linux-kernel - Re: RE: the x86 sysret_rip test fails on the Intel FRED architecture

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Message-ID: <b764c6d8-2a84-27f2-31e2-936c357f8111@gnuweeb.org>
Date:   Sun, 22 Jan 2023 15:54:56 +0700
From:   Ammar Faizi <ammarfaizi2@...weeb.org>
To:     "Li, Xin3" <xin3.li@...el.com>
Cc:     "H. Peter Anvin" <hpa@...or.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        x86 Mailing List <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: RE: the x86 sysret_rip test fails on the Intel FRED architecture

On 1/22/23 3:22 PM, Li, Xin3 wrote:
> The problem is where/how to set %r11 = %rflags in the test code.
> 
> The check happens in the USER1 signal handler, and we could set %r11
> just before calling raise(SIGUSR1).  However, the C library implementation
> of raise() modifies %r11, thus we can't preserve %r11 until the SYSCALL
> instruction. And the test still fails.

 From "man 3 raise":

"""
The raise() function sends a signal to the calling process or thread.
In a single-threaded program it is equivalent to

         kill(getpid(), sig);
"""

Implementing kill syscall with %r11 modified before entering the kernel
may look like this?

static void __raise(int sig)
{
         __asm__ volatile (
                 "pushf\n\t"
                 "popq %%r11\n\t"
                 "syscall"
                 :
                 : "D"(getpid()),        /* %rdi */
                   "S"(sig),             /* %rsi */
                   "a"(__NR_kill)        /* %rax */
                 : "rcx", "r11", "memory"
         );
}

-- 
Ammar Faizi