[RFC] scsi: target: tcmu: running 32bit userspace on 64bit kernel

Bodo Stroesser
2020-06-30T16:49:28Z

Hi,

When using tcmu it might happen, that userspace application cannot be
built as 64 bit program even on a 64 bit host due to existing 32 bit
libraries that must be used, e.g. for compression, encryption,
deduplication, ...

Currently this only works with manual changes in userspace include
file target_core_user.h due to a missing padding field in
struct tcmu_cmd_entry.

Here are field offsets printed by a small program on a 64 bit host,
compiled as 64 bit program and as 32 bit:

Devel:~ # gcc -o print_offsets print_offsets.c
Devel:~ # ./print_offsets
Offset of tcmu_cmd_entry.hdr.len_op = 0000
Offset of tcmu_cmd_entry.hdr.cmd_id = 0004
Offset of tcmu_cmd_entry.hdr.kflags = 0006
Offset of tcmu_cmd_entry.hdr.uflags = 0007

Offset of tcmu_cmd_entry.req.iov_cnt = 0008
Offset of tcmu_cmd_entry.req.iov_bidi_cnt = 000c
Offset of tcmu_cmd_entry.req.iov_dif_cnt = 0010
Offset of tcmu_cmd_entry.req.cdb_off = 0018
Offset of tcmu_cmd_entry.req.iov[0] = 0030

Offset of tcmu_cmd_entry.rsp.scsi_status = 0008
Offset of tcmu_cmd_entry.rsp.read_len = 000c
Offset of tcmu_cmd_entry.rsp.sense_buffer[0] = 0010

Size of struct tcmu_cmd_entry = 0070

Devel:~ # gcc -m32 -o print_offsets print_offsets.c
Devel:~ # ./print_offsets
Offset of tcmu_cmd_entry.hdr.len_op = 0000
Offset of tcmu_cmd_entry.hdr.cmd_id = 0004
Offset of tcmu_cmd_entry.hdr.kflags = 0006
Offset of tcmu_cmd_entry.hdr.uflags = 0007

Offset of tcmu_cmd_entry.req.iov_cnt = 0008
Offset of tcmu_cmd_entry.req.iov_bidi_cnt = 000c
Offset of tcmu_cmd_entry.req.iov_dif_cnt = 0010
Offset of tcmu_cmd_entry.req.cdb_off = 0014
Offset of tcmu_cmd_entry.req.iov[0] = 002c

Offset of tcmu_cmd_entry.rsp.scsi_status = 0008
Offset of tcmu_cmd_entry.rsp.read_len = 000c
Offset of tcmu_cmd_entry.rsp.sense_buffer[0] = 0010

Size of struct tcmu_cmd_entry = 0070

The offset of the fields req.cdb_off and req.iov differ for 64-bit
and 32-bit compilation.

That means:
- 64-bit application on 64-bit host works well
- 32-bit application on 32-bit host works well
- 32-bit application on 64-bit host fails.

Unfortunately I don't see a way to fix this problem such, that
32-bit application runs fine on 32-bit and 64-bit host without
breaking compatibility.

So I'm wondering whether the following change would be a viable
solution:

diff --git a/include/uapi/linux/target_core_user.h b/include/uapi/linux/target_core_user.h
--- a/include/uapi/linux/target_core_user.h
+++ b/include/uapi/linux/target_core_user.h
@@ -114,6 +114,9 @@ struct tcmu_cmd_entry {
__u32 iov_cnt;
__u32 iov_bidi_cnt;
__u32 iov_dif_cnt;
+#ifdef APPL32BIT_ON_KERNEL64BIT
+ __u32 __pad9;
+#endif
__u64 cdb_off;
__u64 __pad1;
__u64 __pad2;

Using this change we can do:

Devel:~ # gcc -m32 -DAPPL32BIT_ON_KERNEL64BIT -o print_offsets print_offsets.c
Devel:~ # ./print_offsets
Offset of tcmu_cmd_entry.hdr.len_op = 0000
Offset of tcmu_cmd_entry.hdr.cmd_id = 0004
Offset of tcmu_cmd_entry.hdr.kflags = 0006
Offset of tcmu_cmd_entry.hdr.uflags = 0007

Offset of tcmu_cmd_entry.req.iov_cnt = 0008
Offset of tcmu_cmd_entry.req.iov_bidi_cnt = 000c
Offset of tcmu_cmd_entry.req.iov_dif_cnt = 0010
Offset of tcmu_cmd_entry.req.cdb_off = 0018
Offset of tcmu_cmd_entry.req.iov[0] = 0030

Offset of tcmu_cmd_entry.rsp.scsi_status = 0008
Offset of tcmu_cmd_entry.rsp.read_len = 000c
Offset of tcmu_cmd_entry.rsp.sense_buffer[0] = 0010

Size of struct tcmu_cmd_entry = 0070

So we can compile a 32-bit application that works on 64-bit kernel without
need to manipulate the include file prepared by the kernel.

What do you think? Do you know a better solution?

Thank you,
Bodo