Messages in this thread | | | Date | Tue, 14 Nov 2017 15:59:21 +0530 | From | "Naveen N. Rao" <> | Subject | Re: [PATCH v4 2/3] powerpc/modules: Don't try to restore r2 after a sibling call |
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Kamalesh Babulal wrote: > From: Josh Poimboeuf <jpoimboe@redhat.com> > > When attempting to load a livepatch module, I got the following error: > > module_64: patch_module: Expect noop after relocate, got 3c820000 > > The error was triggered by the following code in > unregister_netdevice_queue(): > > 14c: 00 00 00 48 b 14c <unregister_netdevice_queue+0x14c> > 14c: R_PPC64_REL24 net_set_todo > 150: 00 00 82 3c addis r4,r2,0 > > GCC didn't insert a nop after the branch to net_set_todo() because it's > a sibling call, so it never returns. The nop isn't needed after the > branch in that case. > > Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com> > Signed-off-by: Kamalesh Babulal <kamalesh@linux.vnet.ibm.com> > --- > arch/powerpc/kernel/module_64.c | 4 ++++ > 1 file changed, 4 insertions(+) > > diff --git a/arch/powerpc/kernel/module_64.c b/arch/powerpc/kernel/module_64.c > index 39b01fd..9e5391f 100644 > --- a/arch/powerpc/kernel/module_64.c > +++ b/arch/powerpc/kernel/module_64.c > @@ -489,6 +489,10 @@ static int restore_r2(u32 *instruction, struct module *me) > if (is_early_mcount_callsite(instruction - 1)) > return 1; > > + /* Sibling calls don't return, so they don't need to restore r2 */ > + if (instruction[-1] == PPC_INST_BRANCH) > + return 1; > +
This looks quite fragile, unless we know for sure that gcc will _always_ emit this instruction form for sibling calls with relocations.
As an alternative, does it make sense to do the following check instead? if ((instr_is_branch_iform(insn) || instr_is_branch_bform(insn)) && !(insn & 0x1))
- Naveen
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