Re: [PATCH 1/9] kernel: add a PF_FORCE_COMPAT flag

On Sat, 19 Sep 2020, Arnd Bergmann wrote:

On Sat, Sep 19, 2020 at 6:21 PM Andy Lutomirski [off-list ref] wrote:

quoted

On Fri, Sep 18, 2020 at 8:16 AM Christoph Hellwig [off-list ref] wrote:

quoted

On Fri, Sep 18, 2020 at 02:58:22PM +0100, Al Viro wrote:

quoted

Said that, why not provide a variant that would take an explicit 
"is it compat" argument and use it there?  And have the normal one 
pass in_compat_syscall() to that...

That would help to not introduce a regression with this series yes. 
But it wouldn't fix existing bugs when io_uring is used to access 
read or write methods that use in_compat_syscall().  One example 
that I recently ran into is drivers/scsi/sg.c.

Ah, so reading /dev/input/event* would suffer from the same issue, and 
that one would in fact be broken by your patch in the hypothetical case 
that someone tried to use io_uring to read /dev/input/event on x32...

For reference, I checked the socket timestamp handling that has a number 
of corner cases with time32/time64 formats in compat mode, but none of 
those appear to be affected by the problem.

quoted

Aside from the potentially nasty use of per-task variables, one thing 
I don't like about PF_FORCE_COMPAT is that it's one-way.  If we're 
going to have a generic mechanism for this, shouldn't we allow a full 
override of the syscall arch instead of just allowing forcing compat 
so that a compat syscall can do a non-compat operation?

The only reason it's needed here is that the caller is in a kernel 
thread rather than a system call. Are there any possible scenarios where 
one would actually need the opposite?

Quite possibly. The ext4 vs. compat getdents bug is still unresolved. 
Please see,
https://lore.kernel.org/lkml/CAFEAcA9W+JK7_TrtTnL1P2ES1knNPJX9wcUvhfLwxLq9augq1w@mail.gmail.com/ (local)

       Arnd