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([2804:1b3:a7c0:a9f4:4405:8c19:a65e:e640]) by smtp.gmail.com with ESMTPSA id y81-20020a4a4554000000b00480dac71228sm380148ooa.24.2022.11.03.09.39.58 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 03 Nov 2022 09:40:00 -0700 (PDT) Message-ID: Date: Thu, 3 Nov 2022 13:39:57 -0300 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:102.0) Gecko/20100101 Thunderbird/102.4.1 Subject: Re: [PATCH v2 4/9] aarch64: Add the clone3 wrapper Content-Language: en-US To: Szabolcs Nagy Cc: libc-alpha@sourceware.org, Christian Brauner , "H.J. Lu" References: <20220930192613.3491147-1-adhemerval.zanella@linaro.org> <20220930192613.3491147-5-adhemerval.zanella@linaro.org> <1d4ce210-2b28-b061-9780-f643eaa80a27@linaro.org> <8a3dc5d9-b731-4c45-7252-8157ba0be6c2@linaro.org> From: Adhemerval Zanella Netto Organization: Linaro In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Spam-Status: No, score=-5.9 required=5.0 tests=BAYES_00,DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,NICE_REPLY_A,RCVD_IN_DNSWL_NONE,SPF_HELO_NONE,SPF_PASS,TXREP autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on server2.sourceware.org List-Id: On 03/11/22 13:31, Szabolcs Nagy wrote: > The 11/03/2022 13:22, Adhemerval Zanella Netto wrote: >> >> >> On 03/11/22 11:01, Szabolcs Nagy wrote: >>> The 11/03/2022 10:15, Adhemerval Zanella Netto wrote: >>>> On 02/11/22 09:12, Szabolcs Nagy wrote: >>>>> The 09/30/2022 16:26, Adhemerval Zanella via Libc-alpha wrote: >>>>>> It follow the internal signature: >>>>>> >>>>>> extern int clone3 (struct clone_args *__cl_args, size_t __size, >>>>>> int (*__func) (void *__arg), void *__arg); >>>>>> >>>>>> And x86_64 semantics to return EINVAL if either cl_args or func >>>>>> is NULL. The stack is 16-byte aligned prior executing func. >>>>> >>>>> "x86_64 semantics" sounds wrong: maybe this should be documented? >>>>> i'd expect 0 cl_args/func to be UB like in most posix apis. >>>> >>>> Right, I think it is worth to document the function semantic >>>> properly at least on its internal header (include/clone_internal.h). >>>> H.J also added a new clone3.h headers, which is not currently installed >>>> that I am inclined to just remove it from now. We might reinstate >>>> if/when we decide to provide the clone3 as an ABI. >>>> >>>> And returning EINVAL for 0 cl_args/func aligns with our exported clone >>>> interface, where EINVAL is also returned for 0 function argument. >>> >>> ok. >>> >>>>> >>>>> and aligning sp in the child fails if signals are allowed there >>>>> (pthreads does not allow signals now, direct callers might). >>>>> i dont know if that's a concert (or if unaligned stack is >>>>> something we should fix up in clone3). >>>> >>>> It was overlooked on initial x86_64 clone3 implementation as well. I >>>> think it better to just return EINVAL for unaligned stacks and avoid >>>> to change the stack pointer in the created thread. >>> >>> long time ago linux did that on aarch64, but it was removed: >>> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=e6d9a52543338603e25e71e0e4942f05dae0dd8a >>> >>> i think in clone3 the kernel should have aligned (it knows >>> the bounds now), doing it in the userspace wrapper is weird >>> (should we adjust the stack size?). and not doing it at all >>> makes clone3 hard to use portably (user has to know target >>> specific pcs requirements). >>> >>> not sure what's the best way forward. >> >> I think the stack size won't matter much here, at least not from >> kernel point of view (the resulting stack size will most likely >> be page aligned anyway). But I think this kernel commit makes a good >> point that silently adjusting the stack in userland is not the >> correct approach, I think H.J has done to make it consistent with >> glibc clone implementation which does it. >> >> IMHO the best approach would to just remove the stack alignment, >> since it incurs the signal handling issue. > > current generic clone callers dont align the stack and > e.g. unaligned pthread custom stack should work. > > so we have to do arch specific stack alignment somewhere, > maybe in pthread_create? I am thinking on __clone_internal, where if an unaligned stack is used it creates a new clone_args struct with adjust arguments. It can adjust the struct in place (not sure which is better).