Re: in_compat_syscall() returns from kernel thread for X86_32.

[Andreas Dilger]

On Oct 18, 2018, at 11:26 AM, Andy Lutomirski <luto@xxxxxxxxxx> wrote:
> 
> On Wed, Oct 17, 2018 at 9:36 PM NeilBrown <neilb@xxxxxxxx> wrote:
>> 
>> On Wed, Oct 17 2018, Andy Lutomirski wrote:
>> 
>>> On Wed, Oct 17, 2018 at 6:48 PM NeilBrown <neilb@xxxxxxxx> wrote:
>>>> 
>>>> 
>>>> Was: Re: [tip:x86/asm] x86/entry: Rename is_{ia32,x32}_task() to in_{ia32,x32}_syscall()
>>>> On Tue, Apr 19 2016, tip-bot for Dmitry Safonov wrote:
>>>> 
>>>>> Commit-ID:  abfb9498ee1327f534df92a7ecaea81a85913bae
>>>>> Gitweb:     http://git.kernel.org/tip/abfb9498ee1327f534df92a7ecaea81a85913bae
>>>>> Author:     Dmitry Safonov <dsafonov@xxxxxxxxxxxxx>
>>>>> AuthorDate: Mon, 18 Apr 2016 16:43:43 +0300
>>>>> Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
>>>>> CommitDate: Tue, 19 Apr 2016 10:44:52 +0200
>>>>> 
>>>>> x86/entry: Rename is_{ia32,x32}_task() to in_{ia32,x32}_syscall()
>>>>> 
>>>> ...
>>>>> @@ -318,7 +318,7 @@ static inline bool is_x32_task(void)
>>>>> 
>>>>> static inline bool in_compat_syscall(void)
>>>>> {
>>>>> -     return is_ia32_task() || is_x32_task();
>>>>> +     return in_ia32_syscall() || in_x32_syscall();
>>>>> }
>>>> 
>>>> Hi,
>>>> I'm reply to this patch largely to make sure I get the right people
>>>> .....
>>>> 
>>>> This test is always true when CONFIG_X86_32 is set, as that forces
>>>> in_ia32_syscall() to true.
>>>> However we might not be in a syscall at all - we might be running a
>>>> kernel thread which is always in 64 mode.
>>>> Every other implementation of in_compat_syscall() that I found is
>>>> dependant on a thread flag or syscall register flag, and so returns
>>>> "false" in a kernel thread.
>>>> 
>>>> Might something like this be appropriate?
>>>> 
>>>> diff --git a/arch/x86/include/asm/thread_info.h b/arch/x86/include/asm/thread_info.h
>>>> index 2ff2a30a264f..c265b40a78f2 100644
>>>> --- a/arch/x86/include/asm/thread_info.h
>>>> +++ b/arch/x86/include/asm/thread_info.h
>>>> @@ -219,7 +219,7 @@ static inline int arch_within_stack_frames(const void * const stack,
>>>> #ifndef __ASSEMBLY__
>>>> 
>>>> #ifdef CONFIG_X86_32
>>>> -#define in_ia32_syscall() true
>>>> +#define in_ia32_syscall() (!(current->flags & PF_KTHREAD))
>>>> #else
>>>> #define in_ia32_syscall() (IS_ENABLED(CONFIG_IA32_EMULATION) && \
>>>>                           current_thread_info()->status & TS_COMPAT)
>>>> 
>>>> This came up in the (no out-of-tree) lustre filesystem where some code
>>>> needs to assume 32-bit mode in X86_32 syscalls, and 64-bit mode in kernel
>>>> threads.
>>>> 
>>> 
>>> I could get on board with:
>>> 
>>> ({WARN_ON_ONCE(current->flags & PF_KTHREAD); true})
>>> 
>>> The point of these accessors is to be used *in a syscall*.
>>> 
>>> What on Earth is Lustre doing that makes it have this problem?
>> 
>> Lustre uses it in the ->getattr method to make sure ->ino, ->dev and
>> ->rdev are appropriately sized.  This isn't very different from the
>> usage in ext4 to ensure the seek offset for directories is suitable.
>> 
>> These interfaces can be used both from systemcalls and from kernel
>> threads, such as via nfsd.
>> 
>> I don't *know* if nfsd is the particular kthread that causes problems
>> for lustre.  All I know is that ->getattr returns 32bit squashed inode
>> numbers in kthread context where 64 bit numbers would be expected.
>> 
> 
> Well, that looks like Lustre is copying an ext4 bug.
> 
> Hi ext4 people-
> 
> ext4's is_32bit_api() function is bogus.  You can't use
> in_compat_syscall() unless you know you're in a syscall
> 
> The buggy code was introduced in:
> 
> commit d1f5273e9adb40724a85272f248f210dc4ce919a
> Author: Fan Yong <yong.fan@xxxxxxxxxxxxx>
> Date:   Sun Mar 18 22:44:40 2012 -0400
> 
>    ext4: return 32/64-bit dir name hash according to usage type
> 
> I don't know what the right solution is.  Al, is it legit at all for
> fops->llseek to care about the caller's bitness?  If what ext4 is
> doing is legit, then ISTM the VFS needs to gain a new API to tell
> ->llseek what to do.  But I'm wondering why FMODE_64BITHASH by itself
> isn't sufficient,
> 
> I'm quite tempted to add a warning to the x86 arch code to try to
> catch this type of bug.  Fortunately, a bit of grepping suggests that
> ext4 is the only filesystem with this problem.

We need to know whether the readdir cookie returned to userspace
should be a 32-bit cookie or a 64-bit cookie.  Trying to return
a 64-bit value will result in -EOVERFLOW for a 32-bit application,
but is preferable (if possible) because it reduces the chance of
hash collisions causing readdir to have problems.

Cheers, Andreas

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