[PATCH v6 0/1] MR2118: ntdll: Explaining with wine requires a 3G/1G split on 32bit

[Steve Schnepp (＠steveschnepp)]

Windows has a 2G/2G split by default, but some addresses are fixed
in the win32 API, so we have to ensure that those addresses are still
available in the virtual memory space and not taken by Linux.

Kudos to stefand for taking the time to explain on IRC.

--
  v6: ntdll: Explaining why 3G/1G split is needed on 32bit
 
https://gitlab.winehq.org/wine/wine/-/merge_requests/2118

[PATCH v6 1/1] ntdll: Explaining why 3G/1G split is needed on 32bit

[Steve Schnepp]

From: Steve Schnepp <steve.schnepp(a)pwkf.org>

Windows has a 2G/2G split by default, but some addresses are fixed
in the win32 ABI, so we have to ensure that those addresses are still
available in the virtual memory space and not taken by Linux.

Kudos to stefand for taking the time to explain on IRC.
---
 dlls/ntdll/unix/loader.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/dlls/ntdll/unix/loader.c b/dlls/ntdll/unix/loader.c
index 65934acfc36..5a12ee64038 100644
--- a/dlls/ntdll/unix/loader.c
+++ b/dlls/ntdll/unix/loader.c
@@ -2309,6 +2309,22 @@ static int pre_exec(void)
 
 #elif defined(__linux__) && (defined(__i386__) || defined(__arm__))
 
+/* In the Win32 ABI, KUSER_SHARED_DATA is at a fixed address (0x7ffe0000), so
+ * it needs to be mapped precisely there in every virtual process space by the
+ * wine process loader.
+ *
+ * It is used to map kernel address space pages to user space, so that it is
+ * user-space readable without any syscall overhead.
+ *
+ * For that, we need to ensure that Linux doesn't map anything internal in that
+ * area, which it does if configured with a 2G/2G split. Unfortunately, Linux
+ * does initially map the stack there, along with its own vDSO.
+ *
+ * More information:
+ *
+ * https://learn.microsoft.com/en-us/windows-hardware/drivers/ddi/ntddk/ns-ntdd...
+ * https://msrc-blog.microsoft.com/2022/04/05/randomizing-the-kuser_shared_data...
+ */
 static void check_vmsplit( void *stack )
 {
     if (stack < (void *)0x80000000)
-- 
GitLab

https://gitlab.winehq.org/wine/wine/-/merge_requests/2118