linux-kernel - Re: [PATCH 1/2] x86/stacktrace: do not fail when regs on stack for ORC

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Message-ID: <73526a6b-5083-78f1-8f76-d5f3ac52f958@suse.cz>
Date:   Thu, 14 Dec 2017 22:58:35 +0100
From:   Jiri Slaby <jslaby@...e.cz>
To:     Josh Poimboeuf <jpoimboe@...hat.com>, Jiri Slaby <jslaby@...e.cz>
Cc:     mingo@...hat.com, linux-kernel@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org
Subject: Re: [PATCH 1/2] x86/stacktrace: do not fail when regs on stack for
 ORC

On 11/30/2017, 08:57 PM, Josh Poimboeuf wrote:
> So with those changes in mind, how about something like this (plus
> comments)?
> 
> 	for (unwind_start(&state, task, NULL, NULL); !unwind_done(&state);
> 	     unwind_next_frame(&state)) {
> 
> 		regs = unwind_get_entry_regs(&state);
> 		if (regs) {
> 			if (user_mode(regs))
> 				goto success;
> 
> 			if (IS_ENABLED(CONFIG_FRAME_POINTER))
> 				return -EINVAL;
> 		}
> 
> 		addr = unwind_get_return_address(&state);
> 		if (!addr)
> 			return -EINVAL;
> 
> 		if (save_stack_address(trace, addr, false))
> 			return -EINVAL;
> 	}
> 
> 	return -EINVAL;

Kthreads and idle tasks hit this error as they have no user regs on the
stack obviously :).

So making it:
        if (!(task->flags & (PF_KTHREAD | PF_IDLE)))
                return -EINVAL;

works, but is not reliable now. So I believe, we cannot live without
unwind->error to differentiate between "unwind_done() == true" because:
* full stack unwound and the stack type is set to UNKNOWN
* unwinding failed and the stack type is set to UNKNOWN

Or perhaps introduce stack type BOTTOM, NONE, or NOMORE meaning the
bottom of the stacks reached?

> success:
> 	if (trace->nr_entries < trace->max_entries)
> 		trace->entries[trace->nr_entries++] = ULONG_MAX;
> 
> 	return 0;
> 
> After these changes I believe we can enable
> CONFIG_HAVE_RELIABLE_STACKTRACE for ORC.

thanks,
-- 
js
suse labs