linux-kernel - Re: [RFC PATCH v2 2/2] selftests/x86: sysret_rip: Add more syscall tests with respect to `%rcx` and `%r11`

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Message-ID: <ce25e53f-91d4-d793-42a5-036d6bce0b4c@zytor.com>
Date:   Mon, 23 Jan 2023 22:16:01 -0800
From:   "H. Peter Anvin" <hpa@...or.com>
To:     Ammar Faizi <ammarfaizi2@...weeb.org>,
        x86 Mailing List <x86@...nel.org>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Xin Li <xin3.li@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrew Cooper <Andrew.Cooper3@...rix.com>,
        Brian Gerst <brgerst@...il.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Shuah Khan <shuah@...nel.org>, Ingo Molnar <mingo@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Linux Kselftest Mailing List 
        <linux-kselftest@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH v2 2/2] selftests/x86: sysret_rip: Add more syscall
 tests with respect to `%rcx` and `%r11`



On 1/23/23 18:27, Ammar Faizi wrote:
> From: Ammar Faizi <ammarfaizi2@...weeb.org>
> 
> Test that:
> 
>   - "syscall" in a FRED system doesn't clobber %rcx and %r11.
>   - "syscall" in a non-FRED system sets %rcx=%rip and %r11=%rflags.
> 
> Test them out with a trivial system call like __NR_getppid and friends
> which are extremely likely to return with SYSRET on an IDT system; check
> that it returns a nonnegative value and then save the result.
> 

"Nonnegative" here should be "valid"; it is an implementation detail 
that the error value is -1.

However, you are not checking that you don't get a mix of REGS_SAVED and 
REGS_SYSRET, which is a major part of the point.

	-hpa