Messages in this thread | | | Date | Wed, 20 Oct 2021 12:43:52 -0700 | From | Josh Poimboeuf <> | Subject | Re: [PATCH v2 08/14] x86/retpoline: Create a retpoline thunk array |
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On Wed, Oct 20, 2021 at 09:22:29PM +0200, Peter Zijlstra wrote: > On Wed, Oct 20, 2021 at 10:09:56AM -0700, Josh Poimboeuf wrote: > > On Wed, Oct 20, 2021 at 05:46:39PM +0100, Andrew Cooper wrote: > > > On 20/10/2021 16:57, Josh Poimboeuf wrote: > > > > On Wed, Oct 20, 2021 at 12:44:50PM +0200, Peter Zijlstra wrote: > > > >> Stick all the retpolines in a single symbol and have the individual > > > >> thunks as inner labels, this should guarantee thunk order and layout. > > > > How so? > > > > > > > > Just wondering what the purpose of the array is. It doesn't seem to be > > > > referenced anywhere. > > > > > > The array property is what makes: > > > > > > > + reg = (target - &__x86_indirect_thunk_rax) / > > > > + (&__x86_indirect_thunk_rcx - &__x86_indirect_thunk_rax); > > > > > > safe in the next path. > > > > The thunks were already 32-byte aligned. I don't see how slapping a few > > unused symbols around them does anything. > > Previously there were 16 (or rather 15 without rsp) separate symbols and > a toolchain might reasonably expect it could displace them however it > liked, with disregard for the relative position. > > However, now they're part of a larger symbol. Any change to their > relative position would disrupt this larger _array symbol and thus not > be sound. > > This is I think the same reasoning used for data symbols. On their own > there is no guarantee about their relative position wrt to one aonther, > but we're still able to do arrays because an array as a whole is a > single larger symbol.
Makes sense, I think (and good fodder for the commit log).
-- Josh
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