[llvm] [BOLT] Fix build-time assertion in RewriteInstance (PR #90540)

[Maksim Panchenko via llvm-commits]

https://github.com/maksfb created https://github.com/llvm/llvm-project/pull/90540

We use pwrite() in RewriteInstance to update contents of existing sections. pwrite() requires file position to be set past the written offset which we guarantee at the start of rewriteFile(). Then we had an implicit assumption in patchBuildID() that the file position will be set again in patchELFSymTabs() after being reset in patchELFPHDRTable(). That assumption was broken in #90300. The fix is to save and restore file position in patchELFPHDRTable(). Then we don't have to update it again in patchELFSymTabs().

>From 7aad7f859936f69d4db92812f893c61c4ab46dcc Mon Sep 17 00:00:00 2001
From: Maksim Panchenko <maks at fb.com>
Date: Mon, 29 Apr 2024 18:00:54 -0700
Subject: [PATCH] [BOLT] Fix build-time assertion in RewriteInstance

We use pwrite() in RewriteInstance to update contents of existing
sections. pwrite() requires file position to be set past the written
offset which we guarantee at the start of rewriteFile(). Then we had an
implicit assumption in patchBuildID() that the file position will be set
again in patchELFSymTabs() after being reset in patchELFPHDRTable().
That assumption was broken in #90300. The fix is to save and restore
file position in patchELFPHDRTable(). Then we don't have to update it
again in patchELFSymTabs().
---
 bolt/lib/Rewrite/RewriteInstance.cpp | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/bolt/lib/Rewrite/RewriteInstance.cpp b/bolt/lib/Rewrite/RewriteInstance.cpp
index 644d87eeca42e6..23f79e3c135a78 100644
--- a/bolt/lib/Rewrite/RewriteInstance.cpp
+++ b/bolt/lib/Rewrite/RewriteInstance.cpp
@@ -4047,6 +4047,7 @@ void RewriteInstance::patchELFPHDRTable() {
     NewWritableSegmentSize = NextAvailableAddress - NewWritableSegmentAddress;
   }
 
+  const uint64_t SavedPos = OS.tell();
   OS.seek(PHDRTableOffset);
 
   auto createNewTextPhdr = [&]() {
@@ -4151,6 +4152,8 @@ void RewriteInstance::patchELFPHDRTable() {
         << "BOLT-ERROR: could not find PT_GNU_STACK program header to modify\n";
     exit(1);
   }
+
+  OS.seek(SavedPos);
 }
 
 namespace {
@@ -5041,10 +5044,6 @@ void RewriteInstance::patchELFSymTabs(ELFObjectFile<ELFT> *File) {
   assert((DynSymSection || BC->IsStaticExecutable) &&
          "dynamic symbol table expected");
   if (DynSymSection) {
-    // Set pointer to the end of the section, so we can use pwrite to update
-    // the dynamic symbol table.
-    Out->os().seek(DynSymSection->sh_offset + DynSymSection->sh_size);
-
     updateELFSymbolTable(
         File,
         /*IsDynSym=*/true,