[PATCH 16/16] x86_64, entry: Remove a bogus ret_from_fork optimization

[Andy Lutomirski]

ret_from_fork checks TIF_IA32 to determine whether pt_regs and the
related state make sense for ret_from_sys_call.  This is entirely
the wrong check.  TS_COMPAT would make a little more sense, but
there's really no point in keeping this optimization at all.

This fixes a return to the wrong user CS if we came from int 0x80 in
a 64-bit task.

Acked-by: Ingo Molnar <mingo@xxxxxxxxxx>
Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
---
 arch/x86/kernel/entry_64.S | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
index 4670fcdb5150..519498d1c8ec 100644
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -550,11 +550,14 @@ ENTRY(ret_from_fork)
 	testl $3,CS(%rsp)			# from kernel_thread?
 	jz   1f
 
-	testl $_TIF_IA32, TI_flags(%rcx)	# 32-bit compat task needs IRET
-	jnz  int_ret_from_sys_call
-
-	RESTORE_TOP_OF_STACK %rdi
-	jmp ret_from_sys_call			# go to the SYSRET fastpath
+	/*
+	 * By the time we get here, we have no idea whether our pt_regs,
+	 * ti flags, and ti status came from the 64-bit SYSCALL fast path,
+	 * the slow path, or one of the ia32entry paths.
+	 * Use int_ret_from_sys_call to return, since it can safely handle
+	 * all of the above.
+	 */
+	jmp  int_ret_from_sys_call
 
 1:
 	movq %rbp, %rdi
-- 
2.1.0

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