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

[Peter Smith via llvm-commits]

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In-Reply-To: <llvm.org/llvm/llvm-project/pull/125689 at github.com>


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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
----------------
smithp35 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.

The `.plt` is a synthetic linker generated section that reads data from the `.plt.got` section (which should be in read-write data). Another example of linker generated executable sections are the long-branch extension thunks. Of these the position independent ADRP thunk is execute only, but the absolute non-pic one is not.

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.

We may have to add an XO variant of the AARCH64ABSLongThunk https://github.com/llvm/llvm-project/blob/main/lld/ELF/Thunks.cpp#L637 that uses MOVZ/MOVK to synthesize the immediate. Or just use the ADRP thunk when XO is being used.

There's a precedent for this with v6-m which adds a somewhat horrific XO thunk (v6-m lacks MOVT and MOVW) https://github.com/llvm/llvm-project/blob/main/lld/ELF/Thunks.cpp#L937 

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