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

[Andy Lutomirski]

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.

--Andy