On Tue, 2020-06-30 at 18:49 +0200, Bodo Stroesser wrote: > 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? Can you not use something similar to the compat_ioctl mechanism? the job of the compat layer is to re-layout the input and output structures to impedance match between 32 and 64 bit. James