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

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Message-ID: <87o9bi6632.fsf@notabene.neil.brown.name>
Date:   Thu, 25 Oct 2018 14:46:57 +1100
From:   NeilBrown <neilb@...e.com>
To:     "Theodore Y. Ts'o" <tytso@....edu>
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, Theodore Y. Ts'o wrote:

> 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.

NFSd uses FMODE_32BITHASH or FMODE64BITHASH to allow ext4 to do the
right thing.  FMODE_32BITHASH is set for NFSv2 only.

Maybe sys_getdents needs to set FMODE_32BITHASH, and sys_getdent64 needs
to set FMODE_64BITHASH - or something like that.

For lustre it is a bit more complex.  The internal "inode number" is 128
bits and we (sort of) hash it to 32 or 64 bits.  cp_compat_stat() just
says -EOVERFLOW if we give a 64 bit number when 32 are expected, and
there is no flag to say "this is a 32-bit 'stat' request".

But I need to dig into exactly what that "sort-of" means - maybe there
is an answer in there.

NeilBrown

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