ucontext

[Ian Lepore]

On Fri, 2019-10-25 at 11:38 +0300, Konstantin Belousov wrote:
> On Fri, Oct 25, 2019 at 10:44:21AM +0300, Nick Kostirya wrote:
> > On Thu, 24 Oct 2019 17:54:36 +0300
> > Konstantin Belousov <kostikbel at gmail.com> wrote:
> > 
> > > 
> > > I believe you want
> > > 	uc_context.__gregs[_REG_PC]
> > > on arm (32bit) and
> > > 	uc_context.mc_gpregs.gp_elr
> > > on arm64 for aarch64.
> > > 
> > > Sometimes the thumb bit (lowest bit in PC) leaks there, then you should
> > > mask it.
> > 
> > Thanks!
> > 
> > Although I did not understand your last phrase.
> > There is leak of what?
> 
> Leak of the thumb bit.  ARM ARM specifies that in non-thumb mode, pc must
> be word-aligned, in thumb it is half-word aligned.  A way to enter thumb
> mode is to execute BX or BLX instruction with the lowest bit of the target
> PC set to 1.
> 
> Sometimes you might get pc with the bit 0 set, which should
> be masked out then.  This is a bigger issue for unwinders than for simple
> profilers.
> 
> > Where can I read about it?
> 
> ARM ARM (ARM architecture reference manual), available from arm.com.
> Or Google for it.
> 

The kernel has some support for running thumb binaries, but I've never
heard of anybody actually doing so on freebsd.  Nobody has ever
reported a bug related to running a thumb binary, and it would be
astounding to me if we accidentally got everything in the kernel thumb
support right on the first try without ever testing it.

-- Ian