Mitigation for "clone on sparc might fail with -EFAULT for no valid reason" (bz 31394)

It seems the kernel can not deal with uncommitted stack space in the area intended
for the register window when executing the clone() system call. So create a nested
frame (proxy for the kernel frame) and flush it from the processor to memory to
force committing pages to the stack before invoking the system call.

Bug: https://www.mail-archive.com/[email protected]/msg62592.html
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31394
See-also: https://lore.kernel.org/sparclinux/62f9be9d-a086-4134-9a9f-5df8822708af@mkarcher.dialup.fu-berlin.de/
Signed-off-by: Michael Karcher <[email protected]>
Reviewed-by: DJ Delorie <[email protected]>

Author: Michael Karcher

sysdeps/unix/sysv/linux/sparc/sparc32/clone.S

@@ -28,6 +28,9 @@
 	.text
 ENTRY (__clone)
 	save	%sp,-96,%sp
+	save	%sp,-96,%sp
+	flushw
+	restore
 	cfi_def_cfa_register(%fp)
 	cfi_window_save
 	cfi_register(%o7, %i7)

sysdeps/unix/sysv/linux/sparc/sparc64/clone.S

@@ -32,6 +32,9 @@
 
 ENTRY (__clone)
 	save	%sp, -192, %sp
+	save	%sp, -192, %sp
+	flushw
+	restore
 	cfi_def_cfa_register(%fp)
 	cfi_window_save
 	cfi_register(%o7, %i7)