Re: [PATCH] [AArch64] Sanitize the address before working with allocation tags

[Luis Machado via Gdb-patches <gdb-patches@sourceware.org>]

On 5/18/21 5:33 PM, Simon Marchi wrote:
> On 2021-05-18 4:19 p.m., Luis Machado via Gdb-patches wrote:
>> Remove the logical tag/top byte from the address whenever we have to work with
>> allocation tags.
> 
> Can you explain a bit more why this is needed?  What down the line
> doesn't like to receive an address with a logical tag?

We shouldn't be passing an address with a non-zero top byte (or tag) to 
a ptrace request, for example. It may work (in fact, it works) but we 
are not supposed to rely on it. So we sanitize the pointer before it 
gets to fetch_memtags/store_memtags.

This is clarified in the AArch64 Tagged Address ABI document 
(https://www.kernel.org/doc/html/latest/arm64/tagged-address-abi.html).

In an upcoming patch to support memory tags in core files 
(https://sourceware.org/pipermail/gdb-patches/2021-May/178973.html), 
this address also gets passed down to the core target's fetch_memtags 
implementation. It needs to compare addresses, so it doesn't make sense 
to let through an address with a non-zero top byte, or else we risk not 
having a match due to differences in the upper byte.