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Wed, 3 Jun 2020 18:09:40 +0000 (UTC) Subject: Re: [RFC PATCH 2/2] linux-user/mmap: Fix Clang 'type-limit-compare' warning To: Richard Henderson , Eric Blake , =?UTF-8?Q?Philippe_Mathieu-Daud=c3=a9?= , qemu-devel@nongnu.org References: <20200503113220.30808-1-f4bug@amsat.org> <20200503113220.30808-3-f4bug@amsat.org> <7529763d-1f4a-5077-14ed-98753c8db288@redhat.com> <555fa273-336a-1e46-77f6-e3057dea36ca@linaro.org> From: Thomas Huth Openpgp: preference=signencrypt Message-ID: Date: Wed, 3 Jun 2020 20:09:38 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.9.0 MIME-Version: 1.0 In-Reply-To: <555fa273-336a-1e46-77f6-e3057dea36ca@linaro.org> Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=205.139.110.120; envelope-from=thuth@redhat.com; helo=us-smtp-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/06/03 01:04:35 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_PASS=-0.001 autolearn=_AUTOLEARN X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: qemu-trivial@nongnu.org, Riku Voipio , Laurent Vivier , Gerd Hoffmann Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 03/06/2020 20.01, Richard Henderson wrote: > On 6/3/20 9:06 AM, Eric Blake wrote: >> Instead of using #if, the following suffices to shut up clang: >> >> diff --git c/linux-user/mmap.c w/linux-user/mmap.c >> index e37803379747..8d9ba201625d 100644 >> --- c/linux-user/mmap.c >> +++ w/linux-user/mmap.c >> @@ -715,7 +715,7 @@ abi_long target_mremap(abi_ulong old_addr, abi_ulong old_size, >>              host_addr = MAP_FAILED; >>          } >>          /* Check if address fits target address space */ >> -        if ((unsigned long)host_addr + new_size > (abi_ulong)-1) { >> +        if ((unsigned long)host_addr > (abi_ulong)-1 - new_size) { >>              /* Revert mremap() changes */ >>              host_addr = mremap(g2h(old_addr), new_size, old_size, flags); >>              errno = ENOMEM; >> >> >> That is, it is no longer a tautological type compare if you commute the >> operations so that neither side is a compile-time constant. > > To some extent the tautological compare is a hint to the compiler that the > comparison may be optimized away. If sizeof(abi_ulong) >= sizeof(unsigned > long), then the host *cannot* produce an out-of-range target address. > > We could add the sizeof test to the if, to preserve the optimization, but that > by itself doesn't prevent the clang warning. > > Which is why I have repeatedly suggested that we disable this warning globally. I guess most people (like me) don't have a strong opinion about this. So maybe simply suggest a patch to disable the warning? I don't think that anybody will object. Thomas