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

From: Frederic Weisbecker
Date: Wed Mar 28 2012 - 12:11:43 EST

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.)


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.

We probably need to check what is saved in irqs (set_irq_regs())
and exceptions as well.
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