linux-kernel - Re: [patch V4 part 3 13/29] x86/traps: Prepare for using DEFINE

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Message-ID: <874kshxqjm.fsf@nanos.tec.linutronix.de>
Date:   Fri, 15 May 2020 15:42:53 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        Brian Gerst <brgerst@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Will Deacon <will@...nel.org>
Subject: Re: [patch V4 part 3 13/29] x86/traps: Prepare for using DEFINE_IDTENTRY

Andy,

Thomas Gleixner <tglx@...utronix.de> writes:
> In historic kernels si_addr was simply set to regs->ip and the uprobe
> muck changed that in commit b02ef20a9fba08 ("uprobes/x86: Fix the wrong
> ->si_addr when xol triggers a trap")
>     
>     If the probed insn triggers a trap, ->si_addr = regs->ip is technically
>     correct, but this is not what the signal handler wants; we need to pass
>     the address of the probed insn, not the address of xol slot.
>
> Now that I filled my own blanks, I think that I can come up with a
> halfways useful comment.

That's what I came up with (delta patch)

Thanks,

        tglx

8<---------------
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -205,6 +205,16 @@ static void do_error_trap(struct pt_regs
 	}
 }
 
+/*
+ * Posix requires to provide the address of the faulting instruction for
+ * SIGILL (#UD) and SIGFPE (#DE) in the si_addr member of siginfo_t.
+ *
+ * This address is usually regs->ip, but when an uprobe moved the code out
+ * of line then regs->ip points to the XOL code which would confuse
+ * anything which analyzes the fault address vs. the unmodified binary. If
+ * a trap happened in XOL code then uprobe maps regs->ip back to the
+ * original instruction address.
+ */
 static __always_inline void __user *error_get_trap_addr(struct pt_regs *regs)
 {
 	return (void __user *)uprobe_get_trap_addr(regs);