On Tue, Mar 11, 2025 at 11:45:21AM +0000, Catalin Marinas wrote: > On Mon, Mar 10, 2025 at 07:37:32PM +0000, Mark Rutland wrote: > > I was worried that ex_handler_load_unaligned_zeropad() might not do the > > right thing in response to a tag check fault (e.g. access the wrong 8 > > bytes), but it looks as though that's ok due to the way it generates the > > offset and the aligned pointer. > > > > If load_unaligned_zeropad() is handed a string that starts with an > > unexpected tag (and even if that starts off aligned), > > ex_handler_load_unaligned_zeropad() will access that and cause another > > tag check fault, which will be reported. > > Yes, it will report an async tag check fault on the > exit_to_kernel_mode() path _if_ load_unaligned_zeropad() triggered the > fault for other reasons (end of page). Sorry, yes. The aligned case I mentioned shouldn't apply here. > It's slightly inconsistent, we could set TCO for the async case in > ex_handler_load_unaligned_zeropad() as well. Yep, I think that'd be necessary for async mode. > For sync checks, we'd get the first fault ending up in > ex_handler_load_unaligned_zeropad() and a second tag check fault while > processing the first. This ends up in do_tag_recovery and we disable > tag checking after the report. Not ideal but not that bad. Yep; that's what I was describing in the second paragraph above, though I forgot to say that was assuming sync or asymm mode. > We could adjust ex_handler_load_unaligned_zeropad() to return false if > the pointer is already aligned but we need to check the semantics of > load_unaligned_zeropad(), is it allowed to fault on the first byte? IIUC today it's only expected to fault due to misalignment, and the gneral expectation is that for a sequence of load_unaligned_zeropad() calls, we should get at least one byte without faulting (for the NUL terminator). I reckon it'd be better to figure this out based on the ESR if possible. Kristina's patches for MOPS would give us that. Mark.
