[PATCH v3 0/6] Replace one-element arrays with flexible-array members

[Gustavo A. R. Silva]

Hi!

This series aims to replace one-element arrays with flexible-array
members in drivers/scsi/megaraid/

I followed the below steps in order to verify the changes don't
significally impact the code (.text) section generated by the compiler,
for each object file involved:

1. Prepare the build with the following settings and configurations:

        linux$ KBF="KBUILD_BUILD_TIMESTAMP=1970-01-01 KBUILD_BUILD_USER=user
               KBUILD_BUILD_HOST=host KBUILD_BUILD_VERSION=1"
        linux$ make $KBF allyesconfig
        linux$ ./scripts/config -d GCOV_KERNEL -d KCOV -d GCC_PLUGINS \
                         -d IKHEADERS -d KASAN -d UBSAN \
                         -d DEBUG_INFO_NONE \
                         -e DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT
        linux$ make $KBF olddefconfig

2. Build drivers/scsi/megaraid/ with the same settings and configurations
   as in Step 1, and copy the generated object files in directory before/

        linux$ make -j128 $KBF drivers/scsi/megaraid/
        linux$ mkdir -p before
        linux$ cp drivers/scsi/megaraid/*.o before/

3. Implement all the needed changes and create the patch series. In this
   case, six patches.

        linux$ vi code.c
               ...do the magic :)
        linux$ git format-patch ...all the rest

4. Apply a patch at a time (of the previously created series) and, after
   applying EACH patch, build (as in Step 2) drivers/scsi/megaraid/ and
   copy the generated object files in directory after/

5. Compare the code section (.text) of each before/file.o and
   after/file.o. I use the following bash script:

   compare.sh:
        ARGS="--disassemble --demangle --reloc --no-show-raw-insn --section=.text"
        for i in $(cd before && echo *.o); do
                echo $i
                diff -u <(objdump $ARGS before/$i | sed "0,/^Disassembly/d") \
                        <(objdump $ARGS after/$i  | sed "0,/^Disassembly/d")
        done

   linux$ ./compare.sh > code_comparison.diff

6. Open the code_comparison.diff file from the example above, look for
   any differences that might show up and analyze them in order to
   determine their impact, and what (if something) should be changed
   or study further.

The above process (code section comparison of object files) is based on
this[0] blog post by Kees Cook. The compiler used to build the code was
GCC-12.

In this series I only found the following sorts of differences in files
megaraid_sas.o and megaraid_sas_base.o:

...
...@@ -7094,24 +7094,24 @@
     6302:      movq   $0x0,0x1e20(%rbx)
     630d:      test   %r15,%r15
     6310:      je     6316 <megasas_aen_polling+0x56>
-                       6312: R_X86_64_PC32     .text.unlikely+0x3ae3
+                       6312: R_X86_64_PC32     .text.unlikely+0x3ae0
     6316:      mov    0x0(%rip),%eax        # 631c <megasas_aen_polling+0x5c>
                        6318: R_X86_64_PC32     event_log_level-0x4
     631c:      mov    0xc(%r15),%r14d
     6320:      lea    0x2(%rax),%edx
     6323:      cmp    $0x6,%edx
     6326:      ja     632c <megasas_aen_polling+0x6c>
-                       6328: R_X86_64_PC32     .text.unlikely+0x3ac3
+                       6328: R_X86_64_PC32     .text.unlikely+0x3ac0
     632c:      mov    %r14d,%edx
     632f:      sar    $0x18,%edx
     6332:      mov    %edx,%ecx
     6334:      cmp    %eax,%edx
     6336:      jge    633c <megasas_aen_polling+0x7c>
-                       6338: R_X86_64_PC32     .text.unlikely+0x399c
+                       6338: R_X86_64_PC32     .text.unlikely+0x3999
...

All of them have to do with the relocation of symbols in the
.text.unlikely subsection and they don't seem to be of any actual
relevance. So, we can safely ignore them.

Also, notice there is an open issue in bugzilla.kernel.org [1] that's
seems could be fixed by this series. :)

This helps with the ongoing efforts to tighten the FORTIFY_SOURCE
routines on memcpy() and help us make progress towards globally
enabling -fstrict-flex-arrays [2].

Link: https://en.wikipedia.org/wiki/Flexible_array_member
Link: https://www.kernel.org/doc/html/v5.10/process/deprecated.html#zero-length-and-one-element-arrays
Link: https://github.com/KSPP/linux/issues/79
Link: https://github.com/KSPP/linux/issues/109
Link: https://bugzilla.kernel.org/show_bug.cgi?id=215943 [1]
Link: https://reviews.llvm.org/D126864 [2]

Thanks

[0] https://outflux.net/blog/archives/2022/06/24/finding-binary-differences/

Changes in v3:
 - Split the struct_size() changes into a couple of separate patches.
 - Use objdump to compare the code (.text) sections of the object
   files before and after the changes.
 - Modify MR_FW_RAID_MAP_ALL and MR_DRV_RAID_MAP_ALL structures. Change
   suggested by Kees Cook.

Changes in v2:
 - Revert changes in struct MR_FW_RAID_MAP_ALL.

Gustavo A. R. Silva (6):
  scsi: megaraid_sas: Replace one-element array with flexible-array
    member in MR_FW_RAID_MAP
  scsi: megaraid_sas: Replace one-element array with flexible-array
    member in MR_FW_RAID_MAP_DYNAMIC
  scsi: megaraid_sas: Replace one-element array with flexible-array
    member in MR_DRV_RAID_MAP
  scsi: megaraid_sas: Replace one-element array with flexible-array
    member in MR_PD_CFG_SEQ_NUM_SYNC
  scsi: megaraid_sas: Use struct_size() in code related to struct
    MR_FW_RAID_MAP
  scsi: megaraid_sas: Use struct_size() in code related to struct
    MR_PD_CFG_SEQ_NUM_SYNC

 drivers/scsi/megaraid/megaraid_sas_base.c   | 20 ++++++++++----------
 drivers/scsi/megaraid/megaraid_sas_fp.c     |  6 +++---
 drivers/scsi/megaraid/megaraid_sas_fusion.c |  2 +-
 drivers/scsi/megaraid/megaraid_sas_fusion.h | 12 ++++++------
 4 files changed, 20 insertions(+), 20 deletions(-)

-- 
2.34.1