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Message-ID: <20240414021555.GQ2118490@ZenIV> References: <878r1ibpdn.fsf@all.your.base.are.belong.to.us> <20240413164318.7260c5ef@namcao> <22E65CA5-A2C0-44A3-AB01-7514916A18FC@dilger.ca> Precedence: bulk X-Mailing-List: linux-ext4@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <22E65CA5-A2C0-44A3-AB01-7514916A18FC@dilger.ca> Sender: Al Viro On Sat, Apr 13, 2024 at 07:46:03PM -0600, Andreas Dilger wrote: > As to whether the 0xfffff000 address itself is valid for riscv32 is > outside my realm, but given that RAM is cheap it doesn't seem unlikely > to have 4GB+ of RAM and want to use it all. The riscv32 might consider > reserving this page address from allocation to avoid similar issues in > other parts of the code, as is done with the NULL/0 page address. Not a chance. *Any* page mapped there is a serious bug on any 32bit box. Recall what ERR_PTR() is... On any architecture the virtual addresses in range (unsigned long)-512.. (unsigned long)-1 must never resolve to valid kernel objects. In other words, any kind of wraparound here is asking for an oops on attempts to access the elements of buffer - kernel dereference of (char *)0xfffff000 on a 32bit box is already a bug. It might be getting an invalid pointer, but arithmetical overflows are irrelevant.