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([2a01:cb14:58d:8400:ecf6:58e2:9c06:a308]) by smtp.gmail.com with ESMTPSA id t124sm16359wmf.19.2020.04.08.09.04.47 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 08 Apr 2020 09:04:48 -0700 (PDT) Subject: Re: [PATCH V2 5/9] objtool: Add support for intra-function calls To: Alexandre Chartre , Peter Zijlstra Cc: x86@kernel.org, linux-kernel@vger.kernel.org, jpoimboe@redhat.com, tglx@linutronix.de References: <20200407073142.20659-1-alexandre.chartre@oracle.com> <20200407073142.20659-6-alexandre.chartre@oracle.com> <20200407130729.GZ20730@hirez.programming.kicks-ass.net> <40b19a8e-ae5e-623e-fb3f-261f9fec2ea5@oracle.com> <7ba6d4c9-c1dc-fa83-2ade-b7d3fba9e7fb@oracle.com> From: Julien Thierry Message-ID: <981d7f79-3429-776f-7356-345adc6332bf@redhat.com> Date: Wed, 8 Apr 2020 17:04:46 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.4.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 4/8/20 5:03 PM, Alexandre Chartre wrote: > > > On 4/8/20 4:19 PM, Julien Thierry wrote: >> >> >> On 4/8/20 3:06 PM, Alexandre Chartre wrote: >>> >>> >>> On 4/7/20 3:28 PM, Alexandre Chartre wrote: >>>> >>>> On 4/7/20 3:07 PM, Peter Zijlstra wrote: >>>>> On Tue, Apr 07, 2020 at 09:31:38AM +0200, Alexandre Chartre wrote: >>>>> >>>>>> index a62e032863a8..7ee1561bf7ad 100644 >>>>>> --- a/tools/objtool/arch/x86/decode.c >>>>>> +++ b/tools/objtool/arch/x86/decode.c >>>>>> @@ -497,3 +497,15 @@ void arch_initial_func_cfi_state(struct >>>>>> cfi_state *state) >>>>>>       state->regs[16].base = CFI_CFA; >>>>>>       state->regs[16].offset = -8; >>>>>>   } >>>>>> + >>>>>> + >>>>>> +void arch_configure_intra_function_call(struct stack_op *op) >>>>>> +{ >>>>>> +    /* >>>>>> +     * For the impact on the stack, make an intra-function >>>>>> +     * call behaves like a push of an immediate value (the >>>>>> +     * return address). >>>>>> +     */ >>>>>> +    op->src.type = OP_SRC_CONST; >>>>>> +    op->dest.type = OP_DEST_PUSH; >>>>>> +} >>>>> >>>>> An alternative is to always set up stack ops for CALL/RET on >>>>> decode, but >>>>> conditionally run update_insn_state() for them. >>>>> >>>>> Not sure that makes more logical sense, but the patch would be >>>>> simpler I >>>>> think. >>>> >>>> Right, this would avoid adding a new arch dependent function and the >>>> patch >>>> will be simpler. This probably makes sense as the stack impact is >>>> the same >>>> for all calls (but objtool will use it only for intra-function calls). >>>> >>> >>> Actually the processing of the ret instruction is more complicated >>> than I >>> anticipated with intra-function calls, and so my implementation is not >>> complete at the moment. >>> >>> The issue is to correctly handle how the ret is going to behave >>> depending how >>> the stack (or register on arm) is modified before the ret. Adjusting >>> the stack >>> offset makes the stack state correct, but objtool still needs to >>> correctly >>> figure out where the ret is going to return and where the code flow >>> continues. >>> >> >> A hint indicating the target "jump" address could be useful. It could >> be used to add the information on some call/jump dynamic that aren't >> associated with jump tables. Currently when objtool finds a jump >> dynamic, if no branches were added to it, it will just return. >> >> Having such a hint could help make additional links (at least on >> arm64). I don't know what Peter and Josh would think of that (if that >> helps in your case of course). >> > > Yes, I am thinking about tracking intra-function call return address, > and having hints to specify a return address changes. For example, > on x86, when we push the branch address on the stack we overwrite the > last return address (the return address of the last intra-function call). > Then the return instruction can figure out where to branch. I see, I was thinking about a more generic hint, that would just indicate "this instruction actually jumps here". So in your case it would just point that a certain return instruction causes to branch somewhere. This way the hint could also be used for other instructions (e.g. INSN_JUMP_DYNAMIC). -- Julien Thierry