Messages in this thread | | | From | Ard Biesheuvel <> | Date | Fri, 9 Nov 2018 18:53:21 +0100 | Subject | Re: [RFC PATCH 1/3] static_call: Add static call infrastructure |
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On 9 November 2018 at 18:52, Ard Biesheuvel <ard.biesheuvel@linaro.org> wrote: > On 9 November 2018 at 18:46, Josh Poimboeuf <jpoimboe@redhat.com> wrote: >> 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? >> > > That doesn't really happen. vmlinux is never over 128 MB, and the > modules are only partially linked, so the linker never gets to the > stage where it needs to emit veneers. > > It's a bit fiddly since inline and out-of-line both use > arch_static_call_transform(), but what I need to do is basically: > > - for out-of-line, the trampoline needs to be patched into a > movn/movk/movk/br sequence if the target is too far > - for inline, the trampoline itself needs to be patched from > adrp/ldr/br (which does a load and a indirect branch) to > movn/movk/movk/br (which uses immediates), and the call sites need to > be patched into calls to the veneer if the target is out of range. > > So arch_static_call_transform() needs to know where the trampoline is > for this use case, so could we perhaps add a 'void *orig' in the key > struct that keeps track of the original value of 'addr' ?
... and pass it to arch_static_call_transform
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