Messages in this thread | | | Date | Mon, 15 Mar 2021 23:09:15 +0100 | From | Peter Zijlstra <> | Subject | Re: [GIT pull] locking/urgent for v5.12-rc3 |
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On Mon, Mar 15, 2021 at 11:59:12AM -0700, Linus Torvalds wrote: > Is it only the static_call_sites entry itself that needs the > alignment? Or do we end up depending on the static call function being > at least 4-byte aligned too? The way it plays games with the key makes > me worry.
The only thing that absolutely needs to be aligned is the struct static_call_key address. We use the 2 LSB there.
The code address has no alignment requirements, due to x86 instruction coding the actual CALL (or JMP for tail-calls) can be anywhere.
Now, static_call_site is PC32 encoded, that is:
struct static_call_key *key = (void *)((unsigned long)&site->key + site->key);
And assuming &site->key is aligned, then site->key & 3 == key & 3.
Per the missing alignment for modules, the above went side-ways. The patch in question fixed this by not relying on that and always computing the absolute address first, then transfer the LSBs and then re-encoding it.
Anyway, still good to also fix the alignment.
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