Merge pull request #9187 from jasonmolenda/cp/dont-use-host-memory-at-target-vmaddr-0

[lldb] Don't use a vm addr range starting at 0 for local memory (llvm#100288)

Author: jasonmolenda

lldb/docs/resources/lldbgdbremote.md

@@ -1403,6 +1403,12 @@ For instance, with a macOS process which has nothing mapped in the first
 The lack of `permissions:` indicates that none of read/write/execute are valid
 for this region.
 
+The stub must include `permissions:` key-value on all memory ranges
+that are valid to access in the inferior process -- the lack of
+`permissions:` means that this is an inaccessible (no page table
+entries exist, in a system using VM) memory range.  If a stub cannot
+determine actual permissions, return `rwx`.
+
 **Priority To Implement:** Medium
 
 This is nice to have, but it isn't necessary. It helps LLDB
@@ -2434,4 +2440,4 @@ The `0x` prefixes are optional - like most of the gdb-remote packets,
 omitting them will work fine; these numbers are always base 16.
 
 The length of the payload is not provided.  A reliable, 8-bit clean,
-transport layer is assumed.
+transport layer is assumed.

lldb/source/Expression/IRMemoryMap.cpp

@@ -84,7 +84,7 @@ lldb::addr_t IRMemoryMap::FindSpace(size_t size) {
   // any allocations.  Otherwise start at the beginning of memory.
 
   if (m_allocations.empty()) {
-    ret = 0x0;
+    ret = 0;
   } else {
     auto back = m_allocations.rbegin();
     lldb::addr_t addr = back->first;
@@ -116,10 +116,18 @@ lldb::addr_t IRMemoryMap::FindSpace(size_t size) {
     Status err = process_sp->GetMemoryRegionInfo(ret, region_info);
     if (err.Success()) {
       while (true) {
-        if (region_info.GetReadable() != MemoryRegionInfo::OptionalBool::eNo ||
-            region_info.GetWritable() != MemoryRegionInfo::OptionalBool::eNo ||
-            region_info.GetExecutable() !=
-                MemoryRegionInfo::OptionalBool::eNo) {
+        if (region_info.GetRange().GetRangeBase() == 0 &&
+            region_info.GetRange().GetRangeEnd() < end_of_memory) {
+          // Don't use a region that starts at address 0,
+          // it can make it harder to debug null dereference crashes
+          // in the inferior.
+          ret = region_info.GetRange().GetRangeEnd();
+        } else if (region_info.GetReadable() !=
+                       MemoryRegionInfo::OptionalBool::eNo ||
+                   region_info.GetWritable() !=
+                       MemoryRegionInfo::OptionalBool::eNo ||
+                   region_info.GetExecutable() !=
+                       MemoryRegionInfo::OptionalBool::eNo) {
           if (region_info.GetRange().GetRangeEnd() - 1 >= end_of_memory) {
             ret = LLDB_INVALID_ADDRESS;
             break;