linux-kernel - Re: [PATCH 04/15] perf: Add ability to dump user regs

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Date:	Wed, 28 Mar 2012 23:41:34 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Jiri Olsa <jolsa@...hat.com>
Cc:	"Frank Ch. Eigler" <fche@...hat.com>, acme@...hat.com,
	a.p.zijlstra@...llo.nl, mingo@...e.hu, paulus@...ba.org,
	cjashfor@...ux.vnet.ibm.com, eranian@...gle.com,
	gorcunov@...nvz.org, tzanussi@...il.com, mhiramat@...hat.com,
	rostedt@...dmis.org, robert.richter@....com,
	linux-kernel@...r.kernel.org, mjw@...hat.com
Subject: Re: [PATCH 04/15] perf: Add ability to dump user regs

On Wed, Mar 28, 2012 at 07:02:34PM +0200, Jiri Olsa wrote:
> On Wed, Mar 28, 2012 at 06:06:13PM +0200, Frederic Weisbecker wrote:
> > On Wed, Mar 28, 2012 at 11:12:30AM -0400, Frank Ch. Eigler wrote:
> > > Hi, Jiri -
> > > 
> > > > [...]
> > > > > [...]  Upon a normal syscall entry to the kernel, not
> > > > > all user registers are saved explicitly for such easy retrieval.  The
> > > > > others may be spilled to the stack by gcc during the various sys_*
> > > > > functions or elsewhere.  [...]
> > > > 
> > > > Are you reffering to x86_64 where only portion of registers
> > > > is stored by SAVE_ARGS macro? Seems like 32 bits stores the
> > > > whole pt_regs.
> > > 
> > > I believe that's the right area.  I'm not sure even the 32-bit variant
> > > is complete enough, for example exempting MMX/SSE registers.  These
> > > may also contain spilled registers before long.
> > > 
> > > 
> > > > Generally you could need all the registers to start the unwind, but
> > > > I was assuming that for most cases the stack pointer and instruction
> > > > pointer should be enough.. but I might be wrong here.
> > > 
> > > Yeah; the question is how much is missed besides those "most cases".
> > > 
> > > 
> > > > > To recover these registers at run time, we found that the kernel
> > > > > stack itself has to be partially unwound [... Without that, it ...]
> > > > > may accidentally pass garbage data to perf userspace.  Correcting
> > > > > this could require a kernel-space libunwind.
> > > 
> > > > AFAIK not going to happen any time soon ;)
> > > 
> > > Understood.  Then the code needs to ensure that it does not purport to
> > > pass register values that it does not know.  (Back when we were at
> > > this stage in systemtap, we got some reasonable backtraces even
> > > without kernel unwinding, ie. tolerating missing registers.)
> > 
> > Right.
> > 
> > I think in normal syscall case we save rdi, rsi, rdx, rax and rip.
> > If we take the syscall slow path we save rbx, rbp, r12-15.
> > 
> > Unfortunately we don't save rsp, which must be the most important
> > for cfi unwinding.
> 
> hm, I think we always have stack pointer
> 
> should be saved by cpu itself together with other control
> regs like: ip cs eflags sp ss
> 
> For syscalls, we also have the ones stored by SAVE_ARGS
> The rest of the registers (SAVE_REST) are available
> only for the sake of the syscall_trace_enter during
> the slow path, but it's poped out before executing
> the actuall syscall.

Ah good point!
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