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Mon, 2 Sep 2024 07:04:52 +0000 Received: from MN0PR11MB6304.namprd11.prod.outlook.com ([fe80::7f88:f3b1:22ec:f508]) by MN0PR11MB6304.namprd11.prod.outlook.com ([fe80::7f88:f3b1:22ec:f508%6]) with mapi id 15.20.7918.024; Mon, 2 Sep 2024 07:04:51 +0000 Date: Mon, 2 Sep 2024 15:04:37 +0800 From: Feng Tang To: Vlastimil Babka CC: Danilo Krummrich , "cl@linux.com" , "penberg@kernel.org" , "rientjes@google.com" , "iamjoonsoo.kim@lge.com" , "akpm@linux-foundation.org" , "roman.gushchin@linux.dev" , "42.hyeyoo@gmail.com" <42.hyeyoo@gmail.com>, "urezki@gmail.com" , "hch@infradead.org" , "kees@kernel.org" , "ojeda@kernel.org" , "wedsonaf@gmail.com" , "mhocko@kernel.org" , "mpe@ellerman.id.au" , "chandan.babu@oracle.com" , "christian.koenig@amd.com" , "maz@kernel.org" , "oliver.upton@linux.dev" , "linux-kernel@vger.kernel.org" , "linux-mm@kvack.org" , "rust-for-linux@vger.kernel.org" , kasan-dev Subject: Re: [PATCH v2 1/2] mm: vmalloc: implement vrealloc() Message-ID: References: <20240722163111.4766-1-dakr@kernel.org> <20240722163111.4766-2-dakr@kernel.org> <07491799-9753-4fc9-b642-6d7d7d9575aa@suse.cz> <44fa564b-9c8f-4ac2-bce3-f6d2c99b73b7@suse.cz> Content-Type: text/plain; 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Since commit 946fa0dbf2d89 ("mm/slub: extend redzone check to > > extra allocated kmalloc space than requested") and preceding commits, if > > slub_debug is enabled (red zoning or user tracking), only the 56 bytes > > will be zeroed. The rest will be either unknown garbage, or redzone. > > Yes. > > > > > Then we might e.g. krealloc(120) and get a kmalloc-128 object and 64 > > bytes (result of ksize()) will be copied, including the garbage/redzone. > > I think it's fixable because when we do this in slub_debug, we also > > store the original size in the metadata, so we could read it back and > > adjust how many bytes are copied. > > krealloc() --> __do_krealloc() --> ksize() > When ksize() is called, as we don't know what user will do with the > extra space ([57, 64] here), the orig_size check will be unset by > __ksize() calling skip_orig_size_check(). > > And if the newsize is bigger than the old 'ksize', the 'orig_size' > will be correctly set for the newly allocated kmalloc object. > > For the 'unstable' branch of -mm tree, which has all latest patches > from Danilo, I run some basic test and it seems to be fine. when doing more test, I found one case matching Vlastimil's previous concern, that if we kzalloc a small object, and then krealloc with a slightly bigger size which can still reuse the kmalloc object, some redzone will be preserved. With test code like: buf = kzalloc(36, GFP_KERNEL); memset(buf, 0xff, 36); buf = krealloc(buf, 48, GFP_KERNEL | __GFP_ZERO); Data after kzalloc+memset : ffff88802189b040: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ffff88802189b050: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ffff88802189b060: ff ff ff ff cc cc cc cc cc cc cc cc cc cc cc cc ffff88802189b070: cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc Data after krealloc: ffff88802189b040: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ffff88802189b050: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ffff88802189b060: ff ff ff ff cc cc cc cc cc cc cc cc cc cc cc cc ffff88802189b070: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 If we really want to make [37, 48] to be zeroed too, we can lift the get_orig_size() from slub.c to slab_common.c and use it as the start of zeroing in krealloc(). Thanks, Feng