[PATCH] ntdll: Add a shortcut to NtReadVirtualMemory for the local process

[Martin Storsjo]

EnumProcessModules uses ReadProcessMemory (which forwards to
NtReadVirtualMemory) for reading the tables of a process, even for
the current process. When running wine within docker,
NtReadVirtualMemory currently fails unless the SYS_PTRACE capability
is explicitly added.

LLVM's libunwind uses EnumProcessModules for locating .eh_frame
sections in all loaded modules when unwinding dwarf exceptions
(used on i686). As it is used for locating sections within the
current process, libunwind only calls EnumProcessModules for
the current process.

Therefore, exception handling in executables with libunwind on i686
doesn't work when run with wine within docker.

Even for setups where NtReadVirtualMemory is allowed, it is
significantly slower than just reading the memory directly within
the process. Currently, handling 10000 thrown exceptions with
libunwind on i686 takes 24 seconds when run in wine, while it
runs in around 1.4 seconds with this patch.

Signed-off-by: Martin Storsjo <martin at martin.st>
---
 dlls/ntdll/virtual.c | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/dlls/ntdll/virtual.c b/dlls/ntdll/virtual.c
index d15b49f6fd..566b5982a7 100644
--- a/dlls/ntdll/virtual.c
+++ b/dlls/ntdll/virtual.c
@@ -3521,6 +3521,26 @@ NTSTATUS WINAPI NtReadVirtualMemory( HANDLE process, const void *addr, void *buf
 
     if (virtual_check_buffer_for_write( buffer, size ))
     {
+        if (process == GetCurrentProcess())
+        {
+            sigset_t sigset;
+
+            server_enter_uninterrupted_section( &csVirtual, &sigset );
+            if (virtual_check_buffer_for_read( addr, size ))
+            {
+                memcpy(buffer, addr, size);
+                status = STATUS_SUCCESS;
+            }
+            else
+            {
+                size = 0;
+                status = STATUS_ACCESS_VIOLATION;
+            }
+            server_leave_uninterrupted_section( &csVirtual, &sigset );
+            if (bytes_read) *bytes_read = size;
+            return status;
+        }
+
         SERVER_START_REQ( read_process_memory )
         {
             req->handle = wine_server_obj_handle( process );
-- 
2.17.1