[llvm] [llvm][yaml2obj] Modify section header overriding timing (PR #130942)

[Ruoyu Qiu via llvm-commits]

https://github.com/cabbaken created https://github.com/llvm/llvm-project/pull/130942

yaml2obj should determine the program header offset (and other properties) based on the intended values rather than the final sh_offset of the section header.

This change adjusts the timing of when the section header is overridden to ensure that the program headers are set correctly.

More details [here](https://github.com/llvm/llvm-project/pull/126537#issuecomment-2700421989).

>From 701efbe96d8d2bdad6df3d91e2c1bcd6adfc2f8c Mon Sep 17 00:00:00 2001
From: Ruoyu Qiu <cabbaken at outlook.com>
Date: Wed, 12 Mar 2025 18:37:50 +0800
Subject: [PATCH] [llvm][yaml2obj] Modify section header overriding timing

Signed-off-by: Ruoyu Qiu <cabbaken at outlook.com>
---
 llvm/lib/ObjectYAML/ELFEmitter.cpp | 20 ++++++++++++++++----
 1 file changed, 16 insertions(+), 4 deletions(-)

diff --git a/llvm/lib/ObjectYAML/ELFEmitter.cpp b/llvm/lib/ObjectYAML/ELFEmitter.cpp
index 9ae76a71ede5e..48d5d58beea86 100644
--- a/llvm/lib/ObjectYAML/ELFEmitter.cpp
+++ b/llvm/lib/ObjectYAML/ELFEmitter.cpp
@@ -206,6 +206,9 @@ template <class ELFT> class ELFState {
   NameToIdxMap DynSymN2I;
   ELFYAML::Object &Doc;
 
+  std::vector<std::pair<unsigned, ELFYAML::Section>>
+      SectionHeadersOverrideHelper;
+
   StringSet<> ExcludedSectionHeaders;
 
   uint64_t LocationCounter = 0;
@@ -226,6 +229,7 @@ template <class ELFT> class ELFState {
                           StringRef SecName, ELFYAML::Section *YAMLSec);
   void initSectionHeaders(std::vector<Elf_Shdr> &SHeaders,
                           ContiguousBlobAccumulator &CBA);
+  void overrideSectionHeaders(std::vector<Elf_Shdr> &SHeaders);
   void initSymtabSectionHeader(Elf_Shdr &SHeader, SymtabType STType,
                                ContiguousBlobAccumulator &CBA,
                                ELFYAML::Section *YAMLSec);
@@ -845,7 +849,7 @@ void ELFState<ELFT>::initSectionHeaders(std::vector<Elf_Shdr> &SHeaders,
       }
 
       LocationCounter += SHeader.sh_size;
-      overrideFields<ELFT>(Sec, SHeader);
+      SectionHeadersOverrideHelper.push_back({SN2I.get(Sec->Name), *Sec});
       continue;
     }
 
@@ -899,12 +903,17 @@ void ELFState<ELFT>::initSectionHeaders(std::vector<Elf_Shdr> &SHeaders,
     }
 
     LocationCounter += SHeader.sh_size;
-
-    // Override section fields if requested.
-    overrideFields<ELFT>(Sec, SHeader);
+    SectionHeadersOverrideHelper.push_back({SN2I.get(Sec->Name), *Sec});
   }
 }
 
+template <class ELFT>
+void ELFState<ELFT>::overrideSectionHeaders(std::vector<Elf_Shdr> &SHeaders) {
+  for (std::pair<unsigned, ELFYAML::Section> &IndexAndSec :
+       SectionHeadersOverrideHelper)
+    overrideFields<ELFT>(&IndexAndSec.second, SHeaders[IndexAndSec.first]);
+}
+
 template <class ELFT>
 void ELFState<ELFT>::assignSectionAddress(Elf_Shdr &SHeader,
                                           ELFYAML::Section *YAMLSec) {
@@ -2090,6 +2099,9 @@ bool ELFState<ELFT>::writeELF(raw_ostream &OS, ELFYAML::Object &Doc,
   // Now we can decide segment offsets.
   State.setProgramHeaderLayout(PHeaders, SHeaders);
 
+  // Override section fields if requested.
+  State.overrideSectionHeaders(SHeaders);
+
   bool ReachedLimit = CBA.getOffset() > MaxSize;
   if (Error E = CBA.takeLimitError()) {
     // We report a custom error message instead below.