for XSAVES
XSAVES uses compacted format and is a kernel instruction. The kernel
should use standard-format, non-supervisor state data for PTRACE.
So to summarize:
- PTRACE_GETREGSET with NT_X86_XSTATE gets 832 and return 832, with no
error.
- PTRACE_SETREGSET get 832 (sizeof struct _xstate) but wants at least
1088, otherwise it will fail with -EFAULT (why not -EINVAL?)
Ideas?
>
> Thanks,
> //richard
[adding x86 folks]
Am 20.06.2017 um 10:49 schrieb Thomas Meyer:
> Am Dienstag, den 20.06.2017, 08:58 +0200 schrieb Richard Weinberger:
>> Thomas,
>>
>> Am 20.06.2017 um 03:56 schrieb Thomas Meyer:
>>> Hi,
>>>
>>> I finally did figure out where in the host kernel the ptrace
>>> syscall
>>> fails with -EFAULT.
>>
>> Nice! Thanks a lot for digging into this. I still had no chance to
>> setup
>> Ipv6 to connect to your host and figure myself. ;-\
>>
>>> In arch/x86/kernel/fpu/regset.c:130:
>>>
>>> 114 int xstateregs_set(struct task_struct *target, const struct
>>> user_regset *regset,
>>> 115 unsigned int pos, unsigned int count,
>>> 116 const void *kbuf, const void __user *ubuf)
>>> 117 {
>>> 118 struct fpu *fpu = &target->thread.fpu;
>>> 119 struct xregs_state *xsave;
>>> 120 int ret;
>>> 121
>>> 122 if (!boot_cpu_has(X86_FEATURE_XSAVE))
>>> 123 return -ENODEV;
>>> 124
>>> 125 pr_info("in xstateregs_set");
>>> 126
>>> 127 /*
>>> 128 * A whole standard-format XSAVE buffer is needed:
>>> 129 */
>>> 130 if ((pos != 0) || (count < fpu_user_xstate_size)) {
>>> 131 pr_info("EFAULT from xstateregs_set");
>>> 132-> pr_info("pos = %i, count = %i,
>>> fpu_user_xstate_size= %i\n", pos, count, fpu_user_xstate_size);
>>> 133 return -EFAULT;
>>> 134 }
>>>
>>> Sadly I had to fallback to debugging by printk because kgdb/qemu
>>> gdbstub, all didn't work for some unknown reason :-(
>>
>> As always. printk is best debugger ever. ;-)
>>
>>> output is:
>>> [ 69.598349] EFAULT from xstateregs_set
>>> [ 69.598350] pos = 0, count = 832, fpu_user_xstate_size= 1088
>>>
>>> calling code is in arch/x86/um/os-Linux/registers.c:
>>>
>>> 49 int restore_fp_registers(int pid, unsigned long *fp_regs)
>>> 50 {
>>> 51 struct iovec iov;
>>> 52
>>> 53 if (have_xstate_support) {
>>> 54 iov.iov_base = fp_regs;
>>> 55 iov.iov_len = sizeof(struct _xstate);
>>> 56 if (ptrace(PTRACE_SETREGSET, pid,
>>> NT_X86_XSTATE, &iov) < 0)
>>> 57 -> return -errno;
>>> 58 return 0;
>>> 59 } else {
>>> 60 return restore_i387_registers(pid, fp_regs);
>>> 61 }
>>> 62 }
>>>
>>> it looks like _xstate is too short for above operation, I wonder
>>> why
>>> PTRACE_GETREGSET works without a warning of too short size.
>>
>> Does PTRACE_GETREGSET return a size?
>
> Yes, it returns 832. the size of struct _xstate.
>
>> Maybe we have to take this into account.
>> It could be that your host CPU has a smaller set.
>> Also check whether PTRACE_SETREGSET always fails.
>
> In UML the first userspace ptrace always fails, so init get's killed.
>
> The check "count < fpu_user_xstate_size" was introduced by commit:
>
> commit 91c3dba7dbc199191272f4a9863f86ea3bfd679f
> Author: Yu-cheng Yu <[email protected]>
> Date: Fri Jun 17 13:07:17 2016 -0700
>
> x86/fpu/xstate: Fix PTRACE frames for XSAVES
>
> XSAVES uses compacted format and is a kernel instruction. The kernel
> should use standard-format, non-supervisor state data for PTRACE.
