linux-kernel - Re: in_compat_syscall() returns from kernel thread for X86

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Date:   Wed, 24 Oct 2018 09:15:34 -0400
From:   "Theodore Y. Ts'o" <tytso@....edu>
To:     NeilBrown <neilb@...e.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Peter Zijlstra <peterz@...radead.org>,
        Dmitry Safonov <dsafonov@...tuozzo.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...nel.org>,
        Brian Gerst <brgerst@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-tip-commits@...r.kernel.org, jsimmons@...radead.org
Subject: Re: in_compat_syscall() returns from kernel thread for X86_32.

On Wed, Oct 24, 2018 at 12:47:57PM +1100, NeilBrown wrote:
> 
> I doubt it was copied - more likely independent evolution.
> But on reflection, I see that it is probably reasonable that it
> shouldn't be used this way - or at all in this context.
> I'll try to understand what the issues really are and see if I can
> find a solution that doesn't depend on this interface.
> Thanks for your help.

At least for ext4, the primary problem is that we want to use a 64-bit
telldir/seekdir cookie if all 64-bits will make it to user space, and
a 32-bit telldir cookie if only 32 bits will make it userspace.  This
impacts NFS as well because if there are people who are still using
NFSv2, which has 32-bit directory offsets, we need to constrain the
telldir/seekdir cookies we give to NFS to be a 32 has as opposed to a
64-bit hash.

						- Ted