linux-kernel - Re: [RFC][PATCH 1/2] x86: Allow breakpoints to emulate call functions

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Message-ID: <20190502195052.0af473cf@gandalf.local.home>
Date:   Thu, 2 May 2019 19:50:52 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Nicolai Stange <nstange@...e.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Jiri Kosina <jikos@...nel.org>,
        Miroslav Benes <mbenes@...e.cz>,
        Petr Mladek <pmladek@...e.com>,
        Joe Lawrence <joe.lawrence@...hat.com>,
        Shuah Khan <shuah@...nel.org>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Mimi Zohar <zohar@...ux.ibm.com>,
        Juergen Gross <jgross@...e.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nayna Jain <nayna@...ux.ibm.com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Joerg Roedel <jroedel@...e.de>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>, stable <stable@...r.kernel.org>
Subject: Re: [RFC][PATCH 1/2] x86: Allow breakpoints to emulate call
 functions

On Thu, 2 May 2019 19:31:29 -0400
Steven Rostedt <rostedt@...dmis.org> wrote:

> Digging a little further, I pinpointed it out to being kretprobes. The
> problem I believe is the use of kernel_stack_pointer() which does some
> magic on x86_32. kretprobes uses this to hijack the return address of
> the function (much like the function graph tracer does). I do have code
> that would allow kretprobes to use the function graph tracer instead,
> but that's still in progress (almost done!). But still, we should not
> have this break the use of kernel_stack_pointer() either.
> 
> Adding some printks in that code, it looks to be returning "&regs->sp"
> which I think we changed.
>

This appears to fix it!

-- Steve

diff --git a/arch/x86/kernel/ptrace.c b/arch/x86/kernel/ptrace.c
index 4b8ee05dd6ad..600ead178bf4 100644
--- a/arch/x86/kernel/ptrace.c
+++ b/arch/x86/kernel/ptrace.c
@@ -171,8 +171,12 @@ unsigned long kernel_stack_pointer(struct pt_regs *regs)
 	unsigned long sp = (unsigned long)&regs->sp;
 	u32 *prev_esp;
 
-	if (context == (sp & ~(THREAD_SIZE - 1)))
+	if (context == (sp & ~(THREAD_SIZE - 1))) {
+		/* int3 code adds a gap */
+		if (sp == regs->sp - 5*4)
+			return regs->sp;
 		return sp;
+	}
 
 	prev_esp = (u32 *)(context);
 	if (*prev_esp)