On Mon, Nov 22, 2021 at 06:46:44PM +0100, Petr Mladek wrote: > On Thu 2021-11-11 17:50:03, Josh Poimboeuf wrote: > > On Wed, Nov 10, 2021 at 12:20:47PM +0000, David Laight wrote: > > > > > Wouldn't moving part of a function to .text.cold (or .text.unlikely) > > > > > generate the same problems with the stack backtrace code as the > > > > > .text.fixup section you are removing had?? > > > > > > > > GCC can already split a function into func and func.cold today (or > > > > worse: func, func.isra.N, func.cold, func.isra.N.cold etc..). > > > > > > > > I'm assuming reliable unwind and livepatch know how to deal with this. > > > > > > They'll have 'proper' function labels at the top - so backtrace > > > stands a chance. > > > Indeed you (probably) want it to output "func.irsa.n.cold" rather > > > than just "func" to help show which copy it is in. > > > > I guess that livepatch will need separate patches for each > > > version of the function - which might be 'interesting' if > > > all the copies actually need patching at the same time. > > > You'd certainly want a warning if there seemed to be multiple > > > copies of the function. > > > > Hm, I think there is actually a livepatch problem here. > > > > If the .cold (aka "child") function actually had a fentry hook then we'd > > be fine. Then we could just patch both "parent" and "child" functions > > at the same time. We already have the ability to patch multiple > > functions having dependent interface changes. > > > > But there's no fentry hook in the child, so we can only patch the > > parent. > > > > If the child schedules out, and then the parent gets patched, things can > > go off-script if the child later jumps back to the unpatched version of > > the parent, and then for example the old parent tries to call another > > patched function with a since-changed ABI. > > This thread seems to be motivation for the patchset > https://lore.kernel.org/all/20211119090327.12811-1-mbenes@xxxxxxx/ > I am trying to understand the problem here, first. And I am > a bit lost. > > How exactly is child called in the above scenario, please? > How could parent get livepatched when child is sleeping? > > I imagine it the following way: > > parent_func() > fentry > > /* some parent code */ > jmp child > /* child code */ > jmp back_to_parent > /* more parent code */ > ret Right. > In the above example, parent_func() would be on stack and could not > get livepatched even when the process is sleeping in the child code. > > The livepatching is done via ftrace. Only code with fentry could be > livepatched. And code called via fentry must be visible on stack. How would parent_func() be on the stack? If it jumps to the child then it leaves no trace on the stack. -- Josh
