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

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Message-Id: <AD3CD642-C7D4-43C5-AB1B-F65FE916618F@amacapital.net>
Date:   Wed, 24 Oct 2018 21:45:30 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     NeilBrown <neilb@...e.com>
Cc:     "Theodore Y. Ts'o" <tytso@....edu>,
        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 Oct 24, 2018, at 8:46 PM, NeilBrown <neilb@...e.com> wrote:
> 
>> 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.

It’s possible for a 32-bit process and a 64-bit process to share a directory fd, so I don’t think it’s quite that simple.

One option would be to add .llseek and .getdents flags or entire new compat operations to indicate that the caller expects 32-bit offsets.

I wonder how overlayfs interacts with this whole mess.