On 5/21/21 12:18 PM, Mark Brown wrote: > On Sat, May 15, 2021 at 11:00:16PM -0500, madvenka@xxxxxxxxxxxxxxxxxxx wrote: > >> Special cases >> ============= >> >> Some special cases need to be mentioned: > > I think it'd be good if more of this cover letter, especially sections > like this which cover the tricky bits, ended up in the code somehow - > it's recorded here and will be in the list archive but that's not the > most discoverable place so increases the maintainance burden. It'd be > great to be able to compare the code directly with the reliable > stacktrace requirements document and see everything getting ticked off, > actually going all the way there might be too much and loose the code in > the comments but I think we can get closer to it than we are. Given > that a lot of this stuff rests on the denylist perhaps some comments > just before it's called would be a good place to start? > I will add more comments in the code to make it clear. >> - EL1 interrupt and exception handlers end up in sym_code_ranges[]. >> So, all EL1 interrupt and exception stack traces will be considered >> unreliable. This the correct behavior as interrupts and exceptions > > This stuff about exceptions and preemption is a big one, rejecting any > exceptions makes a whole host of things easier (eg, Mark Rutland raised > interactions between non-AAPCS code and PLTs as being an issue but if > we're able to reliably reject stacks featuring any kind of preemption > anyway that should sidestep the issue). > Yes. I will include this in the code comments. >> Performance >> =========== > >> Currently, unwinder_blacklisted() does a linear search through >> sym_code_functions[]. If reviewers prefer, I could sort the >> sym_code_functions[] array and perform a binary search for better >> performance. There are about 80 entries in the array. > > If people are trying to live patch a very busy/big system then this > could be an issue, equally there's probably more people focused on > getting boot times as fast as possible than live patching. Deferring > the initialisation to first use would help boot times with or without > sorting, without numbers I don't actually know that sorting is worth the > effort or needs doing immediately - obvious correctness is also a > benefit! My instinct is that for now it's probably OK leaving it as a > linear scan and then revisiting if it's not adequately performant, but > I'd defer to actual users there. I have followed the example in the Kprobe deny list. I place the section in initdata so it can be unloaded during boot. This means that I need to copy the information before that in early_initcall(). If the initialization must be performed on first use, I probably have to move SYM_CODE_FUNCTIONS from initdata to some other place where it will be retained. If you prefer this, I could do it this way. Thanks! Madhavan
