smb: client: memcpy() with surrounding object base address

[ Upstream commit f69b018 ]

Like commit f1f047b ("smb: client: Fix -Wstringop-overflow issues"),
adjust the memcpy() destination address to be based off the surrounding
object rather than based off the 4-byte "Protocol" member. This avoids a
build-time warning when compiling under CONFIG_FORTIFY_SOURCE with GCC 15:

In function 'fortify_memcpy_chk',
    inlined from 'CIFSSMBSetPathInfo' at ../fs/smb/client/cifssmb.c:5358:2:
../include/linux/fortify-string.h:571:25: error: call to '__write_overflow_field' declared with attribute warning: detected write beyond size of field (1st parameter); maybe use struct_group()? [-Werror=attribute-warning]
  571 |                         __write_overflow_field(p_size_field, size);
      |                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Kees Cook <kees@kernel.org>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>

fs/smb/client/cifssmb.c

@@ -5349,7 +5349,7 @@ CIFSSMBSetPathInfo(const unsigned int xid, struct cifs_tcon *tcon,
 	param_offset = offsetof(struct smb_com_transaction2_spi_req,
 				InformationLevel) - 4;
 	offset = param_offset + params;
-	data_offset = (char *) (&pSMB->hdr.Protocol) + offset;
+	data_offset = (char *)pSMB + offsetof(typeof(*pSMB), hdr.Protocol) + offset;
 	pSMB->ParameterOffset = cpu_to_le16(param_offset);
 	pSMB->DataOffset = cpu_to_le16(offset);
 	pSMB->SetupCount = 1;