[lldb/linux] Make truncated reads work (#106532)

Previously, we were returning an error if we couldn't read the whole
region. This doesn't matter most of the time, because lldb caches memory
reads, and in that process it aligns them to cache line boundaries. As
(LLDB) cache lines are smaller than pages, the reads are unlikely to
cross page boundaries.

Nonetheless, this can cause a problem for large reads (which bypass the
cache), where we're unable to read anything even if just a single byte
of the memory is unreadable. This patch fixes the lldb-server to do
that, and also changes the linux implementation, to reuse any partial
results it got from the process_vm_readv call (to avoid having to
re-read everything again using ptrace, only to find that it stopped at
the same place).

This matches debugserver behavior. It is also consistent with the gdb
remote protocol documentation, but -- notably -- not with actual
gdbserver behavior (which returns errors instead of partial results). We
filed a
[clarification
bug](https://sourceware.org/bugzilla/show_bug.cgi?id=24751) several
years ago. Though we did not really reach a conclusion there, I think
this is the most logical behavior.

The associated test does not currently pass on windows, because the
windows memory read APIs don't support partial reads (I have a WIP patch
to work around that).

1 files changed, 6 insertions, 16 deletions

diff --git a/lldb/source/Plugins/Process/gdb-remote/GDBRemoteCommunicationServerLLGS.cpp b/lldb/source/Plugins/Process/gdb-remote/GDBRemoteCommunicationServerLLGS.cpp
index f6f13a8..504c994 100644
--- a/lldb/source/Plugins/Process/gdb-remote/GDBRemoteCommunicationServerLLGS.cpp
+++ b/lldb/source/Plugins/Process/gdb-remote/GDBRemoteCommunicationServerLLGS.cpp
@@ -2526,28 +2526,18 @@ GDBRemoteCommunicationServerLLGS::Handle_memory_read(
   size_t bytes_read = 0;
   Status error = m_current_process->ReadMemoryWithoutTrap(
       read_addr, &buf[0], byte_count, bytes_read);
-  if (error.Fail()) {
-    LLDB_LOGF(log,
-              "GDBRemoteCommunicationServerLLGS::%s pid %" PRIu64
-              " mem 0x%" PRIx64 ": failed to read. Error: %s",
-              __FUNCTION__, m_current_process->GetID(), read_addr,
-              error.AsCString());
-    return SendErrorResponse(0x08);
-  }
-
-  if (bytes_read == 0) {
-    LLDB_LOGF(log,
-              "GDBRemoteCommunicationServerLLGS::%s pid %" PRIu64
-              " mem 0x%" PRIx64 ": read 0 of %" PRIu64 " requested bytes",
-              __FUNCTION__, m_current_process->GetID(), read_addr, byte_count);
+  LLDB_LOG(
+      log,
+      "ReadMemoryWithoutTrap({0}) read {1} of {2} requested bytes (error: {3})",
+      read_addr, byte_count, bytes_read, error);
+  if (bytes_read == 0)
     return SendErrorResponse(0x08);
-  }
 
   StreamGDBRemote response;
   packet.SetFilePos(0);
   char kind = packet.GetChar('?');
   if (kind == 'x')
-    response.PutEscapedBytes(buf.data(), byte_count);
+    response.PutEscapedBytes(buf.data(), bytes_read);
   else {
     assert(kind == 'm');
     for (size_t i = 0; i < bytes_read; ++i)