[llvm] [BOLT] Refactor NewTextSegmentAddress handling (PR #145950)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-bolt

Author: Maksim Panchenko (maksfb)

<details>
<summary>Changes</summary>

Refactor the code for NewTextSegmentAddress to correctly point at the true start of the segment when PHDR table is placed at the beginning. We used to offset NewTextSegmentAddress by PHDR table plus cache line alignment.

NFC for proper binaries. Some YAML binaries from our tests will diverge due to bad segment address/offset alignment.

---
Full diff: https://github.com/llvm/llvm-project/pull/145950.diff


1 Files Affected:

- (modified) bolt/lib/Rewrite/RewriteInstance.cpp (+15-25) 


``````````diff
diff --git a/bolt/lib/Rewrite/RewriteInstance.cpp b/bolt/lib/Rewrite/RewriteInstance.cpp
index 93bd93b6cb984..dc7591a1d1226 100644
--- a/bolt/lib/Rewrite/RewriteInstance.cpp
+++ b/bolt/lib/Rewrite/RewriteInstance.cpp
@@ -626,6 +626,9 @@ Error RewriteInstance::discoverStorage() {
     NextAvailableAddress += BC->PageAlign;
   }
 
+  NewTextSegmentAddress = NextAvailableAddress;
+  NewTextSegmentOffset = NextAvailableOffset;
+
   if (!opts::UseGnuStack && !BC->IsLinuxKernel) {
     // This is where the black magic happens. Creating PHDR table in a segment
     // other than that containing ELF header is tricky. Some loaders and/or
@@ -652,6 +655,8 @@ Error RewriteInstance::discoverStorage() {
 
     PHDRTableAddress = NextAvailableAddress;
     PHDRTableOffset = NextAvailableOffset;
+    NewTextSegmentAddress = NextAvailableAddress;
+    NewTextSegmentOffset = NextAvailableOffset;
 
     // Reserve space for 3 extra pheaders.
     unsigned Phnum = Obj.getHeader().e_phnum;
@@ -664,14 +669,12 @@ Error RewriteInstance::discoverStorage() {
 
     NextAvailableAddress += Phnum * sizeof(ELF64LEPhdrTy);
     NextAvailableOffset += Phnum * sizeof(ELF64LEPhdrTy);
-  }
 
-  // Align at cache line.
-  NextAvailableAddress = alignTo(NextAvailableAddress, 64);
-  NextAvailableOffset = alignTo(NextAvailableOffset, 64);
+    // Align at cache line.
+    NextAvailableAddress = alignTo(NextAvailableAddress, 64);
+    NextAvailableOffset = alignTo(NextAvailableOffset, 64);
+  }
 
-  NewTextSegmentAddress = NextAvailableAddress;
-  NewTextSegmentOffset = NextAvailableOffset;
   BC->LayoutStartAddress = NextAvailableAddress;
 
   // Tools such as objcopy can strip section contents but leave header
@@ -4133,13 +4136,8 @@ void RewriteInstance::mapAllocatableSections(
     }
 
     if (SType == ST_READONLY) {
-      if (PHDRTableAddress) {
-        // Segment size includes the size of the PHDR area.
-        NewTextSegmentSize = NextAvailableAddress - PHDRTableAddress;
-      } else if (NewTextSegmentAddress) {
-        // Existing PHDR table would be updated.
+      if (NewTextSegmentAddress)
         NewTextSegmentSize = NextAvailableAddress - NewTextSegmentAddress;
-      }
     } else if (SType == ST_READWRITE) {
       NewWritableSegmentSize = NextAvailableAddress - NewWritableSegmentAddress;
       // Restore NextAvailableAddress if no new writable sections
@@ -4186,9 +4184,7 @@ void RewriteInstance::patchELFPHDRTable() {
   // NOTE Currently .eh_frame_hdr appends to the last segment, recalculate
   // last segments size based on the NextAvailableAddress variable.
   if (!NewWritableSegmentSize) {
-    if (PHDRTableAddress)
-      NewTextSegmentSize = NextAvailableAddress - PHDRTableAddress;
-    else if (NewTextSegmentAddress)
+    if (NewTextSegmentAddress)
       NewTextSegmentSize = NextAvailableAddress - NewTextSegmentAddress;
   } else {
     NewWritableSegmentSize = NextAvailableAddress - NewWritableSegmentAddress;
@@ -4201,15 +4197,9 @@ void RewriteInstance::patchELFPHDRTable() {
     SmallVector<ELF64LEPhdrTy, 3> NewPhdrs;
     ELF64LEPhdrTy NewPhdr;
     NewPhdr.p_type = ELF::PT_LOAD;
-    if (PHDRTableAddress) {
-      NewPhdr.p_offset = PHDRTableOffset;
-      NewPhdr.p_vaddr = PHDRTableAddress;
-      NewPhdr.p_paddr = PHDRTableAddress;
-    } else {
-      NewPhdr.p_offset = NewTextSegmentOffset;
-      NewPhdr.p_vaddr = NewTextSegmentAddress;
-      NewPhdr.p_paddr = NewTextSegmentAddress;
-    }
+    NewPhdr.p_offset = NewTextSegmentOffset;
+    NewPhdr.p_vaddr = NewTextSegmentAddress;
+    NewPhdr.p_paddr = NewTextSegmentAddress;
     NewPhdr.p_filesz = NewTextSegmentSize;
     NewPhdr.p_memsz = NewTextSegmentSize;
     NewPhdr.p_flags = ELF::PF_X | ELF::PF_R;
@@ -4270,7 +4260,7 @@ void RewriteInstance::patchELFPHDRTable() {
   };
 
   auto writeNewSegmentPhdrs = [&]() {
-    if (PHDRTableAddress || NewTextSegmentSize) {
+    if (NewTextSegmentSize) {
       SmallVector<ELF64LE::Phdr, 3> NewPhdrs = createNewPhdrs();
       OS.write(reinterpret_cast<const char *>(NewPhdrs.data()),
                sizeof(ELF64LE::Phdr) * NewPhdrs.size());

``````````

</details>


https://github.com/llvm/llvm-project/pull/145950