Re: [PATCH v3 4/4] ARM: Implement PAN for LPAE by TTBR0 page table walks disablement

From: "Arnd Bergmann" <arnd@arndb.de>
To: "Ard Biesheuvel" <ardb@kernel.org>,
	"Russell King" <linux@armlinux.org.uk>
Cc: "Florian Fainelli" <f.fainelli@gmail.com>,
	"Linus Walleij" <linus.walleij@linaro.org>,
	"Geert Uytterhoeven" <geert@linux-m68k.org>,
	"Stefan Wahren" <wahrenst@gmx.net>,
	"Kees Cook" <keescook@chromium.org>,
	linux-arm-kernel@lists.infradead.org,
	"Catalin Marinas" <catalin.marinas@arm.com>
Subject: Re: [PATCH v3 4/4] ARM: Implement PAN for LPAE by TTBR0 page table walks disablement
Date: Wed, 15 May 2024 21:51:45 +0000	[thread overview]
Message-ID: <addc678d-8804-46c8-bcd3-c5dfa645fcd8@app.fastmail.com> (raw)
In-Reply-To: <CAMj1kXG-iTrGn7vbqusroKeYQRFkEnmGjMdgXVP+rSJvaC61RA@mail.gmail.com>

On Wed, May 15, 2024, at 16:36, Ard Biesheuvel wrote:
> On Wed, 15 May 2024 at 18:18, Russell King (Oracle) <linux@armlinux.org.uk> wrote:
>> > and so R0 is preserved, and the issue does not happen.
>> >
>> > Not sure how to reduce this to a reproducer that can be used to report
>> > the issue to the GCC folks, but it is most definitely a compiler
>> > problem, as far as I can tell.
>>
>> Well this has come up before...
>>
>> commit 851140ab0d083c78e5723a8b1cbd258f567a7aff
>> Author: Masahiro Yamada <yamada.masahiro@socionext.com>
>> Date:   Wed Oct 2 11:28:02 2019 +0100
>>
>>     ARM: 8908/1: add __always_inline to functions called from __get_user_check()
>>
>> I assume it's a wontfix on the GCC side.
>>
>
> Yes, that is the exact same issue.
>
> Not sure whether it has been reported - the GCC side might not even be aware.
>
> I managed to reproduce this in godbolt - it happens even with -O2 not
> just with -Os
>
> https://godbolt.org/z/do56voKsa
>
> As far as I can tell, functions that use asm("r#") cannot safely call
> other functions at all unless those are __always_inline. The fact that
> it triggers with -Os first is just because it inlines much less
> aggressively than other optimization levels.

I remember that one now, and I also think this was a wontfix,
since the compiler has no way of doing the right thing here when
a variable is forced into an argument register and then used
after a function call. Spilling the contents on the stack or another
register would break the 'asm("%r0") annotation.

It would be nice to get a compiler warning for code like this, but
I don't know what legitimate use cases that would warn for, e.g. it
may be indistinguishable from the global 'register unsigned long
current_stack_pointer asm("r1");' construct we have on a couple
of architectures.

     Arnd

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