linux-kernel - Re: [PATCH 09/11] x86/fault: Rename no_context() to kernelmode_fixup_or

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Message-Id: <88AA1DD6-615B-4049-B335-F2F40F85EF08@amacapital.net>
Date:   Wed, 3 Feb 2021 11:53:03 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Andy Lutomirski <luto@...nel.org>, x86@...nel.org,
        LKML <linux-kernel@...r.kernel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Yonghong Song <yhs@...com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 09/11] x86/fault: Rename no_context() to kernelmode_fixup_or_oops()



> On Feb 3, 2021, at 11:39 AM, Borislav Petkov <bp@...en8.de> wrote:
> 
> On Sun, Jan 31, 2021 at 09:24:40AM -0800, Andy Lutomirski wrote:
>> The name no_context() has never been very clear.  It's only called for
>> faults from kernel mode, so rename it and change the no-longer-useful
>> user_mode(regs) check to a WARN_ON_ONCE.
>> 
>> Cc: Dave Hansen <dave.hansen@...ux.intel.com>
>> Cc: Peter Zijlstra <peterz@...radead.org>
>> Signed-off-by: Andy Lutomirski <luto@...nel.org>
>> ---
>> arch/x86/mm/fault.c | 28 ++++++++++------------------
>> 1 file changed, 10 insertions(+), 18 deletions(-)
>> 
>> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
>> index 177b612c7f33..04cc98ec2423 100644
>> --- a/arch/x86/mm/fault.c
>> +++ b/arch/x86/mm/fault.c
>> @@ -693,17 +693,10 @@ page_fault_oops(struct pt_regs *regs, unsigned long error_code,
>> }
>> 
>> static noinline void
>> -no_context(struct pt_regs *regs, unsigned long error_code,
>> -       unsigned long address, int signal, int si_code)
>> +kernelmode_fixup_or_oops(struct pt_regs *regs, unsigned long error_code,
>> +             unsigned long address, int signal, int si_code)
> 
> Ew, I don't like functions with "or" in the name - they're probably not
> doing one thing only as they should.
> 
> Why not simply "handle_kernel_fault" ?

Hmm, I could maybe get behind handle_kernelmode_fault.  I’ll contemplate it.  I like the name of the function indicating that either it returns after fixing it or it doesn’t return.

I refuse to say “kernel” without qualification. In this $@!$ file, we have kernel _mode_, kernel _address_, and kernel _privilege_, and they are all different.

> 
> Also, all the callsites now do:
> 
>    if (!user_mode(regs)) {
>        kernelmode_fixup_or_oops
>        ...
> 
> I guess you can push the "user_mode" check inside that function for less
> hairy code at the callsites.

I feel like that would be more obfuscated — then the function would return without fixing anything for usermode faults, return after fixing it for kernel mode faults, or oops.

> 
>> {
>> -    if (user_mode(regs)) {
>> -        /*
>> -         * This is an implicit supervisor-mode access from user
>> -         * mode.  Bypass all the kernel-mode recovery code and just
>> -         * OOPS.
>> -         */
>> -        goto oops;
>> -    }
>> +    WARN_ON_ONCE(user_mode(regs));
> 
> I guess...
> 
> -- 
> Regards/Gruss,
>    Boris.
> 
> https://people.kernel.org/tglx/notes-about-netiquette