Re: [PATCH] AArch64 pauth: Support backtrace in EL1 (kernel space)

[Luis Machado <luis.machado@arm.com>]

Hi,

On 9/28/22 03:59, Koudai Iwahori via Gdb-patches wrote:
> The way to remove the signature bits from the address depends on the
> 55th bit of the address. If 55th bit is zero, the signature bits should
> be all cleared. If the 55th bit is one, the signature bits should be all
> set.
> ---
> I found very similar patches after fixing this issue:
>    https://sourceware.org/pipermail/gdb-patches/2022-July/190507.html
>    https://sourceware.org/pipermail/gdb-patches/2021-October/182859.html
> If this issue will be fixed in the near future, I can wait for it

Yes, it should be fixed by the first link above. It is pending approval from maintainers, which should
hopefully happen soon.

I also have an upcoming patch (that relies on the above patch) to support pauth for user-mode QEMU.

See https://sourceware.org/bugzilla/show_bug.cgi?id=29421.

> 
>   gdb/aarch64-tdep.c | 16 +++++++++++++++-
>   1 file changed, 15 insertions(+), 1 deletion(-)
> 
> diff --git a/gdb/aarch64-tdep.c b/gdb/aarch64-tdep.c
> index d0387044934..16d1e44e903 100644
> --- a/gdb/aarch64-tdep.c
> +++ b/gdb/aarch64-tdep.c
> @@ -244,6 +244,20 @@ class instruction_reader : public abstract_instruction_reader
>   
>   } // namespace
>   
> +/* removes the pauth signature bits from the address. */
> +
> +static CORE_ADDR
> +aarch64_remove_pauth_signature (CORE_ADDR addr, CORE_ADDR mask)
> +{
> +  /* 55th bit in address determines whether the address comes from the top
> +     address range or the bottom address range. */
> +  constexpr CORE_ADDR pauth_va_range_select_mask = CORE_ADDR(1) << 55;
> +  if (addr & pauth_va_range_select_mask)
> +    return addr | mask;
> +  else
> +    return addr & ~mask;
> +}
> +
>   /* If address signing is enabled, mask off the signature bits from the link
>      register, which is passed by value in ADDR, using the register values in
>      THIS_FRAME.  */
> @@ -258,7 +272,7 @@ aarch64_frame_unmask_lr (aarch64_gdbarch_tdep *tdep,
>       {
>         int cmask_num = AARCH64_PAUTH_CMASK_REGNUM (tdep->pauth_reg_base);
>         CORE_ADDR cmask = frame_unwind_register_unsigned (this_frame, cmask_num);
> -      addr = addr & ~cmask;
> +      addr = aarch64_remove_pauth_signature(addr, cmask);
>   
>         /* Record in the frame that the link register required unmasking.  */
>         set_frame_previous_pc_masked (this_frame);