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Venkataraman" Cc: Mark Brown , mark.rutland@arm.com, ardb@kernel.org, jthierry@redhat.com, catalin.marinas@arm.com, will@kernel.org, jmorris@namei.org, pasha.tatashin@soleen.com, linux-arm-kernel@lists.infradead.org, live-patching@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [RFC PATCH v4 1/2] arm64: Introduce stack trace reliability checks in the unwinder Message-ID: <20210521200806.nk3m7aldelmi3l2r@treble> References: <20210521161117.GB5825@sirena.org.uk> <20210521174242.GD5825@sirena.org.uk> <26c33633-029e-6374-16e6-e9418099da95@linux.microsoft.com> <20210521175318.GF5825@sirena.org.uk> <20210521184817.envdg232b2aeyprt@treble> <74d12457-7590-bca2-d1ce-5ff82d7ab0d8@linux.microsoft.com> <20210521191140.4aezpvm2kruztufi@treble> <20210521191608.f24sldzhpg3hyq32@treble> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, May 21, 2021 at 02:41:56PM -0500, Madhavan T. Venkataraman wrote: > > Or is frame->reliable supposed to be checked after all? Looking at the > > code again, I'm not sure. > > > > Either way it would be good to document the interface more clearly in a > > comment above the function. > > > > So, arch_stack_walk_reliable() would do this: > > start_backtrace(frame); > > while (...) { > if (!frame->reliable) > return error; > > consume_entry(...); > > ret = unwind_frame(...); > > if (ret) > break; > } > > if (ret == -ENOENT) > return success; > return error; > > > Something like that. I see. So basically there are six possible combinations of return states: 1) No error frame->reliable 2) No error !frame->reliable 3) -ENOENT frame->reliable 5) -ENOENT !frame->reliable (doesn't happen in practice) 4) Other error frame->reliable (doesn't happen in practice) 6) Other error !frame->reliable On x86 we have fewer combinations: 1) No error state->error 2) No error !state->error 3) Error state->error 4) Error !state->error (doesn't happen in practice) I think the x86 interface seems more robust, because it's more narrow and has fewer edge cases. Also it doesn't have to distinguish between error enums, which can get hairy if a downstream callee happens to return -ENOENT for a different reason. -- Josh