[PATCH v3 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.

--
  v3: ntdll: Explaining with wine requires a 3G/1G split on 32bit

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

[PATCH v3 1/1] ntdll: Explaining with wine requires a 3G/1G split on 32bit

[Steve Schnepp]

From: Steve Schnepp

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.
---
 dlls/ntdll/unix/loader.c | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/dlls/ntdll/unix/loader.c b/dlls/ntdll/unix/loader.c
index 65934acfc36..652f27e5ca3 100644
--- a/dlls/ntdll/unix/loader.c
+++ b/dlls/ntdll/unix/loader.c
@@ -2309,6 +2309,28 @@ static int pre_exec(void)

#elif defined(__linux__) && (defined(__i386__) || defined(__arm__))

+/* In Win32, KUSER_SHARED_DATA is at a fixed address (0x7ffe0000), so it needs
+ * to be mapped there in every virtual process space by wine.
+ *
+ * It is used to map kernel address space pages to user space, so that it is
+ * user-space readable without any syscall overhead. It is very comparable
+ * to Linux's vDSO.
+ *
+ * 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.
+ *
+ * A potentially nice thing to have is to move KUSER_SHARED_DATA if the
+ * hardcoded address does not work.
+ *
+ * Yet there is no official way to communicate the address from ntdll to
+ * kernel32, as is should be at a fixed address in the first place.  The only
+ * alternative might be wine-specific private calls from kernel32 to ntdll,
+ * which is bad.
+ *
+ * Moreover, while it would probably make 99% of applications out there happy,
+ * this would still break the ABI, which is also bad.
+ */
 static void check_vmsplit( void *stack )
 {
     if (stack < (void *)0x80000000)

[Steve Schnepp (＠steveschnepp)]

Amended the commit with the first remarks.