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

From: Andy Lutomirski
Date: Thu Oct 25 2018 - 00:45:38 EST

> On Oct 24, 2018, at 8:46 PM, NeilBrown <neilb@xxxxxxxx> 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.

Itâs possible for a 32-bit process and a 64-bit process to share a directory fd, so I donât think itâs 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.