linux-kernel - Re: ftrace not showing the process names for all processes on syscall events

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20200330143947.33515a16@gandalf.local.home>
Date:   Mon, 30 Mar 2020 14:39:47 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     David Laight <David.Laight@...LAB.COM>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: ftrace not showing the process names for all processes on
 syscall events

On Mon, 30 Mar 2020 18:14:37 +0000
David Laight <David.Laight@...LAB.COM> wrote:

> From: Steven Rostedt
> > Sent: 30 March 2020 19:08
> > On Mon, 30 Mar 2020 15:34:08 +0000
> > David Laight <David.Laight@...LAB.COM> wrote:
> >   
> > > Oh, does the 'function_graph' code ignore tail calls?  
> > 
> > Yes and no ;-)  It works by dumb luck. As it was a year after function
> > graph tracing was live (some time in 2010 I believe) that someone brought
> > up tail calls, and I had to take a look at how it never crashed, and was
> > surprised that it "just worked". Here's a summary:  
> 
> 'Dumb luck' seems to be failing me :-)
> I'll look more closely tomorrow.

The tl;td; version is that the function graph tracer relies on a shadow
stack that it uses to save the original return address, as it replaces the
original return address with the address of the fgraph return trampoline.

Although a tail call causes the real stack to only contain one return
address, the shadow stack will contain a return address for every function,
even if it was a tail call. As that stack gets updated by the entry of the
function not the return side. The difference is, a tail call would cause
the shadow stack to just contain a call to the start of the fgraph return
trampoline, and not an address into the rest of the kernel.

-- Steve