From 6c82b8a378a6f59e94a81d91225db4fabf6e2bff Mon Sep 17 00:00:00 2001
From: Augusto Noronha <augusto2112@me.com>
Date: Tue, 11 May 2021 13:07:02 -0300
Subject: [PATCH] Change Target::ReadMemory to ensure the amount of memory read
 from the file-cache is the amount requested.

This change ensures that if for whatever reason we read less bytes than expected (for example, when trying to read memory that spans multiple sections), we try reading from the live process as well.

Reviewed By: jasonmolenda

Differential Revision: https://reviews.llvm.org/D101390
---
 lldb/source/Target/Target.cpp | 36 ++++++++++++++++++++++++++---------
 1 file changed, 27 insertions(+), 9 deletions(-)

diff --git a/lldb/source/Target/Target.cpp b/lldb/source/Target/Target.cpp
index f41a1528053b..d879c0f6221d 100644
--- a/lldb/source/Target/Target.cpp
+++ b/lldb/source/Target/Target.cpp
@@ -1758,19 +1758,30 @@ size_t Target::ReadMemory(const Address &addr, void *dst, size_t dst_len,
   if (!resolved_addr.IsValid())
     resolved_addr = addr;
 
-  bool is_readonly = false;
+  // If we read from the file cache but can't get as many bytes as requested,
+  // we keep the result around in this buffer, in case this result is the
+  // best we can do.
+  std::unique_ptr<uint8_t[]> file_cache_read_buffer;
+  size_t file_cache_bytes_read = 0;
+
   // Read from file cache if read-only section.
   if (!force_live_memory && resolved_addr.IsSectionOffset()) {
     SectionSP section_sp(resolved_addr.GetSection());
     if (section_sp) {
       auto permissions = Flags(section_sp->GetPermissions());
-      is_readonly = !permissions.Test(ePermissionsWritable) &&
-                    permissions.Test(ePermissionsReadable);
-    }
-    if (is_readonly) {
-      bytes_read = ReadMemoryFromFileCache(resolved_addr, dst, dst_len, error);
-      if (bytes_read > 0)
-        return bytes_read;
+      bool is_readonly = !permissions.Test(ePermissionsWritable) &&
+                         permissions.Test(ePermissionsReadable);
+      if (is_readonly) {
+        file_cache_bytes_read =
+            ReadMemoryFromFileCache(resolved_addr, dst, dst_len, error);
+        if (file_cache_bytes_read == dst_len)
+          return file_cache_bytes_read;
+        else if (file_cache_bytes_read > 0) {
+          file_cache_read_buffer =
+              std::make_unique<uint8_t[]>(file_cache_bytes_read);
+          std::memcpy(file_cache_read_buffer.get(), dst, file_cache_bytes_read);
+        }
+      }
     }
   }
 
@@ -1809,7 +1820,14 @@ size_t Target::ReadMemory(const Address &addr, void *dst, size_t dst_len,
     }
   }
 
-  if (!is_readonly && resolved_addr.IsSectionOffset()) {
+  if (file_cache_read_buffer && file_cache_bytes_read > 0) {
+    // Reading from the process failed. If we've previously succeeded in reading
+    // something from the file cache, then copy that over and return that.
+    std::memcpy(dst, file_cache_read_buffer.get(), file_cache_bytes_read);
+    return file_cache_bytes_read;
+  }
+
+  if (!file_cache_read_buffer && resolved_addr.IsSectionOffset()) {
     // If we didn't already try and read from the object file cache, then try
     // it after failing to read from the process.
     return ReadMemoryFromFileCache(resolved_addr, dst, dst_len, error);