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Violators will be prosecuted for from ; Fri, 17 Nov 2017 08:17:07 -0000 Subject: Re: [PATCH v4.2] 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> <1510737417.g8rnjuztlf.naveen@linux.ibm.com> <20171116012628.6ajxlychto365sf6@treble> <1510837263.5d3ac8knzo.naveen@linux.ibm.com> <20171116174537.duz4x6vfzhp44lfh@treble> Cc: Balbir Singh , linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, live-patching@vger.kernel.org, Michael Ellerman From: Kamalesh Babulal Date: Fri, 17 Nov 2017 13:47:03 +0530 MIME-Version: 1.0 In-Reply-To: <20171116174537.duz4x6vfzhp44lfh@treble> Content-Type: text/plain; charset=utf-8; format=flowed Message-Id: <2094a573-d236-e1b7-6f4d-47049f3d0dfb@linux.vnet.ibm.com> List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On Thursday 16 November 2017 11:15 PM, Josh Poimboeuf wrote: > On Thu, Nov 16, 2017 at 06:39:03PM +0530, Naveen N. Rao wrote: >> Josh Poimboeuf wrote: >>> On Wed, Nov 15, 2017 at 02:58:33PM +0530, Naveen N. Rao wrote: >>>>> +int instr_is_link_branch(unsigned int instr) >>>>> +{ >>>>> + return (instr_is_branch_iform(instr) || instr_is_branch_bform(instr)) && >>>>> + (instr & BRANCH_SET_LINK); >>>>> +} >>>>> + >>>> >>>> Nitpicking here, but since we're not considering the other branch forms, >>>> perhaps this can be renamed to instr_is_link_relative_branch() (or maybe >>>> instr_is_relative_branch_link()), just so we're clear :) >>> >>> My understanding is that the absolute/relative bit isn't a "form", but >>> rather a bit that can be set for either the b-form (conditional) or the >>> i-form (unconditional). And the above function isn't checking the >>> absolute bit, so it isn't necessarily a relative branch. Or did I miss >>> something? >> >> Ah, good point. I was coming from the fact that we are only considering the >> i-form and b-form branches and not the lr/ctr/tar based branches, which are >> always absolute branches, but can also set the link register. > > Hm, RISC is more complicated than I realized ;-) > >> Thinking about this more, aren't we only interested in relative branches >> here (for relocations), so can we actually filter out the absolute branches? >> Something like this? >> >> int instr_is_relative_branch_link(unsigned int instr) >> { >> return ((instr_is_branch_iform(instr) || instr_is_branch_bform(instr)) && >> !(instr & BRANCH_ABSOLUTE) && (instr & BRANCH_SET_LINK)); > > Yeah, makes sense to me. Here's another try (also untested). If this > looks ok, Kamalesh would you mind testing again? > > ----8<---- > > From: Josh Poimboeuf > Subject: [PATCH v4.2] powerpc/modules: Don't try to restore r2 after a sibling call > > 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 Reviewed-and-tested-by: Kamalesh Babulal > --- > arch/powerpc/include/asm/code-patching.h | 1 + > arch/powerpc/kernel/module_64.c | 12 +++++++++++- > arch/powerpc/lib/code-patching.c | 5 +++++ > 3 files changed, 17 insertions(+), 1 deletion(-) > > diff --git a/arch/powerpc/include/asm/code-patching.h b/arch/powerpc/include/asm/code-patching.h > index abef812de7f8..2c895e8d07f7 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_relative_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 759104b99f9f..180c16f04063 100644 > --- a/arch/powerpc/kernel/module_64.c > +++ b/arch/powerpc/kernel/module_64.c > @@ -487,7 +487,17 @@ 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; > + > + /* > + * Make sure the branch isn't a sibling call. Sibling calls aren't > + * "link" branches and they don't return, so they don't need the r2 > + * restore afterwards. > + */ > + if (!instr_is_relative_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..d81aab7441f7 100644 > --- a/arch/powerpc/lib/code-patching.c > +++ b/arch/powerpc/lib/code-patching.c > @@ -304,6 +304,11 @@ int instr_is_relative_branch(unsigned int instr) > return instr_is_branch_iform(instr) || instr_is_branch_bform(instr); > } > > +int instr_is_relative_link_branch(unsigned int instr) > +{ > + return instr_is_relative_branch(instr) && (instr & BRANCH_SET_LINK); > +} > + > static unsigned long branch_iform_target(const unsigned int *instr) > { > signed long imm; >