On 9 November 2018 at 18:31, Josh Poimboeuf <[email protected]> wrote: > On Fri, Nov 09, 2018 at 06:25:24PM +0100, Ard Biesheuvel wrote: >> On 9 November 2018 at 16:14, Ard Biesheuvel <[email protected]> >> wrote: >> > On 9 November 2018 at 16:10, Josh Poimboeuf <[email protected]> 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.
