[lld] [LLD][RISCV] Report error for unsatisfiable RISCV_ALIGN (PR #74121)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-lld-elf

Author: Philip Reames (preames)

<details>
<summary>Changes</summary>

If we have a RISCV_ALIGN relocation which can't be satisfied with the available space provided, report an error rather than silently continuing with a corrupt state.

For context, https://github.com/llvm/llvm-project/pull/73977 fixes an LLD bug which can cause this effect, but that's not the only source of such cases.

Another is our hard-to-fix set of LTO problems.  We can have a single function which was compiled without C in an otherwise entirely C module.  Until we have all of the mapping symbols and related mechanisms implemented, this case can continue to arise.

I think it's very important from a user interface perspective to have non-assertion builds report an error in this case. If we don't report an error here, we can crash the linker (due to the fatal error at the bottom of the function), or if we're less lucky silently produce a malformed binary.

There's a couple of known defects with this patch.

First, there's no test case.  I don't know how to write a stable test case for this that doesn't involve hex editing an object file, or abusing the LTO bug that we hope to fix.

Second, this will report an error on each relax iteration.  I explored trying to report an error only once after relaxation, but ended up deciding I didn't have the context to implement it safely.

I would be thrilled if someone more knowledgeable of this code wants to write a better version of this patch, but in the meantime, I believe we should land this to address the user experience problem described above.

---
Full diff: https://github.com/llvm/llvm-project/pull/74121.diff


1 Files Affected:

- (modified) lld/ELF/Arch/RISCV.cpp (+8-2) 


``````````diff
diff --git a/lld/ELF/Arch/RISCV.cpp b/lld/ELF/Arch/RISCV.cpp
index a556d89c36400d3..435600186143098 100644
--- a/lld/ELF/Arch/RISCV.cpp
+++ b/lld/ELF/Arch/RISCV.cpp
@@ -686,8 +686,14 @@ static bool relax(InputSection &sec) {
       const uint64_t align = PowerOf2Ceil(r.addend + 2);
       // All bytes beyond the alignment boundary should be removed.
       remove = nextLoc - ((loc + align - 1) & -align);
-      assert(static_cast<int32_t>(remove) >= 0 &&
-             "R_RISCV_ALIGN needs expanding the content");
+      // If we can't satisfy this alignment, we've found a bad input.
+      if (LLVM_UNLIKELY(static_cast<int32_t>(remove) < 0)) {
+        error(getErrorLocation((const uint8_t*)loc) +
+              "insufficient padding bytes for " + lld::toString(r.type) +
+              ": " + Twine(r.addend) + " bytes available "
+              + "for requested alignment of " + Twine(align) + " bytes");
+        remove = 0;
+      }
       break;
     }
     case R_RISCV_CALL:

``````````

</details>


https://github.com/llvm/llvm-project/pull/74121