I believe that the basic premise is that you should provide a directly
reachable copy of the save/rstore functions, even if this means that
you need several copies of the functions.
I just fixed a very similar problem with grub2 in fact. It was using r0
and trashing the saved LR that way.
The real fix is indeed to statically link those gcc "helpers", we
shouldn't generate things like cross-module calls inside function prologs
and epilogues, when stackframes aren't even guaranteed to be reliable.
However, in the grub2 case, it was easier to just use r12 :-)
For not just the module loading case, I believe r12 is the only real solution
now. I checked one debugger capable of doing ELF download. It also uses r12 for
trampoline code. I am guessing for the reason that prompted this discussion.
Without r12 we'd have to change standard libraries to automagically link in gcc
helpers for any conceivable non-.text section, which I am not sure is feasible.
How would you write section independent helper functions which link to any
section needing them?!
Asking users to create their own section specific copy of helper functions is
definitely not portable if the module or other code is not architecture
dependent.
It is a normal gcc feature that you can assign specific code to non-.text
sections and it is not documented that it may crash depending on the OS arch
the ELF is built for, so asking for a Power Architecture specific change on
tool libs to make Power Architecture Linux happy seems a bit much to ask.
Using r12 in any Linux related trampoline code seems a reachable goal, and it
would eliminate the conflict to the ABI.
Regards,
Heinz
Hi,
I've tried the fix using register r12 for the trampoline code, instead of
register r11.
I'd to adapt entry_matches() too:
--- arch/powerpc/kernel/module_32.c (revision 1898)
+++ arch/powerpc/kernel/module_32.c (working copy)
@@ -187,8 +187,8 @@
static inline int entry_matches(struct ppc_plt_entry *entry, Elf32_Addr val)
{
- if (entry->jump[0] == 0x3d600000 + ((val + 0x8000) >> 16)
- && entry->jump[1] == 0x396b0000 + (val & 0xffff))
+ if (entry->jump[0] == 0x3d800000 + ((val + 0x8000) >> 16)
+ && entry->jump[1] == 0x398c0000 + (val & 0xffff))
return 1;
return 0;
}
@@ -215,10 +215,9 @@
entry++;
}
- /* Stolen from Paul Mackerras as well... */
- entry->jump[0] = 0x3d600000+((val+0x8000)>>16); /* lis r11,sym@ha */
- entry->jump[1] = 0x396b0000 + (val&0xffff); /* addi r11,r11,sym@l*/
- entry->jump[2] = 0x7d6903a6; /* mtctr r11 */
+ entry->jump[0] = 0x3d800000+((val+0x8000)>>16); /* lis r12,sym@ha */
+ entry->jump[1] = 0x398c0000 + (val&0xffff); /* addi r12,r12,sym@l*/
+ entry->jump[2] = 0x7d8903a6; /* mtctr r12 */
entry->jump[3] = 0x4e800420; /* bctr */
DEBUGP("Initialized plt for 0x%x at %p\n", val, entry);
It looks working, also in the case the trampoline code is used.
Please see also the debug session below.
Let me summarize the situation:
+ According to Freescale, EABI assigns a dedicated function to register r11.
+ The ELF sections .text and .init.text may trigger the usage of the
trampoline code.
+ The trampoline code must not user register r11, too.
+ We must not depend on addresses returned by vmalloc during kernel module
loading.
I think the bug must be fixed !!!
Regards
Steffen
--
(gdb) bt
#0 0xd54990f0 in ?? ()
#1 0xd5499088 in ?? ()
#2 0xc0001d9c in do_one_initcall (fn=0xd76e26ac) at init/main.c:716
#3 0xc0059630 in sys_init_module (umod=0x4802f008, len=<value optimized out>,
uargs=0x10012008 "") at kernel/module.c:2470
#4 0xc000dff8 in syscall_dotrace_cont () at arch/powerpc/kernel/entry_32.S:331
Backtrace stopped: frame did not save the PC
<-- we are returning from the driver init entry point
(gdb) info reg r11
r11 0xcbb57f00 3417669376
(gdb) x/2x 0xcbb57f00
0xcbb57f00: 0xcbb57f20 0xc0001d9c
<-- register r11 is fine here
0xd54990f0: lis r12,-10386
0xd54990f4: addi r12,r12,-20268
0xd54990f8: mtctr r12
<-- this is the trampoline code using now r12, instead of r11
(gdb) info reg r12
r12 0xd76db0d4 3614290132
(gdb) info reg ctr
ctr 0xd76db0d4 3614290132
0xd54990fc: bctr
<-- we are jumping to the epilogue
0xd76db0d4: lwz r29,-12(r11)
0xd76db0d8: lwz r30,-8(r11)
0xd76db0dc: lwz r0,4(r11)
0xd76db0e0: lwz r31,-4(r11)
0xd76db0e4: mtlr r0
0xd76db0e8: mr r1,r11
<- the epilogue
(gdb) info reg lr
lr 0xc0001d9c 0xc0001d9c <do_one_initcall+100>
0xc0001d9c <do_one_initcall+100>: lis r9,-16330
0xc0001da0 <do_one_initcall+104>: lwz r0,12296(r9)
0xc0001da4 <do_one_initcall+108>: lis r9,-16330
0xd76db0ec: blr
<-- the jump back to do_one_initcall()
<-- no crash
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