[lld] [LLD][ELF][AArch64] Add support for SHF_AARCH64_PURECODE ELF section flag (3/3) (PR #125689)

[Csanád Hajdú via llvm-commits]

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@@ -2179,6 +2181,34 @@ template <class ELFT> void Writer<ELFT>::checkExecuteOnly() {
                             "data and code";
 }
 
+// Check that all input sections of .text have the SHF_AARCH64_PURECODE section
----------------
Il-Capitano wrote:

> The outputs from the c runtime make sense as I expect no-one has added SHF_AARCH64_PURECODE to these files. I'm surprised that these didn't get picked up by your earlier search through the .text OutputSection though.

They were there all along in my local testing, I just ignored them because it's expected that they don't have the SHF_AARCH64_PURECODE flag. I was able to get CRT files with the flag set by messing around with `llvm-objcopy`, and I could produce a final executable with PF_R stripped.


> I can think of a couple of ways to handle this:
> 
> * Add the SHF_AARCH64_PURECODE flag to the .plt on AArch64 (and Arm platforms). Add the SHF_AARCH64_PURECODE flag to the thunks that are execute-only. Can do the same on ARM.
> * Selectively filter out the synthetic sections. For example you can do that for the .plt. The thunks are a bit riskier as some may not be execute-only. The Arm thunks have some execute-only specific thunks.

Adding the purecode flag for `.plt` sections seems like the better approach to me. I'm happy to do a separate patch that addresses this for both ARM and AArch64.

As for the function call thunks, I'll add your suggestion of handling them to this patch, since it's AArch64-specific and closely tied to the new section flag.

Thank you for the helpful links!

https://github.com/llvm/llvm-project/pull/125689