Re: [PATCH] tracing/user_events: align uaddr on unsigned long alignment

On 17/09/2023 23:09, David Laight wrote:

From: Clément Léger

quoted

Sent: 14 September 2023 14:11

enabler->uaddr can be aligned on 32 or 64 bits. If aligned on 32 bits,
this will result in a misaligned access on 64 bits architectures since
set_bit()/clear_bit() are expecting an unsigned long (aligned) pointer.
On architecture that do not support misaligned access, this will crash
the kernel. Align uaddr on unsigned long size to avoid such behavior.
This bug was found while running kselftests on RISC-V.

You don't want to do it on x86-64 either.
A locked accesses that crosses a cache line boundary is horrid.
So horrid that recent cpu's can be made to fault.

Hi David,

Thanks for the additional information.

I'd also doubt that other cpu that can do misaligned transfers
can even do locked ones.

For x86 (and LE) the long[] bitmap can be treated as char[]
avoiding all the problems.

Perhaps there ought to be bit a bit-array based on char[]
(not long[]) that would be endianness independent and
use byte-sized atomics.

That would work for a few architectures but I don't think all of them
have byte "grain" atomics. So I guess Masami solution (long aligned
set/clear_bit()) remains the best out there.

Clément

(IIRC that is still an issue on sparc32...)

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)