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

[Joel Brobecker <brobecker at adacore dot com>]

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.


> ---
>  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