[libunwind] [libunwind] Avoid reading OOB for non-existent .eh_frame_hdr (PR #68815)

[Saleem Abdulrasool via cfe-commits]

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@@ -53,8 +53,21 @@ template <typename A> class EHHeaderParser {
 
 template <typename A>
 bool EHHeaderParser<A>::decodeEHHdr(A &addressSpace, pint_t ehHdrStart,
-                                    pint_t ehHdrEnd, EHHeaderInfo &ehHdrInfo) {
+                                    size_t ehHdrSize, EHHeaderInfo &ehHdrInfo) {
   pint_t p = ehHdrStart;
+  pint_t ehHdrEnd = ehHdrStart + ehHdrSize;
+
+  // Ensure that we don't read data beyond the end of .eh_frame_hdr
+  if (ehHdrSize < 4) {
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compnerd wrote:

I wonder if we can keep the `eHdrEnd` and then just use the computation here of:

```c++
if (eHdrEnd - eHdrStart < 4) {
```

I think that giving the region is kinda nice and we need that value anyway.

https://github.com/llvm/llvm-project/pull/68815