Re: [PATCH v4 2/3] powerpc/modules: Don't try to restore r2 after a sibling call
From: Naveen N. Rao <hidden>
Date: 2017-11-14 10:29:39
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Kamalesh Babulal wrote:
quoted hunk ↗ jump to hunk
From: Josh Poimboeuf <redacted> =20 When attempting to load a livepatch module, I got the following error: =20 module_64: patch_module: Expect noop after relocate, got 3c820000 =20 The error was triggered by the following code in unregister_netdevice_queue(): =20 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 =20 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. =20 Signed-off-by: Josh Poimboeuf <redacted> Signed-off-by: Kamalesh Babulal <redacted> --- arch/powerpc/kernel/module_64.c | 4 ++++ 1 file changed, 4 insertions(+) =20diff --git a/arch/powerpc/kernel/module_64.c b/arch/powerpc/kernel/module=
_64.c
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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 modul=
e *me)
if (is_early_mcount_callsite(instruction - 1)) return 1; =20 + /* Sibling calls don't return, so they don't need to restore r2 */ + if (instruction[-1] =3D=3D 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 =