On Sep 15, 2010, at 1:05 PM, Eric W. Biederman wrote:
> Matthew McClintock <msm at freescale.com> writes:
>> 1) Is there any reason we don't just use -O0 instead of -0s? This
>> would provide a consistent set of compiler optimization flags across
>> all versions of gcc and we don't have to worry about a flag being
>> added or removed across versions. Or perhaps explicitly list all
>> desired flags?
>
> So -O0 should do no optimization.
Right, a consistent set of optimizations (none) =)
> Last I looked the trick with the
> purgatory code was to compile it cleanly enough that we had support of
> the different relocation types needed in order to relocate the code
> at link time. Usually this winds up being a small subset of the
> linking types of what the architecture supports.
AFAICT, it's not the linking types themselves. On ppc kexec-tools will die() if it encounters an unknown relocation type. I have also confirmed this with objdump -r. However, I must admit my naivety on the subject so I could be missing something here.
>
>> 2) Has anyone else experienced issue with purgatory code not being
>> generated correctly? Specifically looking at a disassebly it looks
>> like -freorder-blocks is changed how functions return. On ppc for
>> example it appears that the functions are not returning using the link
>> register and instead are just branching.
>
> The only problem I have previously seen is having an unsupported
> relocation type. Can you confirm that isn't happening. Ultimately
> if your gcc isn't compiling the code correctly unless we understand
> exactly why that doesn't work we can't make a sensible judgement
> on a good work around.
I confirmed 'objdump -r purgatory/purgatory.ro' does not have any entries covered by machine_apply_elf_rel(). I took some time to look at the assembly itself too and I notice one thing in common with the code that does not work. Basically, some functions don't use the link register to return... see below for a quick snippet:
Bad broken purgatory (with -Os):
00000000 <verify_sha256_digest>:
0: 94 21 ff 50 stwu r1,-176(r1)
4: 7c 08 02 a6 mflr r0
8: 90 01 00 b4 stw r0,180(r1)
c: bf a1 00 a4 stmw r29,164(r1)
...[snip]...
11c: 48 00 00 01 bl 11c <verify_sha256_digest+0x11c>
120: 48 00 00 00 b 120 <verify_sha256_digest+0x120>
124: 39 61 00 b0 addi r11,r1,176
128: 48 00 00 00 b 128 <verify_sha256_digest+0x128> <-- Not using the link register return...
Good working purgatory (with -O0):
00000000 <verify_sha256_digest>:
0: 94 21 ff 50 stwu r1,-176(r1)
4: 7c 08 02 a6 mflr r0
8: 90 01 00 b4 stw r0,180(r1)
c: 93 e1 00 ac stw r31,172(r1)
...[snip]...
1c8: 7c 08 03 a6 mtlr r0
1cc: 83 eb ff fc lwz r31,-4(r11)
1d0: 7d 61 5b 78 mr r1,r11
1d4: 4e 80 00 20 blr
-M
