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Violators will be prosecuted for from ; Wed, 15 Nov 2017 05:38:43 -0000 Subject: Re: [PATCH v4 2/3] powerpc/modules: Don't try to restore r2 after a sibling call To: Josh Poimboeuf , "Naveen N. Rao" References: <20171114092910.20399-1-kamalesh@linux.vnet.ibm.com> <20171114092910.20399-3-kamalesh@linux.vnet.ibm.com> <1510654928.8xrjtkjm8m.naveen@linux.ibm.com> <20171114155323.3sjxx3eykinnl2ea@treble> Cc: Michael Ellerman , Balbir Singh , linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, live-patching@vger.kernel.org From: Kamalesh Babulal Date: Wed, 15 Nov 2017 11:08:38 +0530 MIME-Version: 1.0 In-Reply-To: <20171114155323.3sjxx3eykinnl2ea@treble> Content-Type: text/plain; charset=utf-8; format=flowed Message-Id: List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On Tuesday 14 November 2017 09:23 PM, Josh Poimboeuf wrote: > On Tue, Nov 14, 2017 at 03:59:21PM +0530, Naveen N. Rao wrote: >> Kamalesh Babulal wrote: >>> From: Josh Poimboeuf >>> >>> 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 >>> 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 >>> Signed-off-by: Kamalesh Babulal >>> --- >>> 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)) > > Yes, good point. How about something like this? > > (completely untested because I don't have access to a box at the moment) Reviewed-and-tested-by: Kamalesh Babulal > > > diff --git a/arch/powerpc/include/asm/code-patching.h b/arch/powerpc/include/asm/code-patching.h > index abef812de7f8..302e4368debc 100644 > --- a/arch/powerpc/include/asm/code-patching.h > +++ b/arch/powerpc/include/asm/code-patching.h > @@ -33,6 +33,7 @@ int patch_branch(unsigned int *addr, unsigned long target, int flags); > int patch_instruction(unsigned int *addr, unsigned int instr); > > int instr_is_relative_branch(unsigned int instr); > +int instr_is_link_branch(unsigned int instr); > int instr_is_branch_to_addr(const unsigned int *instr, unsigned long addr); > unsigned long branch_target(const unsigned int *instr); > unsigned int translate_branch(const unsigned int *dest, > diff --git a/arch/powerpc/kernel/module_64.c b/arch/powerpc/kernel/module_64.c > index 9cb007bc7075..b5148a206b4d 100644 > --- a/arch/powerpc/kernel/module_64.c > +++ b/arch/powerpc/kernel/module_64.c > @@ -487,11 +487,13 @@ static bool is_early_mcount_callsite(u32 *instruction) > restore r2. */ > static int restore_r2(u32 *instruction, struct module *me) > { > - if (is_early_mcount_callsite(instruction - 1)) > + u32 *prev_insn = instruction - 1; > + > + if (is_early_mcount_callsite(prev_insn)) > return 1; > > /* Sibling calls don't return, so they don't need to restore r2 */ > - if (instruction[-1] == PPC_INST_BRANCH) > + if (!instr_is_link_branch(*prev_insn)) > return 1; > > if (*instruction != PPC_INST_NOP) { > diff --git a/arch/powerpc/lib/code-patching.c b/arch/powerpc/lib/code-patching.c > index c9de03e0c1f1..4727fafd37e4 100644 > --- a/arch/powerpc/lib/code-patching.c > +++ b/arch/powerpc/lib/code-patching.c > @@ -304,6 +304,12 @@ int instr_is_relative_branch(unsigned int instr) > return instr_is_branch_iform(instr) || instr_is_branch_bform(instr); > } > > +int instr_is_link_branch(unsigned int instr) > +{ > + return (instr_is_branch_iform(instr) || instr_is_branch_bform(instr)) && > + (instr & BRANCH_SET_LINK); > +} > + > static unsigned long branch_iform_target(const unsigned int *instr) > { > signed long imm; > -- cheers, Kamalesh.