[PATCH] smb: client: memcpy() with surrounding object base address

[Kees Cook <kees@kernel.org>]

Like commit f1f047bd7ce0 ("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>
---
Cc: Steve French <sfrench@samba.org>
Cc: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Cc: Paulo Alcantara <pc@manguebit.com>
Cc: Ronnie Sahlberg <ronniesahlberg@gmail.com>
Cc: Shyam Prasad N <sprasad@microsoft.com>
Cc: Tom Talpey <tom@talpey.com>
Cc: Bharath SM <bharathsm@microsoft.com>
Cc: linux-cifs@vger.kernel.org
Cc: samba-technical@lists.samba.org
---
 fs/smb/client/cifssmb.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/smb/client/cifssmb.c b/fs/smb/client/cifssmb.c
index b96ca9be5352..026d6b5f23a9 100644
--- a/fs/smb/client/cifssmb.c
+++ b/fs/smb/client/cifssmb.c
@@ -5337,7 +5337,7 @@ 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;
-- 
2.34.1