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Date:   Mon, 6 May 2019 16:22:54 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Andy Lutomirski <luto@amacapital.net>,
        Steven Rostedt <rostedt@goodmis.org>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andy Lutomirski <luto@kernel.org>,
        Nicolai Stange <nstange@suse.de>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        the arch/x86 maintainers <x86@kernel.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Jiri Kosina <jikos@kernel.org>,
        Miroslav Benes <mbenes@suse.cz>,
        Petr Mladek <pmladek@suse.com>,
        Joe Lawrence <joe.lawrence@redhat.com>,
        Shuah Khan <shuah@kernel.org>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Mimi Zohar <zohar@linux.ibm.com>,
        Juergen Gross <jgross@suse.com>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Nayna Jain <nayna@linux.ibm.com>,
        Masahiro Yamada <yamada.masahiro@socionext.com>,
        Joerg Roedel <jroedel@suse.de>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@vger.kernel.org>, stable <stable@vger.kernel.org>
Subject: Re: [RFC][PATCH 1/2] x86: Allow breakpoints to emulate call functions
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On Mon, May 06, 2019 at 10:19:51AM +0200, Peter Zijlstra wrote:
> +.Lfrom_usermode_no_fixup_\@:
> +.endm
> +
> +.macro IRET_FRAME
> +
> +	/* orig_eax is already POP'ed when we're here */
> +
> +	testl $CS_FROM_KERNEL, 1*4(%esp)
> +	jz .Lfinished_frame_\@
> +
> +	pushl %eax
> +

From there..

> +	lea 10*4(%esp), %eax	# address of <previous context>
> +	cmpl %eax, 4*4(%esp)	# if ->sp is unmodified
> +	jnz .Lmodified_sp_do_fixup_\@
> +
> +	/*
> +	 * Fast path; regs->sp wasn't modified, reuse the original IRET frame.
> +	 */
> +	pop %eax
> +	add $6*4, %esp
> +	jmp .Lfinished_frame_\@;
> +
> +.Lmodified_sp_do_fixup_\@:

... until here, needs to go, it is buggy. While a clever idea, it looses
updates to regs->ip and ->flags.

> +
> +	/*
> +	 * Reconstruct the 3 entry IRET frame right after the (modified)
> +	 * regs->sp without lowering %esp in between, such that an NMI in the
> +	 * middle doesn't scribble our stack.
> +	 */
> +	pushl	%ecx
> +	movl	5*4(%esp), %eax		# (modified) regs->sp
> +
> +	movl	4*4(%esp), %ecx		# flags
> +	movl	%ecx, -4(%eax)
> +
> +	movl	3*4(%esp), %ecx		# cs
> +	andl	$0x0000ffff, %ecx
> +	movl	%ecx, -8(%eax)
> +
> +	movl	2*4(%esp), %ecx		# ip
> +	movl	%ecx, -12(%eax)
> +
> +	movl	1*4(%esp), %ecx		# eax
> +	movl	%ecx, -16(%eax)
> +
> +	popl	%ecx
> +	lea	-16(%eax), %esp
> +	popl	%eax
> +
> +.Lfinished_frame_\@:
> +.endm