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

[Steven Rostedt]

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

> From: Steven Rostedt
> > Sent: 30 March 2020 19:08
> > On Mon, 30 Mar 2020 15:34:08 +0000
> > David Laight <David.Laight@xxxxxxxxxx> 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