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

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Message-ID: <086307ce-2d0d-4cf7-98c8-66a00ddec2aa@rivosinc.com>
Date:   Mon, 18 Sep 2023 10:37:29 +0200
From:   Clément Léger <cleger@...osinc.com>
To:     David Laight <David.Laight@...LAB.COM>,
        Steven Rostedt <rostedt@...dmis.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-trace-kernel@...r.kernel.org" 
        <linux-trace-kernel@...r.kernel.org>
Cc:     Beau Belgrave <beaub@...ux.microsoft.com>
Subject: Re: [PATCH] tracing/user_events: align uaddr on unsigned long
 alignment



On 17/09/2023 23:09, David Laight wrote:
> From: Clément Léger
>> 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
> 
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