>
> So to summarize:
>
> - PTRACE_GETREGSET with NT_X86_XSTATE gets 832 and return 832, with no
> error.
>
> - PTRACE_SETREGSET get 832 (sizeof struct _xstate) but wants at least
> 1088, otherwise it will fail with -EFAULT (why not -EINVAL?)
>
> Ideas?
>
>>
>> Thanks,
>> //richard
On Tue, 2017-06-20 at 11:05 +0200, Richard Weinberger wrote:
> [adding x86 folks]
>
> Am 20.06.2017 um 10:49 schrieb Thomas Meyer:
> >
> > In UML the first userspace ptrace always fails, so init get's killed.
> >
> > The check "count < fpu_user_xstate_size" was introduced by commit:
> >
> > commit 91c3dba7dbc199191272f4a9863f86ea3bfd679f
> > Author: Yu-cheng Yu <[email protected]>
> > Date: Fri Jun 17 13:07:17 2016 -0700
> >
> > x86/fpu/xstate: Fix PTRACE frames for XSAVES
> >
> > XSAVES uses compacted format and is a kernel instruction. The kernel
> > should use standard-format, non-supervisor state data for PTRACE.
> >
> > So to summarize:
> >
> > - PTRACE_GETREGSET with NT_X86_XSTATE gets 832 and return 832, with no
> > error.
> >
> > - PTRACE_SETREGSET get 832 (sizeof struct _xstate) but wants at least
> > 1088, otherwise it will fail with -EFAULT (why not -EINVAL?)
> >
> > Ideas?
We considered allowing a partial XSAVE buffer for PTRACE_SETREGSET, but
it was that the XSAVE instruction requires a full-size buffer led to
this choice. Using a smaller buffer for XSAVE causes a fault.
Yu-cheng
Yu-cheng,
Am 20.06.2017 um 20:04 schrieb Yu-cheng Yu:
>>> So to summarize:
>>>
>>> - PTRACE_GETREGSET with NT_X86_XSTATE gets 832 and return 832, with no
>>> error.
>>>
>>> - PTRACE_SETREGSET get 832 (sizeof struct _xstate) but wants at least
>>> 1088, otherwise it will fail with -EFAULT (why not -EINVAL?)
>>>
>>> Ideas?
>
> We considered allowing a partial XSAVE buffer for PTRACE_SETREGSET, but
> it was that the XSAVE instruction requires a full-size buffer led to
> this choice. Using a smaller buffer for XSAVE causes a fault.
So, this code is not supposed to work?
iov.iov_base = fp_regs;
iov.iov_len = sizeof(struct _xstate);
ptrace(PTRACE_GETREGSET, pid, NT_X86_XSTATE, &iov);
ptrace(PTRACE_SETREGSET, pid, NT_X86_XSTATE, &iov);
This is what UML does and on Thomas's new Laptop PTRACE_SETREGSET is failing.
Thanks,
//richard
Yu-cheng,
Am 20.06.2017 um 20:17 schrieb Richard Weinberger:
> Yu-cheng,
>
> Am 20.06.2017 um 20:04 schrieb Yu-cheng Yu:
>>>> So to summarize:
>>>>
>>>> - PTRACE_GETREGSET with NT_X86_XSTATE gets 832 and return 832, with no
>>>> error.
>>>>
>>>> - PTRACE_SETREGSET get 832 (sizeof struct _xstate) but wants at least
>>>> 1088, otherwise it will fail with -EFAULT (why not -EINVAL?)
>>>>
>>>> Ideas?
>>
>> We considered allowing a partial XSAVE buffer for PTRACE_SETREGSET, but
>> it was that the XSAVE instruction requires a full-size buffer led to
>> this choice. Using a smaller buffer for XSAVE causes a fault.
>
> So, this code is not supposed to work?
>
> iov.iov_base = fp_regs;
> iov.iov_len = sizeof(struct _xstate);
> ptrace(PTRACE_GETREGSET, pid, NT_X86_XSTATE, &iov);
> ptrace(PTRACE_SETREGSET, pid, NT_X86_XSTATE, &iov);
>
> This is what UML does and on Thomas's new Laptop PTRACE_SETREGSET is failing.
