issue with null sections

[ohnekopf]

the bug

assertion at the bottom of rewriteSectionsLibrary tripped when theres a null section at position 1 on the sht.

patchelf: patchelf.cc:961: void ElfFile<Elf_Ehdr, Elf_Phdr, Elf_Shdr, Elf_Addr, Elf_Off, Elf_Dyn, Elf_Sym, Elf_Versym, Elf_Verdef, Elf_Verdaux, Elf_Verneed, Elf_Vernaux, Elf_Rel, Elf_Rela, ElfClass>::rewriteSectionsLibrary() [with Elf_Ehdr = Elf64_Ehdr; Elf_Phdr = Elf64_Phdr; Elf_Shdr = Elf64_Shdr; Elf_Addr = long unsigned int; Elf_Off = long unsigned int; Elf_Dyn = Elf64_Dyn; Elf_Sym = Elf64_Sym; Elf_Versym = short unsigned int; Elf_Verdef = Elf64_Verdef; Elf_Verdaux = Elf64_Verdaux; Elf_Verneed = Elf64_Verneed; Elf_Vernaux = Elf64_Vernaux; Elf_Rel = Elf64_Rel; Elf_Rela = Elf64_Rela; unsigned int ElfClass = 64]: Assertion `curOff == startOffset + neededSpace' failed.
Aborted

Steps To Reproduce

1. take an elf executable (but with ET_DYN, havent tried other cases) that has a SHT_NULL entry (apart from the mandatory one at the top of the sht). i created one by appending 64 null bytes to a random gcc executable, because the section header table was at the end i could just add 1 to the counter on the elf header to get one.

2. run patchelf --add-needed libX.so.Y on it, at least twice. the first time wont trip the assert if the null entry is not at position 1.

Expected behavior

no errors

patchelf --version output

current git master, also patchelf 0.18.0 from debian repo

running the program under a debugger shows that the problem comes from calls to replaceSection with a null section as the target, i dont understand quite well how that ends messing up the offset calculation, but adding a guard against it seems to help. also, using the --no-sort option prevents this.

this is my workaround, but im not sure if it makes sense, also its possible some non SHT_NULL sections might have an empty name?

diff --git a/src/patchelf.cc b/src/patchelf.cc
index 73f1b8d..693ac51 100644
--- a/src/patchelf.cc
+++ b/src/patchelf.cc
@@ -866,7 +866,7 @@ void ElfFile<ElfFileParamNames>::rewriteSectionsLibrary()
             const auto & sectionName = getSectionName(shdrs.at(i));
             const auto sectionSize = rdi(shdrs.at(i).sh_size);
 
-            if (!hasReplacedSection(sectionName)) {
+            if (sectionName != "" && !hasReplacedSection(sectionName) ) {
                 replaceSection(sectionName, sectionSize);
             }
 
@@ -1015,7 +1015,7 @@ void ElfFile<ElfFileParamNames>::rewriteSectionsExecutable()
             lastReplaced = i - 1;
             break;
         }
-        if (!replacedSections.count(sectionName)) {
+        if (sectionName != "" && !replacedSections.count(sectionName)) {
             debug("replacing section '%s' which is in the way\n", sectionName.c_str());
             replaceSection(sectionName, rdi(shdr.sh_size));
         }

another workaround is striping the executable which removes all null sections