[PING**2] [PATCH] Fix handling of null stap semaphores

[George Barrett <bob@bob131.so>]

On Thu, Dec 19, 2019 at 04:20:50AM +1100, George Barrett wrote:
> Ping
> 
> On Wed, Dec 04, 2019 at 06:59:25AM +1100, George Barrett wrote:
> > According to the SystemTap documentation on user-space probes[0], stap
> > probe points without semaphores are denoted by setting the semaphore
> > address in the probe's note to zero. At present the code does do a
> > comparison of the semaphore address against zero, but only after it's
> > been relocated; as such it will (almost?) always fail, commonly
> > resulting in GDB trying to overwrite the ELF magic located at the
> > image's base address.
> >
> > This commit tests the address as specified in the SDT note rather than
> > the relocated value in order to correctly detect absent probe
> > semaphores.
> >
> > [0]: https://sourceware.org/systemtap/wiki/UserSpaceProbeImplementation
> >
> > gdb/Changelog:
> >
> > 	* stap-probe.c: Fix handling of null stap semaphores
> > ---
> >  gdb/stap-probe.c | 7 ++++---
> >  1 file changed, 4 insertions(+), 3 deletions(-)
> >
> > diff --git a/gdb/stap-probe.c b/gdb/stap-probe.c
> > index ba927790a5..e5e3cceacd 100644
> > --- a/gdb/stap-probe.c
> > +++ b/gdb/stap-probe.c
> > @@ -1425,9 +1425,6 @@ stap_modify_semaphore (CORE_ADDR address, int set, struct gdbarch *gdbarch)
> >    struct type *type = builtin_type (gdbarch)->builtin_unsigned_short;
> >    ULONGEST value;
> >  
> > -  if (address == 0)
> > -    return;
> > -
> >    /* Swallow errors.  */
> >    if (target_read_memory (address, bytes, TYPE_LENGTH (type)) != 0)
> >      {
> > @@ -1461,6 +1458,8 @@ stap_modify_semaphore (CORE_ADDR address, int set, struct gdbarch *gdbarch)
> >  void
> >  stap_probe::set_semaphore (struct objfile *objfile, struct gdbarch *gdbarch)
> >  {
> > +  if (m_sem_addr == 0)
> > +    return;
> >    stap_modify_semaphore (relocate_address (m_sem_addr, objfile), 1, gdbarch);
> >  }
> >  
> > @@ -1469,6 +1468,8 @@ stap_probe::set_semaphore (struct objfile *objfile, struct gdbarch *gdbarch)
> >  void
> >  stap_probe::clear_semaphore (struct objfile *objfile, struct gdbarch *gdbarch)
> >  {
> > +  if (m_sem_addr == 0)
> > +    return;
> >    stap_modify_semaphore (relocate_address (m_sem_addr, objfile), 0, gdbarch);
> >  }
> >  
> > --
> > 2.23.0