Hmm, I think we need to do what gdb does, it uses a buffer of size X86_XSTATE_MAX_SIZE.
Thanks,
//richard
On Tue, 2017-06-20 at 20:59 +0200, Richard Weinberger wrote:
> Yu-cheng,
>
> Am 20.06.2017 um 20:17 schrieb Richard Weinberger:
> > Yu-cheng,
> >
> > Am 20.06.2017 um 20:04 schrieb Yu-cheng Yu:
> >>>> So to summarize:
> >>>>
> >>>> - PTRACE_GETREGSET with NT_X86_XSTATE gets 832 and return 832, with no
> >>>> error.
> >>>>
> >>>> - PTRACE_SETREGSET get 832 (sizeof struct _xstate) but wants at least
> >>>> 1088, otherwise it will fail with -EFAULT (why not -EINVAL?)
> >>>>
> >>>> Ideas?
> >>
> >> We considered allowing a partial XSAVE buffer for PTRACE_SETREGSET, but
> >> it was that the XSAVE instruction requires a full-size buffer led to
> >> this choice. Using a smaller buffer for XSAVE causes a fault.
> >
> > So, this code is not supposed to work?
> >
> > iov.iov_base = fp_regs;
> > iov.iov_len = sizeof(struct _xstate);
> > ptrace(PTRACE_GETREGSET, pid, NT_X86_XSTATE, &iov);
> > ptrace(PTRACE_SETREGSET, pid, NT_X86_XSTATE, &iov);
> >
> > This is what UML does and on Thomas's new Laptop PTRACE_SETREGSET is failing.
>
> Hmm, I think we need to do what gdb does, it uses a buffer of size X86_XSTATE_MAX_SIZE.
>
Linux kernel determines XSAVE buffer size from CPUID:
http://elixir.free-electrons.com/linux/latest/source/arch/x86/kernel/fpu/xstate.c#L626
GDB has a fixed X86_XSTATE_MAX_SIZE of 2688. That can become an issue.
Yu-cheng
> Am 20.06.2017 um 21:53 schrieb Yu-cheng Yu <[email protected]>:
>
>> On Tue, 2017-06-20 at 20:59 +0200, Richard Weinberger wrote:
>> Yu-cheng,
>>
>>> Am 20.06.2017 um 20:17 schrieb Richard Weinberger:
>>> Yu-cheng,
>>>
>>> Am 20.06.2017 um 20:04 schrieb Yu-cheng Yu:
>>>>>> So to summarize:
>>>>>>
>>>>>> - PTRACE_GETREGSET with NT_X86_XSTATE gets 832 and return 832, with no
>>>>>> error.
>>>>>>
>>>>>> - PTRACE_SETREGSET get 832 (sizeof struct _xstate) but wants at least
>>>>>> 1088, otherwise it will fail with -EFAULT (why not -EINVAL?)
>>>>>>
>>>>>> Ideas?
>>>>
>>>> We considered allowing a partial XSAVE buffer for PTRACE_SETREGSET, but
>>>> it was that the XSAVE instruction requires a full-size buffer led to
>>>> this choice. Using a smaller buffer for XSAVE causes a fault.
>>>
>>> So, this code is not supposed to work?
>>>
>>> iov.iov_base = fp_regs;
>>> iov.iov_len = sizeof(struct _xstate);
>>> ptrace(PTRACE_GETREGSET, pid, NT_X86_XSTATE, &iov);
>>> ptrace(PTRACE_SETREGSET, pid, NT_X86_XSTATE, &iov);
>>>
>>> This is what UML does and on Thomas's new Laptop PTRACE_SETREGSET is failing.
>>
>> Hmm, I think we need to do what gdb does, it uses a buffer of size X86_XSTATE_MAX_SIZE.
>
> Linux kernel determines XSAVE buffer size from CPUID:
> http://elixir.free-electrons.com/linux/latest/source/arch/x86/kernel/fpu/xstate.c#L626
Hi,
Is there a user space API to get this value?
>
> GDB has a fixed X86_XSTATE_MAX_SIZE of 2688. That can become an issue.
>
> Yu-cheng
>
>