linux-kernel - Re: [Bug report] __arch

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Message-ID: <ce1cc0a5-3393-d16f-eaaf-5e7149d5164f@genode-labs.com>
Date:   Mon, 26 Jun 2023 23:42:51 +0200
From:   Sebastian Sumpf <Sebastian.Sumpf@...ode-labs.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     tglx@...utronix.de, mingo@...hat.com, dave.hansen@...ux.intel.com,
        x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [Bug report] __arch_hweight32/64 x86

Hello Borislav,

On 6/26/23 23:06, Borislav Petkov wrote:
> Hello,
> 
> On Mon, Jun 26, 2023 at 10:49:44PM +0200, Sebastian Sumpf wrote:
>> The 'ALTERNATIVE' macro checks for the popcnt feature. In case this fails
>> the '__sw_hweight' C-function is called from inline assembly with rax and
>> rdi as intput/output operands. However, the code does not contain a clobber
>> list of any callee safe registers that might be touched by the
>> '__sw_height64' C-function.

>> Which registers are those? Can you be more specific?

Registers that are free to use by a called C-function are: rcx, rdx, rsi, rdi, 
and r8-r11 for x86_64. See the "System V Application Binary Interface
AMD64 Architecture Processor Supplement" [1] page 21. x86_32 I would have to 
look up.

>> Therefore, these registers will not be restored
>> upon function return by the compiler. This leads to varying
>> exceptions/bad behavior caused by the thus corrupted registers later
>> on.
> 
> How do I reproduce what you're observing so that I can take a look?

This is hard to tell, I would disable the " X86_FEATURE_POPCNT" feature and use 
the ' CONFIG_ARCH_HAS_FAST_MULTIPLIER' option in order to use the multiplier 
implementation in '__sw_hweight64' in 'lib/hweight.c' At least that is what 
triggered it here.

Regards,

Sebastian

[1] http://www.uclibc.org/docs/psABI-x86_64.pdf

-- 
Sebastian Sumpf
Genode Labs

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