Re: [PATCH v6 6/8] gdb: Add qIsAddressTagged packet

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

On 4/18/24 21:10, Gustavo Romero wrote:
> This commit adds a new packet, qIsAddressTagged, allowing GDB remote
> targets to use it to query the stub if a given address is tagged.
> 
> Currently, the memory tagging address check is done via a read query,
> where the contents of /proc/<PID>/smaps is read and the flags are
> inspected for memory tagging-related flags that indicate the address is
> in a memory tagged region.
> 
> This is not ideal, for example, for QEMU stub and other cases, such as
> on bare-metal, where there is no notion of an OS file like 'smaps.'
> Hence, the introduction of qIsAddressTagged packet allows checking
> if an address is tagged in an agnostic way.
> 
> The is_address_tagged target hook in remote.c attempts to use the
> qIsAddressTagged packet first for checking if an address is tagged and
> if the stub does not support such a packet (reply is empty) it falls
> back to using the current mechanism that reads the contents of
> /proc/<PID>/smaps via vFile requests.
> 
> Signed-off-by: Gustavo Romero <gustavo.romero@linaro.org>
> ---
>  gdb/remote.c | 75 ++++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 75 insertions(+)
> 
> diff --git a/gdb/remote.c b/gdb/remote.c
> index cd271c4b493..3d034bb1ef8 100644
> --- a/gdb/remote.c
> +++ b/gdb/remote.c
> @@ -337,6 +337,9 @@ enum {
>       packets and the tag violation stop replies.  */
>    PACKET_memory_tagging_feature,
>  
> +  /* Support for the qIsAddressTagged packet.  */
> +  PACKET_qIsAddressTagged,
> +
>    PACKET_MAX
>  };
>  
> @@ -15535,6 +15538,50 @@ create_store_memtags_request (gdb::char_vector &packet, CORE_ADDR address,
>    strcpy (packet.data (), request.c_str ());
>  }
>  
> +static void
> +create_is_address_tagged_request (gdbarch *gdbarch, gdb::char_vector &packet,
> +				  CORE_ADDR address)
> +{
> +  int addr_size;
> +  std::string request;
> +
> +  addr_size = gdbarch_addr_bit (gdbarch) / 8;
> +  request = string_printf ("qIsAddressTagged:%s", phex_nz (address, addr_size));
> +
> +  if (packet.size () < request.length () + 1)
> +    error (_("Contents too big for packet qIsAddressTagged."));
> +
> +  strcpy (packet.data (), request.c_str ());
> +}
> +
> +static bool
> +check_is_address_tagged_reply (remote_target *remote, gdb::char_vector &packet,
> +			       bool &tagged)
> +{
> +  gdb_assert (remote != nullptr);
> +  /* Check reply and disable qIsAddressTagged usage if it's not supported.  */
> +  packet_result result = remote->m_features.packet_ok (packet,
> +						       PACKET_qIsAddressTagged);
> +
> +  /* Return false on error (Exx), empty reply (packet not supported), or reply
> +     size doesn't match 2 hex digits.  */
> +  if ((result.status () != PACKET_OK) || (strlen (packet.data ()) != 2))
> +    return false;
> +
> +  gdb_byte reply;
> +  /* Convert only 2 hex digits, i.e. 1 byte in hex format.  */
> +  hex2bin (packet.data (), &reply, 1);
> +
> +  if (reply == 0x00 || reply == 0x01)
> +    {
> +      tagged = !!reply;
> +      return true;
> +    }
> +
> +  /* Invalid reply.  */
> +  return false;
> +}
> +
>  /* Implement the "fetch_memtags" target_ops method.  */
>  
>  bool
> @@ -15581,6 +15628,31 @@ remote_target::store_memtags (CORE_ADDR address, size_t len,
>  bool
>  remote_target::is_address_tagged (gdbarch *gdbarch, CORE_ADDR address)
>  {
> +  /* Firstly, attempt to check the address using the qIsAddressTagged
> +     packet.  */
> +  if (m_features.packet_support (PACKET_qIsAddressTagged) != PACKET_DISABLE)
> +    {
> +      remote_target *remote = get_current_remote_target ();
> +      struct remote_state *rs = get_remote_state ();
> +      bool is_addr_tagged;
> +
> +      create_is_address_tagged_request (gdbarch, rs->buf, address);
> +
> +      putpkt (rs->buf);
> +      getpkt (&rs->buf);
> +
> +      /* If qIsAddressTagged is not supported PACKET_qIsAddressTagged will be
> +	 set to PACKET_DISABLE so no further attempt is made to check addresses
> +	 using this packet and the fallback mechanism below will be used
> +	 instead.  Also, if the check fails due to an error (Exx reply) the
> +	 fallback is used too.  Otherwise, the qIsAddressTagged query succeeded
> +	 and is_addr_tagged is valid.  */
> +      if (check_is_address_tagged_reply (remote, rs->buf, is_addr_tagged))
> +	return is_addr_tagged;
> +    }
> +
> +  /* Fallback to arch-specific method of checking whether an address is tagged
> +     in case check via qIsAddressTagged fails.  */
>    return gdbarch_tagged_address_p (gdbarch, address);
>  }
>  
> @@ -16070,6 +16142,9 @@ Show the maximum size of the address (in bits) in a memory packet."), NULL,
>    add_packet_config_cmd (PACKET_memory_tagging_feature,
>  			 "memory-tagging-feature", "memory-tagging-feature", 0);
>  
> +  add_packet_config_cmd (PACKET_qIsAddressTagged,
> +			 "qIsAddressTagged", "memory-tagging-address-check", 0);
> +
>    /* Assert that we've registered "set remote foo-packet" commands
>       for all packet configs.  */
>    {

Thanks, this is OK.

Approved-By: Luis Machado <luis.machado@arm.com>
Tested-By: Luis Machado <luis.machado@arm.com>