On Thu, 11 May 2017 16:44:07 -0700
Linus Torvalds [off-list ref] wrote:

On Thu, May 11, 2017 at 4:17 PM, Thomas Garnier [off-list ref] wrote:

quoted

Ingo: Do you want the change as-is? Would you like it to be optional?
What do you think?  

I'm not ingo, but I don't like that patch. It's in the wrong place -
that system call return code is too timing-critical to add address
limit checks.

Now what I think you *could* do is:

 - make "set_fs()" actually set a work flag in the current thread flags

 - do the test in the slow-path (syscall_return_slowpath).

Yes, yes, that ends up being architecture-specific, but it's fairly simple.

And it only slows down the system calls that actually use "set_fs()".
Sure, it will slow those down a fair amount, but they are hopefully a
small subset of all cases.

How does that sound to people?  Thats' where we currently do that

        if (IS_ENABLED(CONFIG_PROVE_LOCKING) &&
            WARN(irqs_disabled(), "syscall %ld left IRQs disabled",
regs->orig_ax))
                local_irq_enable();

check too, which is a fairly similar issue.

This is exactly what Heiko did for the s390 backend as a result of this
discussion. See the _CIF_ASCE_SECONDARY bit in arch/s390/kernel/entry.S,
for the hot patch the check for the bit is included in the general
_CIF_WORK test. Only the slow patch gets a bit slower.

git commit b5a882fcf146c87cb6b67c6df353e1c042b8773d
"s390: restore address space when returning to user space".

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.