riscv32 EXT4 splat, 6.8 regression?

[Al Viro]

On Sat, Apr 13, 2024 at 07:46:03PM -0600, Andreas Dilger wrote:

> As to whether the 0xfffff000 address itself is valid for riscv32 is
> outside my realm, but given that RAM is cheap it doesn't seem unlikely
> to have 4GB+ of RAM and want to use it all.  The riscv32 might consider
> reserving this page address from allocation to avoid similar issues in
> other parts of the code, as is done with the NULL/0 page address.

Not a chance.  *Any* page mapped there is a serious bug on any 32bit
box.  Recall what ERR_PTR() is...

On any architecture the virtual addresses in range (unsigned long)-512..
(unsigned long)-1 must never resolve to valid kernel objects.
In other words, any kind of wraparound here is asking for an oops on
attempts to access the elements of buffer - kernel dereference of
(char *)0xfffff000 on a 32bit box is already a bug.

It might be getting an invalid pointer, but arithmetical overflows
are irrelevant.