Move section headers if they would be overwritten

When rewriting the sections of an executable, it can happen that the
section headers occur too early in the file and would be overwritten by
the replaced sections. If this would happen, we move the section headers
to the end of the file.

Author: ttuegel

src/patchelf.cc

@@ -711,11 +711,22 @@ void ElfFile<ElfFileParamNames>::rewriteSectionsExecutable()
     Elf_Addr firstPage = startAddr - startOffset;
     debug("first page is 0x%llx\n", (unsigned long long) firstPage);
 
-    /* Right now we assume that the section headers are somewhere near
-       the end, which appears to be the case most of the time.
-       Therefore they're not accidentally overwritten by the replaced
-       sections. !!!  Fix this. */
-    assert((off_t) rdi(hdr->e_shoff) >= startOffset);
+    if ((off_t) rdi(hdr->e_shoff) < startOffset) {
+        /* The section headers occur too early in the file and would be
+           overwritten by the replaced sections. Move them to the end of the file
+           before proceeding. */
+        off_t shoffNew = fileSize;
+        off_t shSize = rdi(hdr->e_shoff) + rdi(hdr->e_shnum) * rdi(hdr->e_shentsize);
+        growFile (fileSize + shSize);
+        wri(hdr->e_shoff, shoffNew);
+
+        /* Rewrite the section header table.  For neatness, keep the
+           sections sorted. */
+        assert(rdi(hdr->e_shnum) == shdrs.size());
+        sortShdrs();
+        for (unsigned int i = 1; i < rdi(hdr->e_shnum); ++i)
+            * ((Elf_Shdr *) (contents + rdi(hdr->e_shoff)) + i) = shdrs[i];
+    }
 
 
     /* Compute the total space needed for the replaced sections, the