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

[Jiri Olsa <jolsa@redhat.com>]

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.

right, I covered only general registers... need to check about mmc/sse

jirka

> 
> 
> > 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.)
> 
> 
> - FChE