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X-OriginatorOrg: arm.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 18 Aug 2025 19:54:34.2749 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: d4cd4d1c-401f-4578-a0b1-08ddde910e09 X-MS-Exchange-CrossTenant-Id: f34e5979-57d9-4aaa-ad4d-b122a662184d X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=f34e5979-57d9-4aaa-ad4d-b122a662184d;Ip=[4.158.2.129];Helo=[outbound-uk1.az.dlp.m.darktrace.com] X-MS-Exchange-CrossTenant-AuthSource: AMS0EPF000001A8.eurprd05.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: VI1PR08MB5327 Hi Catalin, > On Sat, Aug 16, 2025 at 03:57:49PM +0100, Yeoreum Yun wrote: > > > > > +#ifdef CONFIG_AS_HAS_LSUI > > > > > + > > > > > +#define __LSUI_PREAMBLE ".arch_extension lsui\n" > > > > > + > > > > > +#define LSUI_FUTEX_ATOMIC_OP(op, asm_op, mb) \ > > > > > +static __always_inline int \ > > > > > +__lsui_futex_atomic_##op(int oparg, u32 __user *uaddr, int *oval) \ > > > > > +{ \ > > > > > + int ret = 0; \ > > > > > + int val; \ > > > > > + \ > > > > > + mte_enable_tco(); \ > > > > > > > > > > > The reason uaccess_disable_privileged() sets the MTE TCO (tag check > > > > override) is because the user and the kernel may have different settings > > > > for tag checking. If we use the user instructions provided by FEAT_LSUI, > > > > we leave the MTE checking as is. > > > > > > > > The same comment for all the other functions here. > > > > > > You're right. Thanks for catching this :) > > > > But one bikeshedding question. > > why we need to care about the different settings for tag checking when > > we use uaccess_disable_privileged()? > > Because, for example, the user may not be interested in any tag check > faults (has checking disabled) but the kernel uses KASAN with > synchronous tag check faults. If it uses the privileged instructions as > in the futex API, it either won't make progress or report errors to the > user which it does not expect. > > > IIUC, the reason we uses to uaccess_disaable_privileged() to access > > user memory with copy_from/to_user() and etc. > > We don't use uaccess_disable_privileged() with copy_from_user() since > those use the unprivileged instructions already. Thanks for your explaination :) > > > But, although tag check fault happens in kernel side, > > It seems to be handled by fixup code if user address is wrong. > > The user may know it is wrong and not care (e.g. one wants to keep using > a buggy application). Then Does this example -- ignoring wrong and keep using a buggy application shows us that we need to enable TCO when we runs the LSUI instruction? AFAIK, LSUI instruction also check memory tag -- i.e) ldtadd. if passed user address which has unmatched tag and if user isn't interested in tah check, It can meet the unexpected report from KASAN. Am I missing something? -- Sincerely, Yeoreum Yun