[PATCH] D140035: [X86] Prevent -mibt-seal to work together with -flto=thin

[Joao Moreira via Phabricator via cfe-commits]

joaomoreira added a comment.

In D140035#3998954 <https://reviews.llvm.org/D140035#3998954>, @samitolvanen wrote:

> In D140035#3996066 <https://reviews.llvm.org/D140035#3996066>, @joaomoreira wrote:
>
>> Regarding not being able to reproduce this in kernel -- never mind... I was misled by setup issues while running IBT kernels in QEMU. I managed to fix the setup and confirm that kernel won't boot.
>
> OK, great. Thanks for double checking!
>
> The patch itself looks good to me, but I suspect ibt-seal in general has the same issue as D138337 <https://reviews.llvm.org/D138337> where it can drop endbr instructions from `isUsedInRegularObj` symbols that are not address-taken in the bitcode (e.g. functions whose address is only taken in stand-alone assembly). I saw this issue only in the arm64 Linux kernel, but there's always a chance a similar code pattern emerges on the x86 side at some point in future too. This can obviously be worked around in the kernel, but just something to keep in mind.

Ugh, yeah. I reproduced the behavior with a .s and a .C file on a similar scheme as the repro you wrote for the thin lto problem. I'll start looking for a fix once I'm back from vacation in Jan. Tks for pointing this out.


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