Re: [PATCH] [PR gdb/23210] Unset gdbarch significant_addr_bit by default

[Omair Javaid <omair.javaid@linaro.org>]

On Wed, 23 May 2018, 3:32 PM Joel Brobecker, <brobecker@adacore.com> wrote:

> Hi Omair,
>
>
> > This patch fixes a bug introduced by fix to AArch64 pointer tagging.
> >
> > In our fix for tagged pointer support our agreed approach was to sign
> > extend user-space address after clearing tag bits. This is not same
> > for all architectures and this patch allows sign extension for
> > addresses on targets which specifically set significant_addr_bit.
> >
> > More information about patch that caused the issues and discussion
> > around tagged pointer support can be found in links below:
> >
> > https://sourceware.org/ml/gdb-patches/2018-05/msg00000.html
> > https://sourceware.org/ml/gdb-patches/2017-12/msg00159.html
> >
> > gdb/ChangeLog:
> >
> > 2018-05-23  Omair Javaid  <omair.javaid@linaro.org>
> >
> >       * gdbarch.c (verify_gdbarch): Update.
> >       * utils.c (address_significant): Update.
>
> I haven't delved into the actual patch and whether the approach
> used is correct, but skimming it, I did notice a couple of things.
>
> The first one is that gdbarch.c is a generated file, so you should
> adjust gdbarch.sh instead so that executing gdbarch.sh gives you
> the gdbarch.c file with the behavior you want. In particular, I think
> you probably need to remove the default value for significant_addr_bit.
>

I will update gdbarch as suggested. Are there any other issues in the fix I
should address.?


>
> > ---
> >  gdb/gdbarch.c | 3 +--
> >  gdb/utils.c   | 5 +++--
> >  2 files changed, 4 insertions(+), 4 deletions(-)
> >
> > diff --git a/gdb/gdbarch.c b/gdb/gdbarch.c
> > index c430ebe..5593911 100644
> > --- a/gdb/gdbarch.c
> > +++ b/gdb/gdbarch.c
> > @@ -615,8 +615,7 @@ verify_gdbarch (struct gdbarch *gdbarch)
> >    /* Skip verify of stabs_argument_has_addr, invalid_p == 0 */
> >    /* Skip verify of convert_from_func_ptr_addr, invalid_p == 0 */
> >    /* Skip verify of addr_bits_remove, invalid_p == 0 */
> > -  if (gdbarch->significant_addr_bit == 0)
> > -    gdbarch->significant_addr_bit = gdbarch_addr_bit (gdbarch);
> > +  /* Skip verify of significant_addr_bit, invalid_p == 0 */
> >    /* Skip verify of software_single_step, has predicate.  */
> >    /* Skip verify of single_step_through_delay, has predicate.  */
> >    /* Skip verify of print_insn, invalid_p == 0 */
> > diff --git a/gdb/utils.c b/gdb/utils.c
> > index 9c5bf68..91c0f2b 100644
> > --- a/gdb/utils.c
> > +++ b/gdb/utils.c
> > @@ -2708,10 +2708,11 @@ address_significant (gdbarch *gdbarch, CORE_ADDR
> addr)
> >    /* Clear insignificant bits of a target address and sign extend
> resulting
> >       address, avoiding shifts larger or equal than the width of a
> CORE_ADDR.
> >       The local variable ADDR_BIT stops the compiler reporting a shift
> overflow
> > -     when it won't occur.  */
> > +     when it won't occur. Skip updating of target address if current
> target has
> > +     not set gdbarch significant_addr_bit.  */
>
> Small nit (GNU Coding Style): Two spaces after the period.
>
> >    int addr_bit = gdbarch_significant_addr_bit (gdbarch);
> >
> > -  if (addr_bit < (sizeof (CORE_ADDR) * HOST_CHAR_BIT))
> > +  if (addr_bit && (addr_bit < (sizeof (CORE_ADDR) * HOST_CHAR_BIT)))
> >      {
> >        CORE_ADDR sign = (CORE_ADDR) 1 << (addr_bit - 1);
> >        addr &= ((CORE_ADDR) 1 << addr_bit) - 1;
> > --
> > 2.7.4
>
> --
> Joel
>