Messages in this thread | | | Date | Fri, 9 Nov 2018 11:46:08 -0600 | From | Josh Poimboeuf <> | Subject | Re: [RFC PATCH 1/3] static_call: Add static call infrastructure |
| |
On Fri, Nov 09, 2018 at 06:33:03PM +0100, Ard Biesheuvel wrote: > On 9 November 2018 at 18:31, Josh Poimboeuf <jpoimboe@redhat.com> wrote: > > On Fri, Nov 09, 2018 at 06:25:24PM +0100, Ard Biesheuvel wrote: > >> On 9 November 2018 at 16:14, Ard Biesheuvel <ard.biesheuvel@linaro.org> wrote: > >> > On 9 November 2018 at 16:10, Josh Poimboeuf <jpoimboe@redhat.com> wrote: > >> >> On Fri, Nov 09, 2018 at 02:39:17PM +0100, Ard Biesheuvel wrote: > >> >>> > + for (site = start; site < stop; site++) { > >> >>> > + struct static_call_key *key = static_call_key(site); > >> >>> > + unsigned long addr = static_call_addr(site); > >> >>> > + > >> >>> > + if (list_empty(&key->site_mods)) { > >> >>> > + struct static_call_mod *mod; > >> >>> > + > >> >>> > + mod = kzalloc(sizeof(*mod), GFP_KERNEL); > >> >>> > + if (!mod) { > >> >>> > + WARN(1, "Failed to allocate memory for static calls"); > >> >>> > + return; > >> >>> > + } > >> >>> > + > >> >>> > + mod->sites = site; > >> >>> > + list_add_tail(&mod->list, &key->site_mods); > >> >>> > + > >> >>> > + /* > >> >>> > + * The trampoline should no longer be used. Poison it > >> >>> > + * it with a BUG() to catch any stray callers. > >> >>> > + */ > >> >>> > + arch_static_call_poison_tramp(addr); > >> >>> > >> >>> This patches the wrong thing: the trampoline is at key->func not addr. > >> >> > >> >> If you look at the x86 implementation, it actually does poison the > >> >> trampoline. > >> >> > >> >> The address of the trampoline isn't actually known here. key->func > >> >> isn't the trampoline address; it's the destination func address. > >> >> > >> >> So instead I passed the address of the call instruction. The arch code > >> >> then reads the instruction to find the callee (the trampoline). > >> >> > >> >> The code is a bit confusing. To make it more obvious, maybe we should > >> >> add another arch function to read the call destination. Then this code > >> >> can pass that into arch_static_call_poison_tramp(). > >> >> > >> > > >> > Ah right, so I am basically missing a dereference in my > >> > arch_static_call_poison_tramp() code if this breaks. > >> > > >> > >> Could we call it 'defuse' rather than 'poision'? On arm64, we will > >> need to keep it around to bounce function calls that are out of range, > >> and replace it with a PLT sequence. > > > > Ok, but doesn't that defeat the purpose of the inline approach? > > > > It does. But this only occurs when a module is loaded far away, and > this will only happen if you have 2 GB range KASLR enabled, or your > 128 MB module region gets exhausted for some reason, so the majority > of calls should use a single relative branch.
Makes sense. Do you also account for the possibility that the original call emitted by GCC was far away and thus used the PLT?
-- Josh
| |