ucontext

[Konstantin Belousov]

On Fri, Oct 25, 2019 at 08:26:19AM -0600, Ian Lepore wrote:
> 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.

I am curious as well, isn't thumb completely transparent to the kernel ?
I.e. my impression was that some code might be compiled into thumb, and
then a thunk which does BX to the location, is used to switch to thumb
mode.  There is no new ELF machine type involved, or different exception
entry mode, so it should just work ?

And this is why I remember about this bit 0 issue, it caused some problems
to libunwind on arm.